Tuesday, December 31, 2013

Using Agilent Command Expert to Help Write Programs

Happy New Year everyone!  

I am writing this on New Years Eve 2013 and I want to wish everyone a happy and safe 2014.  Ed, Gary, and I had a goal of getting 4 blog posts out a month this year and with this being the fourth blog post this month, we have met that goal!  Thank you to everyone for taking the time to read our blog.

Today, I am going to provide a short intro to  Agilent Command Expert (hereto referred to as ACE). ACE is a free tool that Agilent offers that helps you program your instruments. You can write simple scripts in ACE that you can save and share with other programmers.  The coolest feature of ACE is that it allows you to export the sequence to different programming languages.   You can find the latest version of Command Expert at: 

Using ACE is pretty easy.  The first thing that you need to do is set up your instrument.  My instrument is an Agilent N6700B.  Here is what the setup/edit screen looks like:  

Once you choose the instrument, ACE will download the command set so you do not need to have the programming guide handy.  This is a great feature since you can easily see all of the commands as well as all of the inputs and outputs of the commands. 

Here is the set voltage screen:

You can see here that it shows you all of the programmable items in the command (in this case the voltage and the channel list).  Once you set everything up as you'd want, click add step and it adds it to the sequence.

You can also do queries with ACE.  Here is how you would do a MEAS:VOLT? query:

Again, it shows you all of the programmable items (in this case only the channel list).  It also shows you that there is returned data.  Click add step to get it added to the sequence.

I am going to continue using the same SCPI command as I used in my example last week, here is completed sequence from the example from last month:

Now for the fun part!  You can export the sequence to just straight SCPI:

To C#:

To C++:


To Matlab:

It gives you the option to copy the text to the clipboard so that you can paste it into the text editor for your programming language.  

There is also a tool included that allows you to create sequences in Labview.  There is a really good tutorial included with the software that shows you how to do this.

There is a ton more info on the ACE webpage link at the beginning of this blog post.

I hope that this short intro to Agilent Command Expert was useful to everyone.  Please feel free to leave a comment.  Happy New Year everyone!

Sunday, December 29, 2013

Shower by cell phone and the attitude of gratitude

As I type this, I’m sitting on an airplane, clean, comfortable (as much as that’s possible sitting in economy), clean shaven, well fed, flying home to New Jersey from Frankfurt. Things could have been different…
I started my travels today from a hotel in Sindelfingen, Germany, drove to the Stuttgart airport, and took a short flight to Frankfurt where I connected with this much longer flight home (8.5 hours). I was visiting some of Agilent’s distribution partners in Germany and France over the last ten days to present training on our newest power supply products.

Last night, before I went to sleep, I called the front desk of the hotel to ask for a 6 am wake-up call. I wanted to have time to shower, shave, dress, eat breakfast, check out, gas-up the car, drive to the airport, return the car, exchange my leftover Euros for US dollars, and finally make my morning flight. I also set my new cell phone alarm for the same time (6 am). I recently purchased my first smart phone, so I’m happy to be using its features!

6 am rolls around, my cell phone alarm goes off (yes, I managed to set it correctly), and I wake up momentarily not knowing exactly where I am or what is happening (typical for trips where you change hotels a lot). 5 seconds after the alarm starts its intermittent beeping, I come to my senses in my nearly pitch-black, normally very quiet room. The only light is a dim glow from a tiny LED on the room thermostat and the only sound is a low hum from the heating system fan in between the alarm beeps. 10 seconds after the alarm started beeping, the room suddenly become absolutely pitch-black. No light at all! I wonder if I’m dreaming and I feel like I have completely lost my eyesight! But then I noticed that the heating system fan went from humming to silent, and the thermostat LED is no longer glowing. Totally dark and totally quiet (between phone beeps) with no thermostat LED and no fan? OK, perhaps I’m not dreaming and most likely, this is a power failure! In the absolute darkness, I fumble for my beeping cell phone and manage to push a button on it partially illuminating the screen. I turn off the alarm, and now can see just enough to try the lamp switch next to the bed. Click, click on the lamp….no light. Yeah, definitely a power failure. I know that the cell phone, an Apple 5S, has a flashlight feature that is a nice, bright white LED. Turning that on, I’m now able to easily maneuver around the room. I pick up the room phone, but it is dead. I look outside, and it is dark everywhere. Yep, it’s not just my room, nor is it just the hotel. The power is out everywhere. Guess I won’t be getting my 6 am wake-up call!

Next, it is time to shower. I strategically place the cell phone on the bathroom counter angled just right so the light bounces off the ceiling and I can see reasonably well in the shower. Shaving is a bit more difficult with the limited and oddly angled light, but I manage. Then the lights come back on. Whoo-hoo!  About 10 minutes later, the hotel phone rings and the manager is very apologetic about the wake-up call being 40 minutes late. I tell him “no problem” and explain my phone alarm still woke me on time.
So had my cell phone alarm not awakened me, I may have slept too long resulting in a missed flight. And if I did not have the flashlight feature on the phone, I would not have been able to shower and shave making me more comfortable during a full day of travel. Earlier in my trip, the cell phone GPS also saved my colleague and me by guiding us to our hotel, to the office, and enabling us to find our way back to the hotel after walking around town for lunch.

Cell phones play a very large role in our lives today. While we humans survived the vast majority of our existence without them, smart phones and all electronic technology have vastly changed our lives. While I regularly wonder how my life would have been different had I lived in a time without electricity, today, my smart phone changed my experiences. I could have missed my flight had my cell phone alarm not gone off. I could have been less comfortable during my long travel day had I not been able to shower by cell phone light. Despite a power failure preventing a wake-up call and no light in my hotel room for the early morning, my day was not really disrupted thanks to my cell phone. While these things are minor in the grand scheme of things, I am still grateful for the technology we have that allows us to do the things we do. And I’m glad to be a part of that technology by working with power products. Many cell phone manufacturers use Agilent power products during their design, verification, and manufacturing processes and I am happy to be a part of that chain.

May all of your travels during this holiday season be uneventful…and Happy New Year!

Friday, November 29, 2013

Using Labview without Using an Instrument Specific Driver

Hi everyone!

Labview is presently one of the most popular programming languages for programming test and measurement equipment.   Here at Power and Energy Central, we often get requests for more Labview programming examples for our products (which is definitely something on my agenda).   We also get requests for Labview drivers (which do exist for many of our products).  I thought that I would use this month’s blog posting to demonstrate how to program without using a driver.  There are a few advantages to this approach.  The first and main one is that it gives you access to the full SCPI command set of the instrument.  Anything you can do with the instrument is available to you.  The second advantage is that you do not need to worry about downloading and setting up drivers. 

I am going to work through an example using my Agilent N6700B on LAN.  We are going to use VISA calls in Labview to communicate with the instrument.  The first thing that we are going to need to do is get the VISA init string from the Agilent IO Libraries (or whatever IO Library you are using).  You can see the init string from my N6700B below (from the Agilent IO Libraries):

With the VISA address in hand, start up Labview and choose a blank VI.  Go to the Functions Pallette -> Instrument IO -> VISA ->Advanced and choose Open.  This function will open up a VISA session with your instrument.  There are quite a few inputs to this function but I usually just set up the instrument address and the VISA Open timeout:

After opening a session, we are ready to send our first commands.  I usually like to send a *RST and a *IDN?  so I know that I am in a known state and fully communicating with my instrument.   To send a command, you are going to go to the VISA menu and choose Write.  There are a few lines that you will need to connect here.  In Labview, you will always connect the “VISA Resource Name Out” and “error out” lines through your entire program (you will see that throughout this example).   The command is the other input.  This will need to be a string.  

Since we sent a query, we need to read out the output buffer.  This is done by choosing read in the VISA menu.   You need to do with the read are set the byte count to be read (I set it to 100 bytes so it is totally out of the way).   You also need a string indicator so that you can read and display the results of the *IDN query.

I am going to finish out my program by setting my supply to 4 V, turning the output on, and measuring the voltage.  All of these steps will use the same reads and writes that we used before.  The last thing I will do is use a VISA Close.  Using a Close will de-allocate all the resources and release the instrument.  This is generally good programming practice and is often overlooked.  Here is what the final program looks like:

After I run the completed program, I get the following results:

We can see that the results are as we expected and our program is working.

From this example, you can see that doing simple things is pretty easy in Labview.   If you are interested in downloading the example, please leave a comment here and I will post it so that you can get it.  As always, if you have any questions please feel free to post in our comments.  Take care!

Wednesday, November 27, 2013

Why can’t you put electronic loads in series?

The quick answer to the above question is: because you will likely damage at least one of the loads with excessive voltage! For the longer answer, read on….

[By the way…this is a milestone post for Watt’s Up? since it is post # 100, so thank you to our readers…and Happy Thanksgiving to those in the US or celebrating elsewhere!]

I’ll start with a brief explanation of what an electronic load is, and what it is used for. I am specifically talking about DC electronic loads here. A DC electronic load is a two-terminal electrical instrument that draws power from a DC source. Loads are used to test DC sources. Any device that has a source of DC output power, such as a DC power supply, a DC-to-DC converter, a battery, a fuel cell, or a solar panel, can have power drawn from it with an electronic load. Click here to see Agilent’s DC electronic loads.

For example, to test a fixed-output DC power supply that is rated for 20 V, 5 A, 100 W, you would connect the power supply output to an electronic load with ratings that are equal to or greater than the power supply ratings and that can draw a constant current from the power supply. Since the power supply is regulating the voltage (20 V), the load must regulate the current it draws from the power supply (up to 5 A). If your DC power supply is a constant current source, the load must be capable of drawing power while regulating voltage. You can set most electronic loads to draw power by regulating either constant voltage (CV) or constant current (CC). You can also set many electronic loads to regulate constant resistance (CR) across their input terminals, and some can regulate constant power (CP).

If the power supply to be tested has a higher output voltage than a single electronic load can handle, you may be tempted to put multiple load inputs in series to accommodate the higher voltage. After all, you can do this will power supply outputs to get higher voltage (click here)….why not with loads?

Putting electronic loads in series can cause one of the load inputs to be exposed to a voltage beyond its capabilities that could result in damage to the load. You are putting loads in series because a single load does not have a high enough voltage rating to handle the voltage of your DC power source. But since one of the load inputs could become a low impedance (nearly a short circuit) during test, all of the voltage from your DC source could appear across the other load input in series. There are several scenarios that can result in this destructive situation. To understand these scenarios, you first have to understand how an electronic load works.

Loads work by controlling the conduction of FETs across their input terminals. The control is realized by using a feedback loop to adjust a measured level (such as the input current) so that it equals a reference level (such as the set current). My colleague, Ed Brorein, posted about this topic last year (click here).

When you put multiple electronic loads in series to accommodate higher voltage, one problem scenario occurs when you set both loads to operate in CC mode. You set the same current on both loads. The exact same current flows through both loads (see figure below), but due to small errors in the accuracy of the settings, the real set values will never be exactly equal. Therefore, one of the loads will be trying to draw a higher current (Load 2 in the figure) than the other load (Load 1 in the figure). Since Load 1 will limit the current at the lower value (9.99 A in this example), Load 2 can never attain its real set point (10.01 A in this example). So its internal feedback loop continues to tell the FETs to conduct more and more current until the FETs are fully on looking nearly like a short circuit. This results in nearly all of the power supply voltage appearing across the Load 1 input which can damage it.

If you operate one load input in CC and one in CV, at first this looks like it will result in a stable operating point. However you have to think about how you get to that stable operating point. If you set the loads first before you connect the voltage, before the voltage is applied, the CC load is not satisfied (no current is flowing) so it goes to a short and the CV load is also not satisfied (no voltage is present) so it goes to an open. When the test voltage is applied, all of the voltage initially appears across the open CV load and can damage it. There are other procedures to follow that could temporarily result in a stable operating point (such as slowly increasing the test voltage if you have that ability), but if any fault condition occurs in any of the loads, they try to protect themselves by either turning the FETs on hard (a short) or opening the FETs. In either case, the large destructive voltage will appear across one of the loads in the series connection resulting in damage.

One of my colleagues, Bob Zollo, wrote an article entitled “Why Can’t You Put Electronic Loads In Series To Get More Voltage?” that appeared in Electronic Design on November 4, 2013. For some additional information about this topic, click here to read the article.

So you can see that putting loads in series can too easily result in damage to at least one of the load inputs. I strongly recommend that you do not do it!

Wednesday, November 13, 2013

How to Make More Accurate Current Measurements

There are a number of ways to make current measurements, including magnetically coupled probes, Hall-effect devices, and even some more exotic field sensing probes, but a good quality resistive shunt really cannot be beat in terms of accuracy, bandwidth, and overall general performance.

We likewise make considerable use of high performance shunts in our DC power products to provide extremely accurate current read-back of load currents, spanning the full range of output loading. Not only is the quality and design of the shunt itself critical, but how you treat it and make use of it are all equally important to get great current measurement performance. At the surface it may seem simple; it’s just measuring the voltage drop across a resistor. In reality it is no simple task. It requires appropriate metrological resources to validate the performance.  There are a lot of potential sources of error to recognize, quantify, and contend with.

When working with folks I sometimes encounter those who prefer to develop in their own current measurement into their test systems, instead of relying on the current read-back system already build into their system DC source. There are times when this is the right thing to do and is fine when done correctly. However some of the time there is the preconception that the DC source cannot provide an accurate measurement. The reality is there is a wide selection of DC sources available spanning a wide range of performance, Most likely something will be available that adequately addresses one’s needs. A second issue is, when developing current measurement capabilities for a test system, is truly recognizing all the potential sources of error. It goes well beyond having a good DVM and a good shunt resistor in the test system.  

A colleague here in our R&D group, Mark Peffley, wrote a comprehensive article that was just published. It covers a myriad of things in depth to be taken into consideration in order to make accurate current measurements, including:
  • Temperature dependencies
  • Self-heating and thermal equilibrium
  • Temperature gradients
  • Thermo-electric effects
  • Additional sources of offset errors
  • Voltage drop considerations
  • Shunt selection practical considerations
  • And more!
So using a shunt is a great foundation for making highly accurate current measurements. That’s why we use them in our power products. But, as Mark points out, there is a lot more to it than just Ohm’s law. When using one of our power products we factor all these things in so that they become a non-issue for the user. However, if you do plan to add current measurement into your test systems then I highly recommend reading Mark’s article “Obtain Accurate Current Measurement” (click here to access) as it is a great reference on the subject!

Wednesday, November 6, 2013

Paralleling power supplies for more power without compromising performance!

A year ago my colleague here, Gary, provided a posting “How can I get more power from my power supplies?” (Click here to review). He describes connecting power supplies in series for higher voltage or in parallel for higher current. Along with suggested set ups a list of requirements and precautions are also provided.

Connecting multiple power supplies in parallel operating as voltage sources is always problematic as there will be some imbalance of voltage between them. That’s why, in this previous posting, one unit operates as a voltage source and the remaining paralleled units operate in constant current. The compliance voltage limit of all the units operating in constant current need to be set higher than the master in operating in constant voltage in order to maintain this operation. This is illustrated in Figure 1.

Figure 1: Operating power supplies in parallel for higher power

As long as a high level of loading is maintained the paralleled units remain in their respective operating modes (in this case at least 2/3 loading). However, what happens if you cannot maintain that high level of loading? It is possible in practice to operate at lighter loads with this approach. In this case it is important to set the voltage levels of all the units the same. Now what happens is when the units are fully loaded they operate as already described, with the lowest voltage unit remaining in constant voltage. But when they are unloaded the lower voltage units transition to unregulated operation and the highest voltage unit then maintains the overall output in constant voltage. This is shown in Figure 2, for 0 to 1/3 loading.

Figure 2: Conditions of power supplies connected in parallel at light loading

There is a bit of performance compromises as a result. The transition between the lowest and highest voltage limits adds to the voltage regulation. Also, due to different units experiencing mode crossover transitions between constant voltage, constant current and unregulated operating modes transient voltage performance suffers considerably.

An improvement on this direct paralleling approach is having a master-slave arrangement with control signals to maintain current sharing across units. Our N5700A and N8700A series power supplies use such a control arrangement as depicted in Figure 3, taken from the N5700A user’s guide.

Figure 3: N5700A Connection for parallel operation (local sensing used)

With this arrangement the master unit, operating in constant voltage, provides an analog current programming output signal to the slave unit, operating in constant current. In this way the two units equally share the load current across a wide range of load current.

Still, having multiple units with only one in constant voltage does not provide as good of dynamic performance as a single voltage source of higher power.  A unique and innovative approach was taken with our N6900A / N7900A series Advance Power System (APS) to support seamless parallel operation without compromising performance. The paralleling arrangement for our N6900A / N7900A series APS is depicted in Figure 4.

Figure 4: N6900A / N7900A series APS Connection for parallel operation

The N6900A / N7900A series APS paralleling arrangement also uses an analog control signal for driving current sharing. However with this arrangement there is no master or slaves. All units remain in constant voltage while equally sharing current. This provides the user with an easy way to scale a power system as required without having to worry about compromising performance.

Thursday, October 31, 2013

Power Supply Programming Part 2: What Type of Programming Language to Use

Happy Halloween Watt’s up fans!  Today I want to look at what programming language you are going to use to write your program.  Instead of recommending a particular language, I am going to break it down by graphical versus text based programming.

Let me start by saying that there really is no correct answer to this question.   This is a matter of personal taste.   

I am going to start with a bit about my background.  Unlike most of my colleagues, I did not specialize in analog electronics in college.  I focused more on computer engineering.  Due to this specialization, I have taken quite a few programming based courses.  I prefer sitting down and programming using a text based programming language because of my background. 

 Graphical programming languages are a very popular option (Agilent VEE is the one that I am most familiar with).     I find these programs are great for doing short programs.  If something can take up less than one page on your computer screen, then it works pretty well here.  These languages also make building user interfaces really easy since there are a lot of easy to access functions for controlling and displaying instrument data.  I personally find that they get very unwieldy if you want to send and read a lot of data with an instrument.  I also find the looping constructs to be strange.   People have told me that these graphical languages look very similar to circuit diagrams and I can see how people would prefer that kind of view to just plain text programming.

I am going to make a confession.  If I have to write a program quickly and I do not have to show it to anybody, I still will write it in HPBASIC.  I find it to be very easy to do simple instrument programming.  There is no need for drivers, once it is set up properly; sending and receiving information with an instrument is a breeze.    Large programs do not fare very well in HPBASIC though.

My preferred way to program these days is Visual Basic (using VISA-COM IO).  If you look at the power supply example programs that we provide, there is a lot of VB in there.  I feel that a text based program allows you to write much more compact code. It takes up a lot less screen space than an equivalent graphically based language.  Something like Visual Basic is also more versatile since it is not only for test and measurement but for more general applications.  The looping constructs work very nicely here and to me the flow makes more sense.  I also find typing quicker than connecting boxes.  Text based programming does have some cons though.  For one, the graphical languages are written from the bottom up to do instrument control.  They have built in functions and data manipulation that make thing easier.  The graphical languages also have some really good libraries for building User Interfaces. 

 The real correct answer to the questions is the best language to use is the one that you are most comfortable programming in.  If you think that I missed any pros or cons please feel free to share in the comments.

Wednesday, October 30, 2013

Protect your DUT from over-current in more ways than one

Last month, I posted about one of our new families of products: the N6900/N7900 Series 1- and 2-kW Advanced Power System (APS) DC Power Supplies (click here). I typically like to post about more general power topics rather than focus on specific Agilent products, but this product has some really interesting features from which you can benefit. After 33 years of working on power here, there aren’t too many new products that get me excited, but this is one of them! So here is a story about an application for it.

Earlier this month, I visited one of our customers that had a device under test (DUT) whose input was sensitive to too much current. That is typically not a difficult issue to protect against using Agilent power supplies with over-current protection (OCP). Set the current limit to a value that you don’t want to exceed, turn on OCP, and the power supply output will go into protect (turn off) when the current limit value is reached. Simple enough! But this customer had an additional requirement. In addition to an OCP value as just described, he also wanted to shut down the output if the current exceeded a lower current for more than a specified amount of time. So he wanted the power supply output to go into protect (turn off) if either of the following conditions occurred on his DUT (I changed this example to protect his information):

       1.  DUT input current exceeds 6 A for any amount of time, or
       2.  DUT input current exceeds 4.5 A for 80 ms

To be honest, at the time of the visit, I wasn’t sure if our new product could do this. The product is so new and so feature-rich that I am not yet familiar with all of its capabilities. But when I returned to my office, I set it up and found it was very easy to do! Here is the solution:

I used the advanced signal routing and logical trigger expressions built into our N7952A APS to setup both requirements. I could have sent SCPI commands to setup the same trigger configuration, but our free Power Assistant Software (N7906A) made this even easier. Figure 1 shows the software with the configuration.

If, after creating the configuration, I want all of the SCPI commands that correspond to it for a program, I could use the software feature “SCPI to clipboard” that creates them from the configuration. See Figure 2.

Take a look at this feature in action. Figure 3 shows a scope trace of the current waveform. As you can see, currents that are less than 4.5 A do not trip the protection. And currents above 4.5 A for less than 80 ms (and below 6 A) also do not trip the protection. But as soon as the current exceeds 4.5 A for 80 ms (and remains below 6 A), the protection tripped – the output shut off causing the current to go to zero amps.

This is just one example of how versatile the N6900/N7900 APS power supplies are. For more information about how these advanced power systems can help you in your power application, please use this link: www.agilent.com/find/aps. To explore this advanced signal routing and logical trigger expressions feature even more, take a look at a post from one of my collegues: http://gpete-neil.blogspot.com/2013/10/protecting-your-dut-during-test-with.html

Thursday, October 17, 2013

Quickly Measure a High Brightness LED’s (HBLED) Forward Electrical Characteristics

It’s not hard to notice (or extremely hard not to notice!) how high brightness LEDs, or HBLEDs, are quickly becoming commonplace all around us in our daily lives. LEDs are no longer relegated to being an indicator light on a display panel. HBLEDs have drastically ratcheted up their output to become sources for illumination.  More and more autos use them for their tail and brake lights. It’s easy to see the “instant on” they have when the auto in front of you hits its brakes, not to mention the deep purity of color they have in comparison to the incandescent predecessors.  They are also turning up in the headlights, the traffic lights, even high power street and parking lot illumination lights, and in countless other places. A lot of testing, characterization, and development work has, and continues to take place, to achieve this level of performance from HBLEDs. This includes making careful measurements of electrical power being provided and the corresponding luminous efficacy outputted, in order to assess its performance.

In my title above I am using the term “quickly” for two reasons in my posting today. First, it is important when trying to capture the forward characteristics of an HBLED that it is performed in a minimum amount of time in order to minimize temperature change due to self-heating.  The temperature an HBLED is running at has in impact on its performance. Minimizing the amount of temperature change improves accuracy of test results in determining the performance of the HBLED, for a given operating temperature. My second reason for using quickly is providing a means to make these HBLED measurements with just a little time and effort.

It turned out using the N6784A four-quadrant SMU module in an N6705B DC power analyzer mainframe worked out really well on both counts of quickly. This set up is depicted in Figure 1.

Figure 1: HBLED test characterization set up

While the N6784A is an extremely fast voltage source it is even a faster current source. With current rise and fall times of just a few microseconds was a simple matter to generate sub-millisecond-long high amplitude pulses of current with fast settling edges to provide the necessary stimulus for performing the forward electrical characterization of the HBLED. This allowed testing to take place in minimum time and avoid significant heating of the HBLED die.

One of the outcomes of the testing is shown in Figure 2, displayed graphically by the 14585A software.  Here a ramped current pulse was used instead of a flat top pulse. The HBLED’s voltage and current were simultaneously digitized as the current was ramped up. This gave a way of characterizing the HBLED’s forward voltage drop for all levels of drive current, from zero to maximum.

Figure 2: HBLED forward characterization results
The N6705B DC Power Analyzer mainframe and 14585A companion software made quick work of the setup, testing, and display of results.  A ramp waveform from the library of pre-defined ARBs was selected and used to generate the current ramp. In this instance it was set to ramp up to 1.2 amps in 1 millisecond. The oscilloscope mode was used to set up the simultaneous capture of voltage and current, synchronized to the current ramp stimulus. As voltage and current were captured it is also a simple matter to display the power, being the point-by-point product of the voltage and current. The electrical power in can then be correlated with a light output measurement on the HBLED for evaluating its performance.

Not only is this setup able to measure the HBLED’s forward characteristics, as the N6784A can source negative voltage and measure down to nanoamp levels it can quickly test the HBLED’s reverse leakage characteristics as well.

Wednesday, October 9, 2013

Using the power supply status subsystem to improve your test throughput

Continuing on my throughput theme here, one recommendation is to take advantage of the power supply’s status subsystem. Some power supply operations take notably longer than most to complete than others. Two notable examples:
  • Initializing a triggered measurement
  • Initializing a triggered output transient or output list event

When developing programs you can include long, fixed wait statements to make certain these operations have completed before proceeding. However, this can easily add many tens of milliseconds or more of unnecessary waiting, increasing overall test time.  A better way is to take advantage of the DC power supply’s status subsystem features that eliminate unnecessary waiting for these operations.

Triggered measurement and output sourcing events can substantially speed up testing by providing actions tightly synchronized with other test activities. But they do have some up-front set up overhead time needed for initializing them. Instead of using a fixed programming delay following an initialization operation it is better to take advantage of the Operation Status Group register in the status subsystem, which is illustrated in Figure 1.

Figure 1: Agilent N6700 series DC power system operation status group

The “WTG meas” bit (#3) or “WTG trans bit (#4) in the condition register can be monitored with a loop in the test program to see when they turn true. At the moment the measurement or output sourcing event is initiated and ready for a trigger the test program will then proceed with its execution without incurring any unnecessary additional waiting. This saves a considerable amount of time as illustrated in Figure 2.

Figure 2: Operation-complete wait time distribution

Instead of waiting for the full worst-case each and every time, the wait is now just the actual time. When repeated over and over for all DUTs being tested, the net result is the average of the actual wait time, which in most cases is just a small fraction of the worst case time! The net result can be many tens of milliseconds test time savings, making an improvement in test throughput.

The first five hints of my compendium “10 Hints for Improving Throughput with your Power Supply” can be viewed here: (click here to access).  For those reading our “Watt’s Up?” blog here are getting the opportunity to preview one of the remaining 5 hints yet to be released!

Monday, September 30, 2013

New Agilent Advanced Power System: More on High-Power!

Last week, I announced two new families of high-power system DC power supplies from Agilent Technologies:
  • N6900/N7900 Series 1- and 2-kW Advanced Power System (APS) DC Power Supplies
  • N8900 Series 5-, 10-, and 15-kW Autoranging DC Power Supplies
Here is the press release on these two new families:

In my post last week, I concentrated on the N8900 Series of autoranging power supplies. Those are basic DC power supplies with outputs up to 15 kW (can be paralleled to 100 kW and more). Today, I am focusing on the N6900/N7900 Series of Advanced Power System DC Power Supplies. These power supplies really do live up to their “Advanced Power System” label. I’ve been working here on power products since 1980 and have supported several feature-rich product families in that time: most notable were our AC source products (6811B, 6812B, and 6813B) and more recently, our battery drain analysis source/measure units (N6705B with N6781A). The new N6900/N7900 Advanced Power System rivals those products for rich features and quite honestly, just like our marketing slogan says, they really should help you “overcome your toughest power test challenges”. Why? Read on…

First the basics:
  • There are ten 1 kW models each in a 1U package
  • There are fourteen 2 kW models each in a 2U package
  • Rated output voltages range from 9 V to 160 V
  • Rated output currents range from 12.5 A to 200 A
  • Outputs can be paralleled up to 10 kW
Here is what these products look like:

Now for a few details. There are two performance levels:
  • N6900 Series is designed for ATE applications where high performance is critical
  • N7900 Series is designed for ATE applications where high-speed dynamic sourcing and measurement is needed
Both performance levels have advanced power features including:

  • Precision voltage and current programming (N6900 is 14-bit; N7900 is 16-bit)
  • Programmable output resistance
  • Current sinking up to 10% of rated current (up to 100% with added N7909A power dissipator)
  • 18-bit voltage and current measurements
  • Power measurements
  • Amp-Hour and Watt-Hour measurements

The higher performance N7900 products add more features to the above:

  • Precision 16-bit voltage and current programming (N6900 is 14-bit)
  • Output lists to quickly step through voltage or current levels
  • Arbitrary waveform generation
  • Low current measurement range
  • Seamless ranging for dynamic current measurements
  • Adjustable sample rate
  • Measurement array readback
  • External data logging

And even more capabilities:
  • Extended current measurement range that measures 2.25 x higher than the rated current
  • Sampling up to 200 kS/s
  • Extensive triggering capability
  • Extensive protection features such as open sense lead detect, over- and under-voltage and current, and over-temperature
  • And my personal favorite: you can track power events by adding a black box recorder (N7908A). The N7908A Black Box Recorder is a user-installable option that performs continuous background logging of output voltage, current, power, and system status to its own dedicated mass storage device. Features of this option include:
    • Automatic logging starts when the power supply is turned on
    • Logs in a circular buffer of about 380 MB
    • Select one record every 10 ms (24 hours of logging) or one record every 100 ms (10 days of logging).
    • Each record saves the average, maximum, and minimum  values for voltage, current, and power in addition to power supply status bits and events
    • Logged data is preserved after a power cycle. A time-stamped event is logged each time power is turned on.

The black box recorder is pretty cool, no? If you have a mission critical power application, this option is a must to keep track of any power related events that might affect your device under test.

One of my colleagues, Neil Forcier, posted about these products on his GPETE blog earlier this month. Here is a link to that post: http://gpete-neil.blogspot.com/2013/09/the-new-advanced-power-system-designed.html

For more detailed information, take a look at the datasheet: http://cp.literature.agilent.com/litweb/pdf/5991-2698EN.pdf

The datasheet contains some fantastic details about the products including 9 tests challenges that are directly addressed by these powerful products. In fact, you can read about each of the 9 test challenges here: www.agilent.com/find/TestChallenges

With all of these advanced features built into this family of products, I think you can now appreciate why we called it the Advanced Power System!

Friday, September 27, 2013

New Agilent Autoranging Power Supplies provide Higher Power

I have stated before that I avoid posting product-only-focused material in this blog since our intention here is to educate about all-things-power rather than to (directly) promote our products. But when we (Agilent….for now…) come out with new power products, I think it is appropriate for me to announce them here. The last time I did this was back on January 23, 2012 in this post:

Coincidentally, those products were autoranging power supplies just like some of the power products we just introduced! Earlier this month, Agilent announced two new families of high-power system power supplies with this press release:


The two new families are the:
·         N6900/N7900 Series 1- and 2-kW Advanced Power System (APS) DC Power Supplies
·         N8900 Series 5-, 10-, and 15-kW Autoranging DC Power Supplies

Today I will focus on the higher power series, the N8900. Next week, I’ll post about the N6900/N7900 series.

What is particularly exciting for us about the N8900 series of products is that it is the highest power level we have ever provided. I’ve been working here at Agilent with power products since 1980 (we were Hewlett-Packard back then and will soon get yet another new name), and until now, the highest-power power supply we offered was the HP SCR-10 Series at 10 kW. This product was discontinued many years ago. Now the new N8900 series has 5, 10, and 15 kW in a single model and outputs can be paralleled for 100 kW or more! Now that’s a lot of power!!

Back when I started, 10 kW came in a much larger package and weighed over 500 lb (227 kg)! Today, we can get 50% more power (15 kW total) in an 80% smaller package (3 U vs 16 U) that is 85% lighter (73 lb vs 500 lb). See the figure below.

The new N8900 series offers 14 different voltage, current, and power combinations with output voltages up to 1500 V and output currents up to 510 A. Outputs can be put in parallel for 100 kW or more. These are basic power supplies, but have autoranging output characteristics so you get more voltage and current combinations from a single output than if you used a power supply with a rectangular output characteristic. See the figure below.

For more information about autorangers and output characteristics, see the post I mentioned earlier (or click here.)

We also created a video introducing the N8900 series (I do a cameo toward the end…can you pick me out?).

These basic power supplies will be used in many different high-power applications, such as hybrid-electric vehicle test and photo-voltaic inverter test. So if you need a basic high-power power supply, check out the Agilent N8900 Series. And don’t hesitate to ask me a question about the product or its suitability for your application.

Thursday, September 12, 2013

How fundamental features of power supplies impact your test throughput – Part 2

In part 1 of” How fundamental features of DC power supplies impact your test throughput” (click here to access) I shared definitions of some of the fundamental power supply features that impact test throughput, including:
  • Command processing time
  • Up-programming response time
  • Down-programming response time

Another fundamental DC power supply feature impacting test throughput is its measurement time. There are actually two aspects to a DC power supply’s measurement time as depicted in Figure 1:
  • Measurement settling time
  • Measurement integration time

Figure 1: DC power supply measurement time

A good indicator of a DC power supply having a high performance measurement system is having programmable measurement integration time, or aperture time, often programmed in power line cycles (PLCs).  One reason for having a programmable integration time is for minimizing any 50 or 60 Hz AC line ripple getting into the DC measurement, by setting the time one or more multiples of a PLC.  Setting the time to 1 PLC provides good ripple rejection with relatively good throughput. When AC line ripple is not an issue the integration time can be set even smaller than 1 PLC, further reducing measurement time. When the DC power supply has a programmable measurement integration time it will no doubt also have a fast-responding measurement system as well, typically just milliseconds, to complement the higher achievable throughput with programmable measurement integration time.

In comparison basic DC power supplies commonly use a 100 millisecond fixed integration time to support AC ripple rejection for both 50 and 60 Hz line frequencies. They also have low bandwidth, slow-responding measurement systems, which can long time to settle after any step change in loading, before a valid measurement can be taken.

We have just introduced our Advanced Power System (APS) DC power supplies. This is a family of high-performance, high power (1 and 2 kW) DC power supplies designed to address the most demanding test challenges. These fundamental throughput-related features for APS are typically more than two orders of magnitude faster compared to more basic-performance DC power supplies, providing much better throughput in manufacturing test. A colleague of mine recently posted details of their introduction on his “General Purpose Electronic Test Equipment (GEPETE)” blog (click here to access) which I believe you will find of interest. Included in this introduction is a link on throughput that takes you to a series of application briefs I have written that go into more detail on improving test throughput with the DC power supply, which you may find very useful.

So how much test throughput improvement might you expect to see by switching from a basic-performance DC source to a high-performance DC source? Well, it really depends on how much the testing makes use of the DC power supply. If it only uses the power supply to provide a fixed DC bias to the device under test (DUT) that never changes for the duration of the test then it will not make a significant difference. More often than not however, a DUT is tested at several bias voltages with several current drain measurements taken for the various bias voltage settings and DUT operating modes. This can add up to a considerable amount of test time. In this case a high-performance DC power supply can more than pay for itself many times over due to improved test throughput.  To get an idea of the kind of difference a high-performance DC power supply can make I set up a representative benchmark test It compares the throughput performance one of our new APS DC power supplies to that of a more basic-performance power supply.  If you are interested in finding out how much difference it made, I made a video of this benchmark testing, entitled “Increasing Test Throughput with Advanced Power System” (click here to access). All I am going to say here is it is an impressive difference but you will need to watch the video to see how much difference!

Friday, September 6, 2013

How fundamental features of power supplies impact your test throughput – Part 1

When it comes to manufacturing of electronic products, reducing test time to improve throughput is virtually always a top priority, because “time is money” as the old saying goes! Usually most all of the attention may be placed on reducing the test time of the banner aspects of the product, such as the RF performance of a wireless device, for example. However, the choice of the DC system power supply can also have a huge impact on your test time and throughput during manufacturing. You may find the lowest cost, more basic-performance DC power supply that meets your immediate needs end up costing you the difference in price many, many times over of that of a higher-performance DC power supply having better throughput performance in the long run!

The DC power supply can incorporate a number of advanced features, such as elaborate triggering and sequencing systems, which will allow you restructure your testing to optimize throughput. However, even fundamental throughput-related features of the power supply can also have a large impact on your test time, including:
  • Command processing time
  • Output up-programming time
  • Output down-programming time
  • Measurement time

Figure 1 illustrates what the command processing and up-programming times are for a DC power supply. The command processing time is the time from when the command is first received to the point where the power supply starts acting on it. In this case it is when power supply’s output starts to change. The up-programming response time is the time the power supply takes for the output to rise and settle within a small band around the final output level, after processing the command instructing it to change its output level.

Figure 1: Power supply command processing and up-programming response times

The down-programming response time is like the up-programming response time except that the power supply is instead being programmed to a lower level. However, you need to look at down-programming independently as short up-programming time does not necessarily guarantee comparably short down-programming time. More basic performance DC power supplies usually lack an active down-programmer circuit that quickly brings down the output. In this case the down-programming response time can be very dependent on how much load the DUT presents to the power supply’s output.

How much difference is there in performance between more basic performance and higher performance DC power supplies on these throughput-related features? It can be considerable; over several orders of magnitude difference. As one example, command processing time can range from up to 100’s of milliseconds for entry-level power supplies to under 1 millisecond for high performance power supplies.
Another fundamental throughput-related feature of a DC power supply is its measurement time. There are a couple of aspects to consider here as well, which I will elaborate on in part 2 of this series on how fundamental features of power supplies impact your test throughput, in an upcoming posting here on “Watt’s Up?” along with tying it all together to show how they affect actual test throughput!

Saturday, August 31, 2013

Power Supply Programming: How Should I Send Commands to my Instrument

Hi everyone!  Happy Labor Day to all you readers in the US!  Every month I struggle with what I am going to write and wind up waiting till the end of the month to do my posting (and I am keeping that streak alive).  In order to combat that, I came up with a series of topics on programming instruments, focusing on our power supplies.  Let’s say that this is the first in a series of three (or maybe four I am not sure).  Please note that anything that I state here is my opinion and not Agilent policy.  Today I am going to focus on the how to send commands to your instrument.  In other words, what sort of IO library do you use to send the commands?

All of my suggestions will be based on the Agilent IO Libraries as that is the environment that I am most familiar with.  There are two major options: direct IO where you use the SCPI from the instrument and drivers where there are functions that you call.

First let’s talk about direct IO.  I learned how to program instrument using HPBASIC as my programming language so this is where it all began with me.  Agilent has two modern standards for doing this.  The first and the older standard is the VISA library.  VISA works very well when you are programming an instrument in the C programming language.  Here is a snippet of C code from a N6700 example with VISA (I have intentionally not provided comments to show the program in its purest form): 

viPrintf(session,"VOLT 5,(@1) \n");
viPrintf(session, "OUTP ON, (@1) \n");
viPrintf(session, "MEAS:VOLT? (@1) \n");

It works pretty well and makes sense once you know it.  The viPrintf and viScanf functions are very similar to some basic C functions so if you are a C programmer, this is really the way to go.

There is also a newer option that works pretty nicely in languages that support COM.  This option is called Agilent VISA COM.  VISA COM works well in Visual Basic and C#.  Here is the same program to the above written in VB:

Set ioMgr = New AgilentRMLib.SRMCls
Set Instrument = New VisaComLib.FormattedIO488
Set Instrument.IO = ioMgr.Open("GPIB0::5")
Instrument.WriteString " VOLT 5,(@1)"
Instrument.WriteString " OUTP ON, (@1)”
Instrument.WriteString "MEAS:VOLT? (@1)”
Result = Instrument.Readstring

In my opinion, this is easier to read than VISA.  When I have to write a program now, I tend to stick with using VISA COM and Visual Basic. 

The other option is to use a driver.  We presently offer two different driver types for our instruments: VXI Plug and Play and IVI COM.  VXI Plug and Play drivers are obsolete now though so I will not reference them further today.  Here is an example of our program using the IVI driver (in C#):

driver = new Agilent.AgilentN67xx.Interop.AgilentN67xx();
IAgilentN67xxProtection2 protectionPtr;
IAgilentN67xxMeasurement measurementPtr;
IAgilentN67xxOutput3 outputPtr;
int channel
driver.Initialize(“GPIB0::5”, idquery, reset, initOptions);
outputPtr = driver.Outputs.get_Item(driver.Outputs.get_Name(channel));
protectionPtr = driver.Protections.get_Item(driver.Protections.get_Name(channel));
measurementPtr = driver.Measurements.get_Item(driver.Measurements.get_Name(channel));
outputPtr.VoltageLevel(3.0, 3.0);
outputPtr.Enabled = true;
mVolt = measurementPtr.Measure(AgilentN67xxMeasurementTypeEnum.AgilentN67xxMeasurementVoltage);

As you can see, the driver is much more complex than the direct IO examples. There are a few reasons to use a driver though.  The first and most common reason is that your system itself it designed to use drivers.  Another good reason is portability.  There are instrument classes in the IVI driver that should work for any DC power supply that is compatible.  One of the main downfall of our IVI drivers is that the functions almost always map 1 to 1 with SCPI so there are not many functions that work at a higher level and you don’t save any time programming there.

My main approach is to use VISA COM in Visual Basic.  I find it to be the easiest for me to program and it is what works for me.  Of course no opinion is wrong though and we are happy that our readers are out there buying and programming our instruments.  Thanks!

Tuesday, August 27, 2013

How to choose a Power Supply Part 1: Student Edition

Note from GaryR: We were fortunate to have an intern, Patrick, in our department for the summer. He has since returned to college, but during his time here at Agilent Technologies, he successfully completed several projects for us. During his last week, he wrote a power supply blog post! Here it is, unedited!!

Hello, I am Patrick, an intern here at Agilent Technologies. This is my first blog post but you might recognize me as “new intern… named Patrick” from the Matthew Carolan insta-classic, “What is Command Processing Time?” And as “Uncredited Waveform Capture Author” from GaryR’s visually-stunning, “Current Limit Setting Affects Voltage Response Time I might not be able to provide the same engineering life lessons Mr. Brorein, Mr. GaryR, and Mr. Carolan provide you monthly but hopefully you will enjoy this post nonetheless.

As you can probably tell from the title, this blog post is the first in a series written to help readers find the right DC power supply for them. For starters, there are a lot of other power supply companies out there; their products are not mentioned here. Actually, I better give you a disclaimer: this is an Agilent blog post about Agilent products written by an Agilent employee. However, my intentions in this post are to inform our readers on how to find a reliable and dependable power supply to the best of my abilities. I simply do not know enough about non-Agilent DC power supplies to be able to confidently recommend you any. With that out of the way, today’s post is written for college students, like myself, but can help give some insight to anyone looking for a power supply.

Agilent Technologies offers over 200 models of DC power supplies. Finding the perfect power supply for your needs is possible but can take some time. Fortunately, you have found your way to this blog post where I have already done all the work.

The first step in finding your power supply to buy is to determine your budget. If you are financially disabled like me, you are really lucky if you can save up $300 - $500 dollars between buying textbooks for a personal power supply. If you need convincing on why you make the expense, Matt’s “The Joys of Owning a DC Power Supply brings up some good points. Personally, I like being able to work where I want and when I want, including at my home during the summer where I would be lucky to find working batteries let alone a power supply. Also with the lifespan these power supplies have, this is a piece of equipment you will be using for decades. So first, figure out your budget!

The next thing to determine is what are you using this power supply for? Are you a hobbyist who only has low power needs? Do you need to power multiple DUTs at once for a senior project? Maybe you need a very precisely regulated and accurate output for a research project? If I wanted to work on labs outside of school I would need a supply with multiple outputs. If I wanted to work on side projects in my spare time my requirements are a lot simpler. Figure out what your performance requirements are, and try to anticipate future projects if you can.

Now, how much space do you have to work with? Power supplies come in many different sizes. In a test rack, a smaller power supply means more room for other test equipment. As a student you probably are not going to have a test rack, but you might have a test bench. If your test bench is anything like mine, it is also your laptop table, study space, dining area, and entertainment center. As you do before you buy a new couch, make sure you measure before you buy your new power supply.

For a student, here would be my priorities when shopping for a power supply: inexpensive, compact, and capable. All Agilent power supplies already provide low noise and well regulated outputs, so you do not have to worry about that. But keep in mind what functions and features you would like. For example, it would be nice to find one with an autoranging output characteristic, which would essentially allow one to perform the job of multiple, and/or a computer interface.

For those with similar priorities as above, shown in Table 1 are the power supplies that you might like. The table was populated with power supplies listed in our “Agilent Power Products Selection Guide”. The row on the top represents the base (no options) price range of the power supply if ordered from Agilent. The column on the left side represents your best choice in that price range. Keep in mind that the only difference in some budget ranges is the voltage/current ratings, which is important for your application but does not necessarily make one power supply better than the other. Within the table, each square states the model number of the power supply, voltage/current/power ratings, number of outputs, and number of ranges, available computer interfaces, and dimensions. The output ratings are sometimes separated by commas or there is another set listed below the first. If the power supply has multiple outputs, then each output is separated by a comma. If the power supply has multiple ranges, then each additional range is listed below the first.

To use Table 1, simply find your budget range and then match your application needs to the available models. Models in the same budget range but have increased functionality are placed higher.

The two families of power supplies that fall into the sub $1,000 price range are the E3600 and U8000 series basic power supplies. The E3600 gives you a lot of voltage/current choices, a number of outputs, as well as the ability to program with it in four of the options listed above. The U8000 is more limited in all these aspects but is more affordable and has features typically found only in programmable supplies such as OVP and OCP protection, recall states, and more. See for yourself how the E3600 and U8000 stack up against other power supplies.

All the power supplies listed above come in a ½ RU w x 2 RU h, with 1 RU being equivalent to 19” width and 1.75” height. This size is compact and will have no trouble fitting on a desk, and when not being used can easily be moved and stored.

It is unfortunate that there is no relatively inexpensive Agilent power supply with autoranging output characteristics. At least there are choices (within the E3600 series) with multiple ranges, giving you more output flexibility for more applications (learn more about rectangular vs. autoranging output characteristics).

Also there is a three output power supply on the list (the E3630A) which is in the middle of the pack in terms of budget. The E3630A is actually the same power supply available at my school laboratories, able to handle every exercise I have done so far.

As a student, these power supplies might be outside your budget, but the least expensive choice, the U8001A, is about the same price as a Play Station 3. Would you rather choose: “fun” (the PS3) or “FUN!” (a power supply)? Please note that the distinguishing lowercase and uppercase variations of the word fun is not a reference to the level of fun but rather how loud I would yell it at you if we were face-to-face.

So, I have listed out your choices. All you have to do is figure out which one fits your applications. If you do not know what your applications are then you already missed a step in this blog post, so start over from the beginning. If you do not have an application then you probably do not need a power supply, but thank you for taking the time to read this. If you cannot find the right power supply for your application in this post then keep saving your money and look out for the next part on, “How to choose a DC power supply”, that is where the real fun starts. If you read this post and succeeded in choosing a power supply then awesome. Thanks for reading!