Saturday, February 28, 2015

Synchronize Your Measurements with Your List Transients

Hi everybody!

My blog post this month is the result of a recent customer question.  The question was: how do you synchronize measurements with list transients?  The short answer is that you use the built in digitizer to generate enough points to sample the measurements over the entire transient.  The rest of this blog will provide the long answer.  The program that I am using here was written for a N6762A DC Power Module but the technique will work with any power supply that has a built in digitizer such as the Advanced Power System or any N6700 module with option 054.

For simplicity’s sake, we are going to use a 5 point list.  The voltage steps are 1 V, 2 V, 3 V, 4 V, and 5 V and the dwell times are 0.1 s, 0.2 s, 0.3 s, 0.4 s, and 0.5 s.  Let’s first set the list up (please note that all programming is done in VB.net with VISA-COM):

The next thing to do is to set up the measurement system.  We need to figure out the total number of points that we need measure so that we can cover the entire transient.  The first thing that we need to do is to calculate the total time of the list transient (you can even do this in your program):

The total time of our transient is 1.5 s.  Now we need to use this to figure out the number of points. I am going to choose a measurement interval of 40.96 us.  This means that we want to take a measurement every 40.96 us for 15 s.  To get the total number of points, you need to divide the total transient time by the measurement time interval:

I’m going to round down and use 36,621 points.  I’m also going to tell the power supply to use the binary data format because as we know from my previous blog posts, this is the fastest way to read back data.   Here is the code to set up the digitizer:

We will set our trigger source to bus for both the transient system and the acquire system:

Next we initiate both systems:

Once the initiate is complete, we send a trigger:

This will start both the list transient and the digitizer.  After everything is completed, we can fetch our measured voltage array:

This array will have all of our measurements.

I hope that this has been useful, have a good month everyone.

Thursday, February 26, 2015

When measurements and mathematics agree, I’m happy!

I have been an electrical engineer my entire career which will span 35 years in June (yikes….I’m too young to be this old!). Despite all of that time, plus the 4 years of undergraduate study before that and 3 years to get my MSEE at night while working full time during the day, I am still amused by some simple engineering principles. When the measurements I make in the real world completely agree with the hard-core theoretical mathematics I learned in school, I am amused.  Perhaps this should simply be expected (it is), but for some reason, I am still delighted when it happens. I recently had one of those simple experiences that I want to share with you today.

I was exploring some of the features on our new Keysight PA2201A IntegraVision power analyzer to better understand its operation and the applications for this new line of power measurement instrumentation. I had a simple desk lamp with a 100 W incandescent light bulb plugged into the wall outlet (120 Vac, 60 Hz here in the United States) and ran the voltage and current to the power analyzer. Given that the bulb presents a nearly pure resistive load to the sinusoidal voltage, as expected, the current was also a sine wave and in phase with the voltage. The power analyzer easily displays these measured waveforms.

What I never had an opportunity to see before was a visualization of the power waveform. For some reason, in all of my 35 years working in the power business, I never looked at a power waveform for a simple resistive load. Voltage? Sure! Current? Many times!! But power? Nope. The IntegraVision power analyzer shows voltage, current, and power waveforms as a typical display (this can be configured in quite a few other ways as well). So I was looking at the waveforms shown below.

The first thing I noticed about the power waveform was that it was sinusoidal and it never went below zero. This quickly made sense to me since I did consider the bulb to be purely resistive meaning it is consuming power 100% of the time, so all of the power flowing to the bulb had to be positive. The bulb is never pushing power back to the AC line as would happen with a reactive load especially for a purely reactive load such as a pure capacitor or inductor. If the load (the bulb) was not purely resistive, some of the power waveform would have dipped below the zero power line indicating that sometimes the load was absorbing power and sometimes it was providing power back to the line.

The next thing I noticed about the power waveform was that its phase was synchronized with the voltage and current, and it showed twice the frequency. Again, this quickly made sense since the power is simply the product of the voltage and the current [P(t) = V(t) * I(t)]. So the positive peaks have to line up (they do), the zero crossings of the voltage and current have to align with zero watts on the power waveform (they do), and the negative peaks in the voltage and current have to line up with another positive peak in the power since a negative voltage times a negative current yields a positive power (they do). This, of course, was the reason for the power waveform being twice the frequency of the voltage and current waveforms.

So I next decided to check the math behind the waveforms. I admit….I had to look up the trigonometric identity, but it was worth it! Since both the voltage and current waveforms are sine waves, and the power is the product of these, I looked up the identity for sine squared:
The voltage is a 120 Vrms, 60 Hz sine wave:
The current is 99.8 VA / Vrms = 0.832 Arms:
The power is V(t) * I(t):
Applying the above sine squared identity:
So you can see there is a 99.8 W fixed offset in the power waveform from which a cosine function is subtracted. The frequency of the cosine is 120 Hz (double the 60 Hz voltage and current waveforms). All of this completely agrees with the power waveform measured by the IntegraVision power analyzer. I am always thrilled when the math agrees with the measurements no matter how simple it is! How about you?

Tuesday, February 24, 2015

Two New Keysight Source Measure Units (SMUs) for Battery Powered Device and Functional Test

Over the past few years here on “Watt’s Up?” I have posted several articles and application pieces on performing battery drain analysis for optimizing run time on mobile wireless devices. The key product we provide for this application space is the N6781A 20V, +/-3A, 20W source measure module for battery drain analysis. A second related product we offer is the N6782A 20V, +/-3A, 20W source measure module for functional test. The N6782A has a few less key features used for battery drain analysis but is otherwise the same as the N6781A. As a result the N6782A is preferred product for testing many of the components used in mobile devices, where the extra battery drain analysis features are not needed. These products are pictured in Figure 1. While at first glance they may appear the same, one thing to note is the N6781A has an extra connector which is independent voltmeter input. This is used for performing a battery run-down test, one of a number of aspects of performing battery drain analysis. Details on these two SMUs can be found on by clicking on: N6781A product page.  N6782A product page,

Figure 1: Keysight N6781A SMU for battery drain analysis and N6782A for functional test

These products have greatly helped customers through their combination of very high performance specialized sourcing and measurement capabilities tailored for addressing the unique test challenges posed by mobile wireless devices and their components. However, things have continued to evolve (don’t they always!). Today’s mobile devices, like smart phones, tablets and phablets, have an amazing amount of capabilities to address all kinds of applications. However, their power consumption has grown considerably as a result. They are now utilizing much larger batteries to support this greater power consumption in order to maintain reasonably acceptable battery run-time. Optimizing battery life continues to be a critical need when developing these products. With their higher power however, there is in turn a greater need for higher power SMUs to power them during test and development. In response we have just added two new higher power SMUs to this family; the N6785A 20V, +/-8A, 80W source measure module for battery drain analysis and the N6785A 20V, +/-8A, 80W source measure module for functional test. These products are pictured in Figure 2. Details on these two new higher power SMUs can be found on by clicking on: N6785A product page.  N6786A product page.

Figure 2: Keysight N6785A SMU for battery drain analysis and N6786A for functional test

A press release went out about these two new SMUs yesterday; Click here to view. With their greater current and power capability, customers developing and producing these advanced mobile wireless devices and their components now have a way to test them to their fullest, not being encumbered by power limitations of lower power SMUs.

This is exciting to me having been working within the industry for quite some time now, helping customers increase battery life by improving how their devices make more efficient use of the battery’s energy. A key part of this has been by using our existing solutions for battery drain analysis to provide critical insights on how their devices are making use of the battery’s energy.  There is a lot of innovation in the industry to make mobile wireless devices operate with even greater efficiency at these higher power and current levels. There is no other choice if they are going to be successful. Likewise, it is great to see continuing to play a key role in this trend in making it a success!

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Monday, February 9, 2015

Consider using an electronic load for generating fast, high-power current pulses

Often there is the need for generating high-power current pulses, typically of short duration, and having rise and fall times on the order of microseconds. This is a common need when testing many types of power semiconductors, for example.

When looking for a DC power supply capable of generating very fast, high-power current pulses, one will find there are not a lot of options readily available that are capable of addressing their needs. There are specialized products dedicated for specific applications like this; an example of this is Keysight’s B1505A purpose-built semiconductor test equipment. They are capable of generating extremely fast, high-power current pulses.

Apart from specialized products however, DC power supplies generally to not offer this kind of speed when operating in a constant current mode (or current priority mode). One exception that comes to mind that we provide is our N6782A and N6782A DC source measure modules. They can create fast current pulses having just a couple of microseconds of rise and fall time. However, they are limited to 20V, 3A, and 20W of output. Most of the higher power, more general-purpose DC sources are not able to generate these kinds of fast, high-power current pulses and most are really more optimized to operate as voltage sources.

One alternative to consider for generating fast, high-power current pulses when working with general-purpose test equipment is to use an electronic load. You may initially say to yourself “an electronic load is for drawing pulses of current, not sourcing them!” but when coupled to a standard DC power supply operating as a voltage source, the setup is able to source fast, high-power current pulses. Most electronic loads are designed to have very fast current response. To illustrate this, I helped one customer needing to test their high brightness LED (HBLED) arrays with fast pulses of current. This was accomplished with the setup shown in Figure 1.

Figure 1: Load setup generating fast, high power current pulses for LED array testing

In this setup the power supply operates as a fixed, static voltage source. The power supply’s output voltage is set to the combined total of the full voltage needed to drive the HBLED array at full current plus the minimum voltage needed for the electronic load. The minimum voltage required for the electronic load is when it conducting maximum current and most of the power supply voltage is then applied across the HBLED array. The electronic load’s required minimum voltage is that which supports its operation in its linear range and maintains full dynamic response characteristics. In the case of Keysight electronic loads this minimum voltage for linear dynamic operation is 3 volts.  Conversely the maximum voltage required for the electronic load is when it drops down to minimum current level, where the power supply’s voltage is instead now being dropped across the electronic load instead of the HBLED array. Note that the electronic load may need to maintain a very small amount of bleed current to maintain linear operation in order to provide truly fast rise and fall times. In this way the electronic load is able to regulate the current across the full range with excellent dynamic response. This can be seen in Figure 2 where we were able to achieve approximately 15 microsecond rise time right from the start.

Figure 2: Pulsed current rise time in HBLED array

One advantage of this setup is the wide range of voltage and power that can be furnished to the DUT using a relatively low power electronic load. A common characteristic of electronic loads is that they can dissipate a given amount of power over an extended range of current and voltage. When the electronic load is at maximum current it is at minimum voltage. Conversely when it is near or at zero current it is then at its maximum voltage. In both cases there is only a small amount of power that the electronic load needs to dissipate. For an HBLED array it does not conduct a lot of current until it reaches about 75% of its full operating voltage. As a result the electronic load does not see a lot of power even on a transient basis. For this particular situation we chose to use the Keysight N3303A 240V, 10A, 250W electronic load. This gave a wide range of voltage, current, and power for testing a comparably wide range of different HBLED array assemblies.

So next time you need to source fast, high-power current pulses, you may want to think “load” instead of “source”!

Friday, January 30, 2015

Using the Passthrough Command in IVI drivers

Hi everybody!

I was working on a customer question yesterday and I thought that it would make an enlightening blog post.  We have a feature for our IVI-COM and IVI-C drivers that allows you to directly send SCPI commands to your instrument.  This is really useful if you run into a situation where you think there is a function is missing from our driver or you run into something unexpected.  Overall, it is pretty easy to use once you know where to find it.

Let's look at the IVI-COM driver first.  You can find the passthrough in the Systems Interface under the IO property.  One you get to the IO level, you can just use the standard VISA-COM commands to send commands to the instrument and read responses back.  For my little example here I am just going to send a voltage measurement and read back the response:

In the code above, agDrvr is the name I gave to the instrument when I initialized it.

We also provide an IVI-C driver.  The IVI-C driver also has a passthrough command that is just a little more complicated than the IVI-COM version.. To send SCPI commands, you use the AgN67xx_SystemWrite function and to read data back you would use the AgN67xx_SystemRead function.  The same example as above would look like this:

In this IVI-C code:
status is where the IVI-C driver will report an error if there is an unsuccessful call
session is the handle that I gave to this instrument when I initialized
32 was my best guess at the size of the response string.  I like to overestimate.
strResp is where I want the response to be stored
respSize is the actual size of the response and is returned by the program

All in all it's not too difficult and will definitely come in handy.

That all I have for you this month.  As always, let us know if you have any questions.

Matt

Thursday, January 29, 2015

New Keysight Power Analyzer called IntegraVision

Back on June 23, 2014, I posted about the last Agilent power products to ever be announced. At that time, we had not yet officially changed our name from Agilent to Keysight. So the AC6800 AC sources we released on that date were released under the Agilent name, soon to be rebranded to Keysight. Well, today, I am announcing the first new Keysight power product: the Keysight Technologies IntegraVision Power Analyzer Model PA2201A.

A press release went out about these products earlier today: click here to view. We here at the Power & Energy Division of Keysight have been involved in power products for decades, and of course, Keysight has an oscilloscope division with commensurate experience producing scopes. I consider the new IntegraVision power analyzers to be a combination of the vast experience of our engineers from these two disciplines combining a power analyzer and an oscilloscope. The power analyzer will enable you to accurately measure parameters such as watts, VA, VAR, power factor, crest factor, efficiency, watt-hours, amp-hours, and harmonics while the oscilloscope will allow you to visualize in real time the voltage, current, and power waveforms that are important in your design.

I am very exciting about this new line of power measurement instruments! I have been working for HP/Agilent/Keysight for nearly 35 years now and have always worked with power products during my career. One of my favorite product families to support has been the older sophisticated 6800 AC Power Source/Analyzers (not to be confused with the newer basic AC6800 series mentioned in the first paragraph above). The older AC sources can produce sine waves, square waves, and arbitrary waveforms (for tests such as cycle dropout tests) as well as measure most of the power analyzer parameters mentioned above since they have a power analyzer built into the AC source. But now the new IntegraVision power analyzer goes well beyond the capabilities of the power measurements built into our AC sources. Adding time-based measurements like watt-hours and amp-hours opens up many more energy measurement application areas for this new product and the visual waveform measurements are a huge benefit when doing things like characterizing AC inrush current or product response to AC line disturbances. I am delighted with the performance of the touch-screen on this product – it will help you gain faster insight into your designs plus it just makes using the product fun! With 0.05% basic accuracy, 5 MSample/second 16-bit digitization, and inputs isolated to 1000 V, the IntegraVision power analyzer really is a superb product for power consumption and power conversion applications. Click here for the IntegraVision web page with links to the individual products.

So the next time you need a power analyzer with great accuracy and you also want to see the power waveforms related to your application, be sure to look at Keysight’s new IntegraVision Power Analyzer Model PA2201A. I’m sure you will not be disappointed! And look here for future posts about some of the interesting applications for this product, such as AC power line disturbance measurements and micro-inverter efficiency measurements. Suggest some of your own power measurements for me to make and I’ll see what I can do for a future post for you!

Wednesday, January 7, 2015

A new current measurement methodology: It’s all about counting the electrons going by!

One thing near and dear to us here at the Power and Energy Division is making accurate current measurements. What exactly is current? It’s basically the flow of electric charge per unit of time. In a conductor it’s the flow of electrons through it per unit of time.

The ampere is the fundamental unit of current in coulombs per second, which equates to 6.241x1018 electrons per second. Accurate current measurement is one of the core values of virtually all of our products. Some of the precision SMU products can measure down to femtoamp (fA) levels (10-15 amps). This is where we tend to muse that we’re getting down to the levels where we’re virtually counting the individual electrons going by.

While there are a few different ways of measuring current, by far the most common is to measure the voltage drop across a resistive shunt. With careful design this provides the most accurate means of current measurement. There are a lot of non-obvious factors that can introduce unexpected errors that many are not aware of, leading them to believe they have better accuracy than what it really is. A good discussion of what it takes to truly make accurate current measurements was covered in a previous posting “How to make more accurate current measurements”(click here to review). We go through great pains in addressing these things in our products in order to provide accurate and repeatable measurements.

Unlike the volt and the ohm, which have quantum standards for their electrical units, the ampere instead relies on the standards for the volt and ohm for measurement, as a quantum standard for the ampere that directly relates it back to charge is still lacking. However, that may change in the not too distant future. A group of scientists were awarded the Helmholtz Prize in metrology for realization of the measurement of the ampere based on fundamental constants. Basically they’ve created an electron charge pump that moves a small, fixed quantity of electrons under control by a clock. You can say they’re literally “counting the electrons as they go by”. This could become the new SI standard reference for current measurement. To me this is very fascinating to find out about. More can be learned on this from the following link to the press release “Helmholtz Prize for the “new” ampere”(click here to review).  I am curious to see how this all plays out in the long run. Maybe it will lead to yet another, and better, way to make more accurate current measurements in products we all use today in our work in electronics!