Friday, May 30, 2014

Arbs! Arbs! Arbs!

Hi everybody,

We have a new intern here and we have recently been talking about the arbitrary waveform capabilities (from now on I will refer to this as arbs)  of our power supplies and I thought that this would make an interesting blog post.  This is a really cool feature that we offer in our products as it give you the ability to create an alternating signal using our DC power supplies.  The two types of arbs are the LIST system and the constant dwell arb.

The LIST arb is a feature that we have in quite a few of our products.  The N6700 family, the N7900 family, and even some of our older power supplies have this feature.  The "Arb" system in the N6705 DC Power Analyzer is similar to the LIST.  These LISTs can contain as many as 512 different points with a timing resolution as low as 1 us.  Each point consists of a voltage or current setting and a time.  The times can be different for each point.  A short example of a programmed LIST is:

VOLT:MODE LIST
LIST:VOLT 10,25,5
LIST:DWEL 5,1,4

In the example above, the voltage will start out at 10 V and stay for 5 seconds, then transition to 25 V for 1 s and then go to 5 V for 4 s.  As you can see there are 3 voltage values with 3 corresponding dwell times.

The second mode for arbs that is only available on the N6705B DC Power Analyzer and the N7900 APS is call the Constant Dwell Arb (CD Arb).  In this mode, you can program as many as 64K points but all of the defined points have the same dwell time.  If we want to do the same waveform as above, we need to choose what will be our dwell time.  Since the smallest dwell I used in my example is 1 s, I will choose that.  Here is what a small part of the code would look like:

VOLT:MODE ARB
ARB:VOLT:CDW:DWEL 1
ARB:VOLT:CDW 10,10,10,10,10,25,5,5,5,5

The code above will produce the same waveform as the LIST example.  CD Arbs can get pretty unwieldy when you have a ton of points but we do offer some tools in our 14585A Control and Analysis software that allow you to import and export csv files to make life a bit easier.

There are advantages and disadvantages to both.  As you can see, in some cases it is easier to program a list since it requires less dwell points and gives you more flexibility with what your dwell can be.  If your waveform has a lot of DC levels in it, then the standard list might work for you.  If you have a long, complex waveform the 64 Kpoints offered in an arb will most likely offer you the best option to replicate your waveform.

Whichever arb you pick, this is a very powerful tool.  I am thinking that I will follow this up at a future date with more information about arbs.  If you have any questions, feel free to leave us some comments.



Powerlifting Agilent style!

I have been working out at a gym including lifting weights since the early 1980’s. We have a small gym here in our office building that I use a few times per week. The other day, while doing incline bench presses, my mind was wandering and I began to wonder how much power it took for me to lift the barbell and weights.
I could put the barbell and weights on a battery operated lift we have here in the office and instead of the battery, use one of our power supplies to power the lift and measure the power while operating the lift. I also wanted to calculate how much power would be required. I admit that I had to take out my old physics book to refresh my memory on how to convert weight moved through a distance to watts, but this turned out to be pretty simple: the power is just the force (weight in newtons) times the velocity. Here is the justification:

Force is mass times acceleration. F = mass*acceleration = kg-m/s^2 = newton = N which is weight when the acceleration is due to gravity (weight = mass*gravity).

Work (energy) is force (weight) applied over distance. Work = F*distance = N-m = joule = J.

Power is work per unit of time. Power = J/s = watt = W.

So power in watts = W = J/s = N-m/s = kg-m/s^2-m/s = mass * acceleration * velocity = kg*gravity*velocity = weight*velocity (gravity = 9.8 m/s^2).

During my investigation, I did go off on a tangent for a short time looking at why we talk about measuring weight in kilograms even though kilograms are units for mass and not weight. It would be proper to measure weight in newtons, not in kilograms, but that’s a different story!

So when I lift 205 lbs (93 kg) a distance of 15 inches (0.38 m) in 1.5 seconds, I use 231 watts of power to do so (mass*gravity*velocity = 93 kg * 9.8 m/s^2 * 0.38 m/1.5 s). As I mentioned above, I wanted to see if I could measure something similar with a power supply connected to a battery operated lift by using our power supplies in place of the 24 V batteries. Here is what I found:
I did a baseline power measurement of just the lift lifting some wooden pallets needed to support the barbell I was about to put on the lift. I used 2 Agilent N7972A (40 V, 50 A, 2kW) power supplies connected in parallel (I needed the extra current capacity) and set to 24 V along with our 14585A Control and Analysis Software to capture the power over time. I could then add weight and measure the incremental power required to lift the added weight.
I found that the lift itself consumes 1502 W as my baseline measurement. Then I added a 288 lb (130.6 kg) battery compartment along with 295 lbs (133.8 kg) of barbell weighs for an added 583 lbs (264.4 kg). Again, I measured the power consumed by the lift while it moved the weights vertically and found it to be 1638 W. Lifting the incremental 264.4 kg consumed an additional 136 W. Let’s see if this makes sense with a calculation. The lift moved 4.5 inches vertically in 2.2 seconds which equals 0.052 m/s. The calculated power is then 264.4 kg * 9.8 m/s^2 * 0.052 m/s = 134.7 W. That’s very close to the measured 136 W!!
It is no surprise that the laws of physics work as expected here and that our power supplies can provide insight into those laws. Agilent has added new meaning to the term “powerlifting”!

Wednesday, May 21, 2014

DC Source Measurement Accuracy and Resolution – With Shorter Measurement Intervals

I had gotten a customer support request a while ago inquiring about what the measurement resolution was on our new family of N6900A and N7900A Advanced Power System (APS) DC sources.  Like many of our newer products they utilize a high-speed digitizing measurement system.

 “I cannot find anything about measurement resolution in the user’s guide, it must have been overlooked!” I was told. Indeed, we have included the measurement resolution in the past on our previous products. We did not include it as a single fixed value this time around, not as an oversight however, but for good reason.

Perhaps the most correct response to the inquiry is “it depends”. Depends on what? The effective measurement resolution depends on the measurement interval that is being used. Why is that? Simply put, there is noise in any measurement system. With older and more basic products that provide low speed measurements and inherently have a long measurement interval that the voltage or current signal is integrated over, measurement system noise is usually not a big factor. However, with the higher speed digitizing measurement systems we now employ in our performance DC sources, factoring in noise based on the measurement interval provides a much more realistic and meaningful answer.

For the N6900A and N7900A APS products we include Table 1 shown below, in our user’s guide to help customers ascertain what the measurement accuracy and resolution is, based on the measurement interval (i.e. measurement integration period) being used is.
  


Table 1: N6900A/N7900A measurement accuracy and resolution vs. Measurement interval

This table is meant to provide an added error term when using shorter measurement intervals. We use 1 power line cycle (1 NPLC) as the reference point at the top of the table, for the measurement accuracy provided in our specifications. This is a result of averaging 3,255 single samples together. By doing this we have effectively spread the measurement system noise over a greater band and filtered it out by the averaging. For voltage measurements the effective resolution is over 20 bits.

Note now at the bottom of the table there is the row for one point averaged. It is for 0.003 NPLCs, which is 5 microseconds, the sampling period of the digitizer in our DC source. For a single sample the effective measurement resolution is now 12.3 bits for voltage. Note also we provide an accuracy error adder term of 0.02%. This is taking into account the measurement repeatability affecting the accuracy.

A convenient expression for converting from number of bits to dB of signal to noise (SNR) for a digitizer is given by:

SNR (dB) = 6.02 x n (# of bits) + 1.76

The 12.3 bits of effective resolution equates to 75.8 dB of SNR, which is very much in line with what to expect from a wide band, high speed digitizing measurement system like what is provided in this product family.

As previously mentioned the effective measurement resolution is over 20 bits for a 1 NPLC measurement interval. This actually happens to be greater than the actual ADC used. While there is less resolution when using shorter measurement intervals, conversely greater resolution can be achieved by using longer measurement intervals, which I expect to talk more about in a future posting here on “Watt’s Up?”!

In the meantime this is just one more example of how we’re trying to do a better job specifying our products to make them more useful and applicable in ascertaining what their true performance will be in one’s end application.

Wednesday, May 14, 2014

European Space Power Conference (ESPC) for 2014

This week’s blog posting is going in a bit of a different direction, as I likewise did last month, to attend and participate in the 2014 European Space Power Conference (ESPC) for 2014. While this was the tenth ESPC, which I understand takes place every couple of years; this was the first time I had opportunity to attend. One thing for certain; this was all about DC power, which is directly aligned with the things I am always involved in. In this particular instance it was all about DC power for satellites and space-bound crafts and probes.

I initially found it just a bit curious that a number of the keynote speeches also focused a fair amount on terrestrial solar power as well, but I supposed I should not be at all surprised. There has been a large amount of innovation and a variety of things that benefit our daily lives that came out of our own space program, fueled by our involvement in the “space race” and still continuing on to this day. (Can you name a few by chance?). This is a natural progression for a vast number of technological advances we enjoy.

At ESPC there were numerous papers presented on solar cells and arrays, batteries and energy storage, nuclear power sources, power conversion and DC/DC converters, super-capacitors, and a variety of other topics related to power. Just a couple of my learnings and observations include:
·         There was a very high level of collaboration of sharing findings and answering questions among peers attending the event
·         While batteries generally have very limited lives, from findings presented, it was interesting to see how well they have performed over extended periods in space, lasting last well in excess of their planned life expectancies. It is a reflection of a combination of several things including careful control and workmanship, understanding life-shortening and failure mechanisms, how to take properly treat them over time, what should be expected, as well as other factors contributing to their longevity. I expect this kind of work will ultimately find its way to being applied to using lithium ion batteries in automotive as well.
·         A lot of innovation likewise continues with solar cell development with higher conversion efficiencies coming from multi-junction devices. Maybe we’ll see this become commonplace for terrestrial applications before long!
·         A number of research papers were presented from participants from universities as well. In all, the quality of work was excellent.

I was there with another colleague, Carlo Canziani. Together we represented some of our DC power solutions there, including our N7905A DC Power Analyzer, N7900 series Advanced Power System (APS), and E4360A series Solar Array Simulator (SAS) mainframe and modules. These are the kinds of advanced power stimulus and measurement test instruments vital for conducting testing on satellite and spacecraft power components and systems.

In all it was a refreshing change of pace to be at an event where power is the primary focus, and if this happens to be an area of interest to you as well, you can find out more about ESPC from their site by clicking on the following link: (ESPC2014). Maybe you will find it worthwhile to attend or even present results of some of your work at the next one as well!

Wednesday, April 30, 2014

New Software Update for the N7900 Advanced Power System

Hi everybody,

Last year, we introduced the Agilent N7900 Advanced Power System (hereon in shortened to N7900 APS).  The N7900 APS is a full of great features that can only be accessed using the instrument's programming interface.  The programming interface works very well but sometimes you just don't want write and troubleshoot a program, you just want something that works.

Well, I have the chance to share some pretty exciting news.  We want to provide you software that makes some of these great features easy for you to use.  The software is the 14585A Control and Analysis Software.  This software was previously only available for the N6705 DC Power Analyzer.

The 14585A software is a standalone application that unlocks three key features: it allows you to look at a graphical representation of the measured data in Scope Mode, create arbitrary waveforms in Arb mode, and log long term data in datalogger mode.  These three advanced features can be setup and run by adjusting a few settings and pressing a few buttons.

 The software comes with a 30 day free trial so feel free to download it to check it out.  Please note that you need at least version A.01.13 of the APS firmware in order to use the software.

You can find the latest APS firmware at:
APS Firmware

You can find the software at:
14585A Software

If you have any questions on the software, feel free to leave us some comments.  Thanks for reading!

Tuesday, April 29, 2014

Measurement of AC plus DC voltage

One of our AC source customers recently asked me to justify the reading on the front panel of one of our AC sources set to produce a sine wave with a DC offset. He had our 6812B AC Power Source/Analyzer set to a sine wave of 100 Vac (60 Hz) and added a DC offset of 50 Vdc. These AC sources can produce output voltages of up to 300 Vrms and DC voltages up to +/- 425 Vdc. With his settings of 100 Vac and 50 Vdc, the front panel meter was reading 111.79 V with the meter set to measure AC+DC. At first this seemed like an odd result to me, but then I realized that we are simply measuring the rms (root-mean-square) of the total waveform (AC plus DC) and that should be the square-root of the sum-of-the-squares of the individual rms values. This can be mathematically proven fairly easily. Since the AC source Vac is set in rms volts and the rms of DC is simply the DC voltage:



This works even if the DC value is set to -50 Vdc instead of +50 Vdc since the value is squared. And sure enough, when I set the AC source output to 100 Vac and -50 Vdc, the front panel measurement shows 111.82 as expected. The small variation in the measured value compared to the exact calculated value is due to the slight inaccuracies in both the output setting and measurement system.


So in summary, measurement of an rms waveform that consists of an AC signal plus a DC signal is the square-root of the sum-of-the-squares of the individual two values. It’s as simple as that!

Monday, April 28, 2014

Upcoming Seminar on Using Your Power Supply to Improve Test Throughput

I have provided here on “Watt’s up?” a number of ideas on how you can improve your test throughput from time to time, as it relates on how to make better use of you system power supplies to accomplish this. I have categorized these ideas on how to improve throughput as either fundamental or advanced.

In “How fundamental features of power supplies impact your test throughput” (click here to review) I shared in a two-part posting definitions of key fundamental power supply features that impact test throughput and ways to make improvements to literally shave seconds off of your test time.

One example (of several) of an advanced idea on improving throughput I previously shared here is “Using the power supply status system to improve test throughput” (click here to review). Here I explain how, by monitoring the status system, you can improve throughput by not relying on using excessively long fixed wait statements in your programming.

I hope you have found these ideas helpful. If you would like to learn more about using your system power supply to improve your test throughput I will be presenting a live web-based seminar this week, in just a couple of days, April 30th, at 1:00 PM EST on this very topic!

In this seminar I will go through a number of things I’ve shared here on “Watt’s up?” in the past, but in greater detail. In addition, I have also prepared several new ideas as well in this seminar that you might find of help for your particular test situation.  You can register online at the following (click here to access seminar description and registration).  In case you miss the live event I expect you will be able to register and listen to seminar afterward as well, as it will be recorded.

So if improving your test throughput is important to you I hope you are able to attend the seminar!