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.

## Friday, May 30, 2014

### 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”!

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”!

Labels:
N7972A,
power calculation,
power supply,
powerlifting,
Usage

## 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?”!

## 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.

Labels:
Advanced Power System,
DC Power Analyzer,
DC/DC converter,
ESPC,
European Space Power Conference,
Misc.,
N6705B DC Power Analyzer,
N7900,
SAS,
solar array simulator,
solar cell test

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