Use the FETCH Command to Minimize Your Measurement Time

Hi everyone,

I am back again with another programming tip for you.  A neat feature on some of our products that many people many not know about is the ability to fetch measurements from a previous acquisition.  Quite a few of our power supplies and loads (the N6700 modules, the N7900 APS power supplies, the 681xB AC Sources, the N3300 loads, and probably some others that I am probably forgetting) have the ability to acquire voltage and current measurements at the same time.  This is done using the FETCH command (in my little example snippets I use the SCPI short form of FETC).  In a previous blog post I used this command to read back an array of current measurements (see Inrush Current Measurements).  In that command, I use a FETCH command to retrieve a triggered measurement.  

There is another, very useful way to use the FETCH commands.  I am not really sure what the best way to phrase it so I am going to take a shot and then illustrate with an example.  When you send a measure command (say for voltage), the measurement system will also acquire the other measurement (in this case current) and you can send a FETCH command to retrieve that acquired data.   Here is a very small example with some comments (all these commands tested on a N7952A Advanced Power System):

Example Snippet 1:
MEAS:VOLT? -> This will start a new acquisition and take the measurements 
<read back the voltage measurement data>
FETC:CURR? -> This will return the current measured during the voltage measurement above
<read back fetched current measurements>

Since we have voltage and current measurements, the instrument can calculate power:
FETC:POW? -> P=V*I
<read back calculated power>

Please note that you can do this with arrays as well. 

How can this save me time in my program you ask?  Well these power supplies all have built in digitizers that you can access with some programming commands.  The default measurement (at 60 Hz line frequency) is 3255 points measured at 5.12 us per point.  That is a total measurement time of  16.67 ms.  You have the ability to change this to fit your needs though.  You can measure up to 512 Kpoints at up to 40,000 s per point.  Every time you send a measure command you need to wait for the measurement to complete.  For instance:

Example Snippet 2:
MEAS:VOLT?
<read back the voltage measurement data> 
MEAS:CURR? 
<read back the current measurement data>

You will need to wait for two acquisition periods because you are initiating two separate measurements.  In the first example snippet, only the MEAS:VOLT? is initiating a measurement, the FETC:CURR is just reading data out of the instrument.    The downside is that the data that you fetch is going to be of the same age as the last measurement you did so if you need something newer, you need to do a new measurement.  Overall though I think that FETCH is a very useful command.  

I hope people find this useful.  Let us know if you have any questions by using the comments.  

What is a floating power supply output?

First let me tell you that a floating power supply output is NOT what is shown below in Figure 1 (haha).

Now some background: earth ground is the voltage potential of the earth and to greatly reduce the risk of subjecting a person to an electrical shock, the outer covering (chassis) of most electrical devices is internally connected to a wire that is connected to earth ground usually through the power cord. The idea here is to ensure that all surfaces a person can touch are at the same voltage potential; namely, the one that he is standing on: earth ground. As long as that is true, the person can freely touch things without the risk of getting shocked due to two of the things he touches at the same time being at different voltage potentials, or one of the things being at a high voltage potential with respect to the earth. If the voltage difference is high enough, the person could be shocked. Earth grounding the chassis also protects the user if there is an internal problem with an electrical device causing its chassis to inadvertently come in contact with an internal high voltage wire. Since the chassis is earth grounded, an internal short to the chassis is really a short to ground and will blow a fuse or trip a circuit breaker to protect the user instead of putting the chassis at the high voltage. If you touched a chassis that had a high voltage with respect to ground on it, your body completes the path to ground and you get shocked!

So to protect the user (and for some other reasons), the chassis of Agilent power supplies are grounded internally through the ground wire (the third wire) in the AC input line cord. Additionally, most if not all of our Agilent power supplies have isolated (floating) outputs. That means that neither the positive output terminal nor the negative output terminal is connected to earth (chassis) ground. See Figure 2.

Figure 3 shows an example of non-floating outputs with the negative output terminal grounded.

For floating DC power supplies, the voltage potential appears from the positive output terminal to the negative output terminal. There is no voltage potential (at least, none with any power behind it) from either the positive terminal to earth ground or from the negative output terminal to earth ground. A power supply with a floating output is more flexible since, if desired, either the positive or negative terminal (or neither) can be connected to earth ground. Some devices under test (DUT) have a DC input with either the positive or negative input terminal connected to earth ground. If one of the power supply outputs was also internally connected to earth ground, when connected to the DUT, it could short out the power supply output. So power supplies with floating output terminals (no connections to earth ground) are more versatile.

If the outputs are floating from earth ground, we need to specify how far above or below earth ground you can float the output terminals. Our power supply documentation provides this information. For example, most Agilent power supply output terminals can float to +/-240 Vdc off of ground. You will frequently see the following in our documentation:

Also, some power supplies have different float ratings for the positive and negative output terminals. For example, for Agilent N5700 models rated for more than 60 Vdc, the following note in the manual means you can float the positive output terminal up to +/-600 Vdc from ground or the negative output terminal up to +/-400 Vdc from ground:

The output characteristic table may list this as “Output Terminal Isolation” as shown below which means the same thing as maximum float voltage:

Figure 4 shows an example of floating a power supply to 200 V above ground. The power supply output is set to 40 V.

You can see from the last example that you have to take the power supply output voltage into consideration when ensuring you are not violating the float voltage rating. If you exceed the float voltage rating of the power supply, you are potentially exceeding the voltage rating of internal parts that could cause the internal parts to fail or break down and present a shock hazard, so don’t violate the float voltage rating!

Upcoming software release unleashes the N7900 APS’s potential without any programming

Our N7900A Advanced Power System (APS) is well named, being the most advanced power product we’ve introduced to date. In many ways it is based on our N6700 series modular DC power system and N6705B DC power analyzer, incorporating their capabilities, including:

• High precision programming and measurement
• Seamless measurement ranging
• High speed measurement digitization of voltage, current, and power
• Long term data logging of voltage, current, and power
• Output ARB and List capabilities
• And quite a bit more

In addition the N7900A APS brings quite a few new and unique capabilities as well, including:

• Much greater output power
• Logic-configurable expression signal routing for advanced custom triggering and control
• Optional external dissipater unit for full two quadrant operation
• Optional black box recorder for post-test diagnostics when needed
• And quite a bit more

To take advantage of these advance capabilities does require a bit of programming, which is to generally be expected for an automated test environment, but in low volume design validation and R&D this can slow down the desired quick time-to-result. The N6705B DC Power Analyzer, in Figure 1, has a full-featured front panel menu and graphical display that lets design validation and R&D users quickly configure and run complex power-related tests on their devices. In comparison, the N6700 series, pictured in Figure 2, does not have all the front panel capabilities of the N6705B and can be looked on as the ATE version of this product platform, requiring programming to take advantage of its advanced capabilities. The N6705B shares all the same DC power modules that the N6700 series uses.

Figure 1: The N6705B DC Power Analyzer, primarily for bench use

Figure 2: The N6700 series Modular DC Power System, primarily for ATE

The N7900A APS is very similar in form and function to the N6700 series, not having all the advanced front panel capabilities that the N6705B has for bench-friendly use of its advanced features. I am really pleased to be able to share with you that this is now changing! While we are not creating a bench version of the N7900 APS, we are upgrading our 14585A Control and Analysis software, which emulates the front panel of an N6705B and more, to work with the N7900 APS as well. The 14585A will soon let you quickly and easily create and configure complex power-related tests based on using the N7900 APS.  I am fortunate enough to be working with a beta version of the software. Some examples of things I was able to do in just a few minutes were to capture the inrush current of an automotive headlight, shown in Figure 3, and superimpose an AC sine wave disturbance on top of the DC output, shown in Figure 4.

Figure 3: Auto headlamp inrush current captured with 14585A software and N7951A APS

Figure 4: Sine wave voltage disturbance on top of DC generated by 14585A software and N7951A APS

The updated release of the 14585A Control and Analysis software is just a few weeks away. More about the 14585A software can be found by clicking on the following link <14585A>. With the 14585A being a great way to implement ideas and tests quickly, using the N7900 APS, look for me and others coming up with some interesting applications in future posts here on “Watt’s Up?”!

R, L, C measurements with an AC source

I recently had a customer ask if there was a way to use one of our AC sources to determine whether the load connected to its output was capacitive or inductive. Agilent’s 6811B, 6812B, and 6813B AC power source/analyzers are all capable of making many different measurements, several of which can be used to calculate the impedance of the connected load. To determine the impedance, the simplest measurements to use are the amplitude and phase measurements of the applied sine wave voltage and resultant current. Agilent’s AC sources can measure harmonic content of the voltage and current, both amplitude and phase, up to the 50th harmonic. From these measurements, you can calculate the impedance of the R, L, or C connected to the AC source output.

I grabbed a few sample parts from our lab area to demonstrate these measurements. First, I used a power resistor that was about 49 ohms. I applied a sine wave of about 20 Vac at 1000 Hz (0 phase) and used the built-in measurement capability to measure about 0.4 Aac at a phase (angle) of very near 0 degrees (the measurement was -7.01E-2 = -0.071 degrees). Of course, 0 degrees of phase between the voltage and current means the sine waves are in phase and the load is resistive as expected. See Figure 1.

The next test I did was with an inductor. Connecting it to the output of the AC source and once again applying about 20 Vac at 1000 Hz from the AC source, using the built-in measurement capability, I measured about 0.129 Aac at a phase of -88.66 degrees. The phase measurement of nearly -90 degrees confirmed that the load on the AC source was an inductor and the magnitude could be calculated to be about 25 mH which is what I expected (I measured it using an Agilent LCR meter). The series resistance in the line cord, clip leads, and inductor wire itself (I did not use remote sense) accounted for the non-ideal phase of -88.66 degrees instead of the ideal phase of -90 degrees. And the R was calculated from the AC source measurements to be about 3.6 ohms and agreed with external verification. See Figure 2.

Finally, I connected a capacitor to the AC source output and applied about 10 Vac at 1000 Hz. The AC source measurement system showed 0.633 Aac at an angle of 87.47 degrees indicating a capacitor was across its output. From these measurements, the capacitance and series R were calculated to be 9.88 uF and 0.711 ohms, consistent with externally verified measurements. See Figure 3.

So you can see that it is possible to determine the impedance (resistance, capacitance, or inductance) of a device connected across the output of an AC source when the right measurement capabilities are built into the AC source, as they are with the Agilent AC sources. These highly capable products not only make measurements like this easy, they also can easily create a large variety of AC output stimulus waveforms.

Use Agilent's BenchVue Software to Save Time

Hi everyone!

I wanted to take the time I have in this month’s blog post to tel you about a great new tool that we have for you here at Agilent.  It is a new software package called BenchVue.  BenchVue offers users the ability to control their power supplies, oscilloscopes, spectrum analyzers,  DMMs, and function generators using their computer without the need to write a program.  It is a free download.  You can download BenchVue at:

Agilent BenchVue Software

Naturally I am going to talk about the power supply potion of BenchVue (this is a power supply blog after all).  Presently the software supports the Agilent N6700 Modular Power Supplies and the Agilent E36xxA Basic Power Supplies.  I do not know if many people remember but we used to have a free software package for the E36xxA power supplies called Intuiilink.   BenchVue is  the modern successor to Intuiilink.  I recently checked BenchVue out on my E3646A DC Power Supply.  

The E3646A is a basic two output DC Power Supply.  BenchVue communicates with it using GPIB (in my case my Agilent 82357 USB/GPIB converter).  Take a look at the below picture:

You can program all of the basic settings (even for multiple channels) on your power supply on one screen.  If you used the front panel to do all of these entries it would take quite a while to navigate through all of the menus.  BenchVue saves you time by putting all of this in one place.  The other neat thing that I noticed is that you can get the voltage and current readback from both channels on the screen at the same time.  You can't see both channels on the front panel, you have to manually switch.

The other main feature of BenchVue that can save you some time is the built in datalogger.  For those of you that are unfamiliar with the term datalog, this is when an instrument takes measurements at a predetermined time interval and stores them in a file that you can look at later.  With BenchVue, you can set a datalog up to take a measurement as fast as every second.  You can then take the stored values and export them to Matlab, Microsoft Excel, Microsoft Word, or to a CSV file.  

You could write your own datalog program but the beauty of BenchVue is that you can set the log up by entering a few values and then just press the "Run" button to make it go.  You also do not have to worry about formatting the data or creating files since BenchVue does all of that for you.  You basically just run it, then export the data and then you can view it in your chosen format.

Here is a sample thirty second datalog where I logged the voltage on Output 1:

After it was complete, I just hit the "Export" button, chose Microsoft Excel and I got the following:

Pretty cool!

That's my short intro to the new Agilent BenchVue software.  Go download it.  Let us know any thoughts you have in the comments section.

How to test the efficiency of DC to DC converters, part 2 of 2

In part 1 of my posting on testing the efficiency of DC to DC converters (click here to review) I went over the test set up, the requirements for load sweep synchronized to the measurements, and details of the choice of the type and set up of the current load sweep itself. In this second part I will be describing details of the measurement set up, setting up the efficiency calculation, and results of the testing. This is based on using the N6705B DC Power Analyzer, N6782A SMUs, and 14585A software as a platform but a number of ideas can be applicable regardless of the platform.

Figure 1: Synchronized measurement and efficiency calculation set up

The synchronized measurement and efficiency calculation set up, and display of results are shown in Figure 1, taking note of the following details corresponding to the numbers in Figure 1:

1. In the 14585A the data logging mode was selected to make and display the measurements. The oscilloscope mode could have just as easily been used but with a 10 second sweep the extra speed of sampling with the oscilloscope mode was not an advantage. A second thing about using the data logging mode is you can set the integration time period for each acquisition point. This can be used to advantage in averaging out noise and disturbances as needed for a smoother and more representative result. In this case an integration period of 50 milliseconds was used.
2. To synchronize the measurements the data log measurement was set to trigger off the start of the load current sweep.
3. Voltage, current, and power for both the input and output SMUs were selected to be measured and displayed. The input and output power are needed for the efficiency calculation.
4. The measurements were set to seamless ranging. In this way the appropriate measurement range for at any given point was used as the loading swept from zero to full load.
5. A formula trace was created to calculate and display the efficiency in %. Note that the negative of the ratio of output power to input power was used. This is because the SMU acting as a load is sinking current and so both its current and power readings are negative.

With all of this completed really all that is left to do is first start the data logging measurement with the start button. It will be “armed” and waiting from a trigger signal from the current load sweep ARB that had been set up. All that is now left to do is press the ARB start button. Figure 2 is a display of all the results after the sweep is completed.

Figure 2: DC to DC Converter efficiency test results

All the input and output voltage, current, and power measurements, and efficiency calculation (in pink) are display, but it can be uncluttered a bit by turning off the voltages and currents traces being displayed and just leave the power and efficiency traces displayed. This happened to be special DC to DC converter designed to give exceptionally high efficiency even down to near zero load, which can be seen from the graph. It’s interesting to note peak efficiency occurred at around 60% of full load and then ohmic losses start becoming more significant.

And that basically sums it all up for performing an efficiency test on a DC to DC converter!

How to test the efficiency of DC to DC converters, part 1 of 2

I periodically get asked to provide recommendations and guidance on testing the efficiency of small DC to DC voltage converters. Regardless of the size of the converter, a DC source is needed to provide input power to the converter under constant voltage, while an electronic load is needed to draw power from the output, usually under constant current loading. The load current needs to be swept from zero to the full load current capability of the DC to DC converter while input power (input voltage times input current) and output power (output voltage times output current) are recorded. The efficiency is then the ratio of power out to power in, most often expressed in a percentage. An illustration of this is shown in Figure 1. In addition to sourcing and sinking power, precision current and voltage measurement on both the input and output, synchronized to the sweeping of the load current is needed.

Figure 1: DC to DC converter efficiency test set up

One challenge for small DC to DC voltage converters is finding a suitable electronic load that will operate at the low output voltages and down to zero load currents, needed for testing their efficiency over their range, from no load to full load output power. It turns out in practice many source measure units (SMUs) will serve well as a DC electronic load for testing, as they will sink current as well as source current.

Perhaps the most optimum choice from us is to use two of our N6782A 2-quadrant SMU modules installed in our N6705B DC Power Analyzer mainframe, using the 14585A software to control the set up and display the results.  This is a rather flexible platform intended for a variety of whatever application one can come up with for the most part. With a little ingenuity it can be quickly configured to perform an efficiency test of small DC to DC converters, swept from no load to full load operation. This is good for converters of 20 watts of power or less and within a certain range of voltage, as the N6782A can source or sink up to 6 V and 3 A or 20 V and 1 A, depending on which range it is set to. One of the N6782A operates as a DC voltage source to power the DUT and the second is operated as a DC current load to draw power from the DUT. A nice thing about the N6782A is it provides excellent performance operated either as a DC source or load, and operated either in constant voltage or constant current.

An excellent video of this set up testing a DC to DC converter was created by a colleague here, which you can review by clicking on the following link: “DC to DC converter efficiency test”.

The video does an excellent job covering a lot of the details. However, if you are interested in testing DC to DC converters using this set up I have a few more details to share here about it which should help you further along with setting it up and running it.

First, the two N6782A SMUs were set up for initial operating conditions. The N6782A providing DC power in was set up as a voltage source at the desired input voltage level and the second N6782A was set to constant current load operation with minimum (near zero) loading current.

Note that the 14585A software does not directly sweep the load current along the horizontal axis. The horizontal axis is time. That is why a time-based current sweep was created in the arbitrary waveform (ARB) section of the 14585A. In that way any point on the horizontal time axis correlates to a certain current load level being drawn from the output of the DUT. The ARB of course was set to run once, not repetitively. The 14585A ARB set up is shown in Figure 2.

Figure 2: Load current sweep ARB set up in 14585A software

This ARB sweep requires a little explanation.  While there are a number of pre-defined ARBs, and they can be used, an x³ power formula was chosen to be used instead. This provided a gradually increasing load sweep that allowed greater resolution of this data and display at light loads, where efficiency more quickly changes. As can be seen, the duration of the sweep, parameter x, was set to 10 seconds. As a full load current needed to be -1 A, using the actual formula (-x/10)³  gave us a gradually increasing load current sweep that topped out at -1A after 10 seconds of duration. The choice of 10 seconds was arbitrary. It only provided an easy way to watch the sweep on the 14585A graphing as it progressed. Finally, a short (0.1 second) pre-defined linear ramp ARB was added as a second part of the ARB sequence, to bring the load current back to initial, near zero, load conditions after the sweep was completed. This is shown in Figure 3.

Figure 3: Second part of ARB sweep to bring DUT load current back to initial conditions

I hope this gives you a number of insights about creative ways you can make use of the ARB. As there is a good amount of subtle details on how to go about making and displaying the measurements I’ll be sharing that in a second part coming up shortly, so keep on the outlook!

More Information on the Awesomeness of binary data

Hi everybody!

This month I am going to do a build on one of Ed's posts from this month.  It was titled "Using Binary Data Transfers to Improve Your Test Throughput".  If you have not read it, go ahead and click on the link. I'll be here when you get back.

I wanted to reiterate how drastic the difference is between using these two interfaces when you are reading large amounts of data.   I did some bench-marking a little while ago and I wanted to share it now with everybody.  Please note that these were quick tests that I did and in no way are official numbers.  In fact if you see anything wrong with my methods, please comment.   

The first thing that I will talk about is my method.  I did the test with a N6700B MPS Mainframe and a N6781A SMU module.  I wrote a program that set up the module to source 5 V and then take an array of voltage measurements.  I set it for the maximum number of measurement points (524288 points) with the fastest sample rate (though for this experiment the sample rate does not really matter).  Before I did the reading of the data from the N6700B to my PC I started a programmatic stopwatch and stopped the stopwatch after the reading was complete.  I looped 20 times and took the average.

One thing that I would highly recommend is to use the Agilent VISA-COM IO library.  The VISA-COM library offers a ReadIEEEBlock function that makes reading binary data really easy for you.  

The screenshot below shows the relevant loop and the calculation.  This program was written in VB and I used LAN to communicate with the instrument.

The other important piece that this is not showing is that I am setting the data format to real using FORM REAL command.  When you use ASCII, the command is FORM ASCII (this is also the default setting).

You can see the commented out ReadString command that I swapped in when I used the ASCII data format.  You can also see my extremely professional (and useful) "I am on line" counter that I put in so that I knew my program was looping correctly.

So now for the times.  ASCII format took around 100 s to read back all 524288 measurements into a string.  When I switched to the binary format, it took under 5 seconds. As you can see, that is a very drastic difference and if you are reading back a lot of data from an instrument that supports the binary format, you really need to use it.

I also did a few other experiments.  I changed the total number of points down to 1000.  The binary format took a little under 20 ms to read the data and the ASCII format took about 125 ms.  The last test that I did was 3 data points.  The binary format took a little less than 15 ms and the ASCII format took under 5 ms to make the measurement.  So you can see that as you read less and less data back, the ASCII format does catch up to the binary format and even exceed it.

Moral of the story is that if it is something more than a few points to read back, use binary because it will save you a ton of time.

That's all I have this month and I will be back next month!

Using a DC Power Supply to Regulate Energy

In a previous 2-part posting I talked about what power and energy is (part 1 – energy) (part 2 – power).  It is pretty straight-forward thing to do to use a DC power supply for regulating voltage or current. Constant voltage (CV) and constant current (CC) regulation are standard features of most all DC power supplies used in testing. However, what if you have an unusual application calling for applying a fixed amount of energy to your device under test (DUT)? For example, adding a fixed amount of energy to a calorimeter or chemical process, or testing the must (or must not) tripping energy of a fuse, or circuit breaker, or squib or detonator perhaps?

When the resistance of a device remains constant, it is relatively straight-forward to apply a fixed amount of energy to a DUT. By applying a fixed voltage or current, the power in the DUT remains constant. Then the energy is simply:

E = (V²/R)_*t = (I²_*R)_*t

Where E is the energy in watt-seconds or joules, V is voltage in volts, R is resistance in ohms, I is the current in amps, and t is time in seconds. All you now need to do is apply the constant voltage or current for a pre-determined amount of time and you will then be delivering a fixed amount of energy to your DUT.

Many times however, a lot of DUTs do not maintain constant loading. The may have a dynamically varying loading by nature or its resistance dramatically increases as it heats up. How do you regulate a fixed amount of energy to your DUT under these circumstances? One possibility is to use one of a few specialized power supplies on the market can regulate their outputs with constant power. As the DUT’s loading decreases or increases the power supply will adjust its output accordingly in order to maintain a constant output power delivered to the DUT.  Again then, by applying this constant power for a pre-determined amount of time you will then be delivering a fixed amount of energy to your DUT.

Still, for DUTs that do not maintain constant loading, it is very often not desirable, or outright unacceptable, to apply constant power sourcing.. It may be you can only apply a fixed voltage or current to your DUT. What can you do for these circumstances? Time can no longer remain a fixed value when trying to regulate a fixed amount of energy. The solution becomes quite a bit more complex, as depicted in Figure 1.

Figure 1: Regulating a fixed amount of energy to a DUT

Putting the solution depicted in Figure 1 into practice can prove challenging. The watt-hour meter needs to provide a trigger out signal when the desired watt-hour (or watt-second) threshold level is reached. This becomes even more challenging if this response time required needs to be just fractions of a second for this set up. More than likely this may become a piece of customized hardware.

Interestingly this very set up can be programmatically configured within our N6900A and N7900A series Advanced Power System (APS) power supplies. These products have Amp-hour and Watt-hour measurement integrated into their measurement systems. Not only can you measure these parameters, there is a programmable way to act on them in a variety of ways as well, which is the expression signal routing. Logical expressions can be programmed and downloaded into APS, which then acts on them at hardware-level speeds.  Creating and loading the signal routing expression into the APS unit is simplified by using the N7906A Power Assistance software, which let me do it graphically, as shown in Figure 2.

Figure 2: Graphically developing and loading an energy limit setting into an Agilent APS unit

In Figure 2 a threshold comparator was set to generate a trigger output at a level of 0.0047 watt-hours. This trigger was then routed to the output transient system, to cause the output to transition to a new output level when triggered. I had entered in zero volts as the triggered output level. Thus when the watt-hour reading reached its trigger point, the output went to zero, cutting off any more power and energy from being delivered to the DUT.

The SCPI command set for this signal routing expression is also generated from this software utility by clicking on “SCPI to clipboard”. This saves on the effort generating the code manually if you are incorporating the expression into a larger test program. For this expression the code generated is:

:SENSe:THReshold1:FUNCtion WHOur

:SENSe:THReshold1:WHOur 0.0047

:SENSe:THReshold1:OPERation GT

:SYSTem:SIGNal:DEFine EXPRession1,"Thr1"

:TRIGger:TRANsient:SOURce EXPRession1

To test things out a 1.18 ohm resistive load was used to draw 84.75 watts for a 10 volt output setting. The output cut back to zero volts at nearly 200 milliseconds, as expected. This is shown in the oscilloscope capture in Figure 3.

Figure 3: APS output for an 84.75 watt load and energy limit set to 0.0047 watt-hours

The load power was then doubled by increasing the output voltage to 14.142 volts. The APS output cut back to zero volts in half the time, delivering the same amount of energy, as expected. This is depicted in the oscilloscope capture in Figure 4.

Figure 4: APS output for a 169.5 watt load and energy limit set to 0.0047 watt-hours

While using a resistor makes it easy to see that a set amount of energy is being delivered to the load. However, being able to act on a real time watt-hour energy measurement makes it very practical to do deliver a fixed amount of energy, regardless of the dynamic nature of the load over time.

Using Binary Data Transfers to Improve Your Test Throughput

From time to time I have shared here on “Watt’s Up?” a number of different ways the system DC power supply in your test set up impacts your test time, and recommendations on how to make significant improvements in the test throughput. Many of these previous posts are based on the first five of ten hints I’ve put together in a compendium entitled “10 Hints on Improving Throughput with your Power Supply” (click here for hints 1-5).

Oscilloscopes, data acquisition, and a variety of other test equipment are often used to capture and digitize waveforms and store large arrays of data during test, the data is then downloaded to a PC. These data arrays can be quite large, from thousands to millions of measurements. For long-term data logging the data files can be many gigabytes in size. These data files can take considerable time to transfer over an instrument bus, greatly impacting your test time.

Advanced system power supplies incorporating digitizing measurement systems to capture waveform measurements like inrush current are no different. This includes a number of system DC and AC power products we provide. Even though you usually have the choice of transferring data in ASCII format, one thing we recommend is instead transfer data in binary format. Binary data transmission requires fewer bytes reducing transfer time by a factor of two or more.

Further details about using binary mode data transfers can be found in hint 7 of another, earlier compendium we did, entitled “10 hints for using your power supply to decrease test time” (click here to access). Between these two compendiums of hints for improving your test throughput I expect you should be able find a few different ideas that will benefit your particular test situation!

Keysight Technologies – What’s in a name, anyway?

This is an exciting time for us! Yesterday, January 7, 2014, we learned what our new company name will be: Keysight Technologies. Here is a link to the official press release: 

http://www.agilent.com/about/newsroom/presrel/2014/07jan-gp14001.html

If you have not heard by now, back in September of 2013, Agilent Technologies announced it would split into two companies by the end of 2014. Since I started in 1980, 33+ years ago, I have worked for and will continue to work for the electronic measurement side of the business. For my first 19 years with the company, we were Hewlett-Packard, a company that formed in 1939 as a test and measurement company and eventually birthed Silicon Valley. It is this electronic measurement side of the business that is getting the new name, just as we did in 1999 when we became Agilent from Hewlett-Packard. The other part of Agilent, the life sciences, diagnostics, and applied markets side will continue to operate under the Agilent name. Note that the electronic measurement side will still have the same great people designing and supporting the same great products and services….just a new name: Keysight Technologies.

So let’s take a closer look at the new name, logo, and tagline for Agilent’s electronic measurement business:

The name combines the words “key”, meaning essential, and “insight” meaning intuitive perception. Essential intuitive perception. Seeing what others cannot. A key can also be used to unlock something; we will continue to unlock measurement insights, as our tagline says. And we have been doing this for 75 years - since the day that Bill Hewlett and Dave Packard released their first electronic test instrument, an audio oscillator, in 1939. By the way, it is purely a coincidence that the Watt’s Up? authors, Ed, Matt, and I have combined experience of more than 75 years as stated in our blog header, although I’d like to think that the three of us have been unlocking measurement insights in the AC and DC power field for a combined total of 75 years! In fact, we now have a total of more than 80 years’ experience between the three of us and I will update the header later this year when we officially begin using the Keysight name. But I digress...

The logo symbol is derived from one of the most fundamental wave shapes there is: a sine wave – a wave shape from which all other wave shapes can be built. This is very appropriate for our 100% focus on electronic measurement, not to mention a tribute to our original founders, Bill and Dave, for their audio oscillator. And the power products and topics that you read about here in Watt’s Up? are equally represented by the logo symbol, especially our AC sources! If you really think about it, Keysight will be doing what Hewlett-Packard originally set out to do and Agilent continued to do for decades: build the world’s leading test and measurement company.

So what’s in a name? Essential intuitive perception is in our name. A clear focus on electronic measurement is in our name. Our strong and deep heritage is in our name. And when we begin to fully operate as Keysight Technologies later this year, what you will find behind our name will be the same high quality products and people that you have come to know. That’s what’s in the name Keysight Technologies!

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: 

 Command Expert Link

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 VB.NET:

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!

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!

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!