Thursday, December 16, 2010

Transducers for making Temperature Measurements with a DMM

Four types of transducers are commonly used for making temperature measurements with a DMM: resistance temperature detectors (RTDs), thermistors, IC sensors and thermocouples. Each type has advantages and disadvantages. The follow is a quick summary of each.

Use thermistors for better sensitivity
A thermistor is a temperature transducer who's resistance changes as a function of temperature. They consist of semiconductor materials and provide excellent sensitivity, but their temperature range is limited, commonly from -80°C to 150°C. Thermistors have highly nonlinear temperature-resistance relationships; therefore, their conversion algorithms are complex. Modern DMMs make thermistor measurements by storing a table of thermistor resistance values and the temperatures they map to. Agilent DMMs like the 34401A, 34410A, and 34411A use the standard Hart-Steinhart approximation to provide accurate conversions, with a typical resolution of .08°C. Check out Agilent’s E2308A thermistor temperature probe

Use RTDs for more accuracy
Resistance temperature detectors (RTDs) work a lot like thermsistors in the sense that they are resistive temperature transducers. They provide high accuracy over a range of roughly -200 to 500°C. There is very little conversion complexity for an RTD since it is so intrinsically linear. Modern DMMs such as the Agilent 34410A provide measurement for the IEC751standard RTD, which has a sensitivity of .0385%/°C.

IC temperature sensors are an easy to use linear solution
Many vendors provide probes that produce a voltage proportional to temperature in degrees C or F. The probes typically use an IC temperature sensor such as the National Semiconductor LM135 series. A temperature IC can cover temperatures from -50°C to +150°C. You can easily compute the temperature from the probe output shown on the DMM display. For example, 270 mV is 27°C.

Thermocouples offer a wide measurement range and ruggedness
Thermocouples can measure the broadest range of temperature, from -210°C to 1100°C, and their rugged construction makes them ideal for harsh environments. Thermocouple temperature measurements are based off the Seebeck Voltage. Take two dissimilar metals and connect one end of each metal together and a small voltage will be present at the open end of the metals, this is the Seebeck voltage. This voltage is a function of the temperature at the junction of the two metals. This relationship between the temperature and the voltage produced by the two dissimilar metals is how thermocouple temperature measurements are made. If you directly connect a thermocouple wire to a DMM connector (which is metal) you create more thermocouple junctions which will distort the original intended measurement junction. Because of this a reference junction is needed between the DMM and the thermocouple cabling. This adds complexity to the measurement. There are DAQ instruments out there with a built-in reference junction like the Agilent 34970A or 34972A. For more technical details on thermocouple temperature measurements reference junctions check out the application note link below. 

By far the most common way I make temperature measurements is with thermocouple because of its ruggedness and its range. For instance I can use the same thermocouple wire to measure the temperature in an environmental chamber, to measure the ambient room temperature, or to measure the heat on a processor chip without having to worry about does its range cover what I am measuring or being delicate when I attach it to a surface. Below is a table that runs through each transducer’s pros and cons (click on to enlarge). 

Friday, December 3, 2010

Creating a Differential Signal with a Function / Arbitrary Waveform Generator

Single-ended signals are referenced to a common level, such as ground, that they can share with other signals, so a single-ended signal requires only a single path or wire. Differential signals are made up of a pair of paths that are both dedicated to a single signal at any given time. One path is used at a higher potential than the other. Differential signals do add complexity, since they require two wires instead of just one, but they provide a number of performance advantages over single-ended signals.
Differential signal advantages include better signal-to-noise ratio, fewer timing errors, and less crosstalk. These advantages make differential signals common in applications such as ADC inputs, instrumentation amplifiers, measurement sensors (like accelerometers), and communication signals. When engineers design and test devices that use differential signals, simulating the differential signals for testing can be challenging. These challenges are caused by the fact that most function/arbitrary waveform generators (FAWGs) have single-ended outputs; instruments that can generate differential signals tend to be fairly costly. In this post I will explain two ways to create a low cost differential signal: using a FAWG with some custom hardware and using a 2-channel FAWG
One way to use custom hardware at the output of a single-ended source to create a differential signal is to use a differential amplifier circuit design as shown it the figure.

The resistors in the differential circuit were chosen to achieve a gain value of 1. I set the DC offset to 0 V. When building the circuit, be careful to keep signal paths and wiring as short as possible to keep parasitic reactive affects low for better signal integrity.
FAWG’s with two single-ended channels (isolated from ground) can have their channels combined into a single differential signal channel. To do this, you need to tie together the two "low" or "common" connections of each channel. The "high" of one channel must be used as the high signal path of the differential channel and the "high" of the other channel must be used as the inverse return wire or low signal path, as shown in the figure.

In addition to the two channels, it is also a lot easier to do this on a two-channel FAWG that has channel tracking capability, like Agilent’s 33522A two-channel function/arb waveform generator. This feature gives you the ability to create an inverted mirror image of the output signal from channel one onto channel two, which is exactly what is needed to create a differential signal. Also, this capability means you only have to set up the arb or built-in waveform on one channel and the inverted version of the waveform automatically tracks to the other channel. Without this feature you would have to setup an arb or built-in waveform on both channels and try to output them in sync using triggering.
As an example I measured and captured three signals with a differential input high-resolution digitizer. The example signal we used was a squarewave at 500 KHz. The figure below shows a digitizer screen shot of the signals. The three signals:
  1. Signal in yellow is a differential signal from the output of the differential amplifier connected to the single-ended FAWG
  2. Signal in green is the differential signal output created by the two channels from the 33522A
  3. Signal in purple is the output of the single-ended FAWG before the differential amplifier input

As you can see from the figure there is quite a bit of ringing on the differential signal created with the custom hardware. When I built my diff amp circuit I was careful to keep wiring as short as possible and I provided a large ground plane. Now with further time and engineer effort I could probably further improve the signal integrity of the circuit. But the point of this example is to show you can save test time and achieve better signal quality by using a 2-chan FAWG with tracking to create a differential signal. Also the cost of a 2-chan FAWG is still typically much cheaper than a differential output waveform generator.

Sunday, November 28, 2010

Tips for Making True RMS AC Measurements with a DMM

True RMS responding DMMs measure the "heating" potential of an applied voltage. Power dissipated in a resistor is proportional to the square of an applied voltage, independent of the waveshape of the signal. Todays general purpose DMMs can accurately measures true RMS voltage or current, as long as the wave shape contains negligible energy above the meter’s effective bandwidth (more on this in a bit). Most DMM's ACV and ACI functions measure the AC–coupled true rms value (DC is rejected). For symmetrical waveforms like sinewaves, triangle waves, and square waves, the ac–coupled and ac+dc values are equal, since these waveforms do not contain a dc offset. However, for non–symmetrical waveforms (such as pulse trains) there is a dc voltage content, which is rejected by ac–coupled true RMS measurements. DC rejection is desirable in certain applications such as when you want to measure the ac ripple present on DC power supplies. For situations where you want to know the AC+DC true rms value, you can determine it by combining results from dc and ac measurements, as shown below:

A common misconception is that "since an ac multimeter is true rms, its sine wave accuracy specifications apply to all waveforms." Actually, the shape of the input signal can dramatically affect measurement accuracy for any multimeter, especially when that input signal contains high–frequency components which exceed the instrument’s bandwidth. As an example, consider a pulse train, one of the most challenging waveforms for a multimeter. The pulse–width of that waveform largely determines its high–frequency content. The frequency spectrum of an individual pulse is determined by its Fourier Integral. The frequency spectrum of the pulse train is the Fourier Series that samples along the Fourier Integral at multiples of the input pulse repetition frequency (PRF).

The below figure shows the Fourier Integral of two different pulses: one of broad width (200 μs); the other narrow (6.7 μs). Agilent's 34410A/11A series of DMMs have an effective AC measurement bandwidth of 300 KHz.  If we used one of these DMMs to measure the RMS ACV value of both pulses in the figure the measured value of the broader pulse will be more accurate than the measured value of the narrow pulse since its frequency components outside of the DMM's bandwidth are larger in amplitude.
When making true RMS measurements on non-symmetrical waveforms, accuracy drops as the crest factor and/or the frequency of the waveform increases (for more info on crest factor click here). Here is a list of other tips when making true RMS AC measurements:
1. For maximum accuracy, measure as close to full scale as you can. You might need to override auto scaling in some cases. Be careful with high-crest-factor signals not to overload and saturate the meter’s input circuitry.
2. Be sure to select your DMMs appropriate low-frequency filter to allow for the fundamental to be captured. The lower the filter the longer the measurement will take.
3. You may not want to use the first measured value because many DMMs have a large-value DC-blocking capacitor in the input path. You need to allow this capacitor to charge, especially when you are measuring low-frequency signals or when you are switching between measurement points that have a large DC offset.
4. When you measure AC voltages less than 100 mV, be aware that these measurements are especially susceptible to errors introduced by extraneous noise sources. An exposed test lead will serve as an antenna and the DMM will measure these unwanted signals as well. Reduce the area of the “antenna,” use
good shielding techniques, and make sure the AC source and the DMM are connected to the same electrical outlet to minimize ground loops.
5. AC loading errors: The input impedance of a DMM is often in the region of 10 MΩ in parallel with 100 pF. The cabling you use to connect signals to the multimeter adds additional capacitance and loading. As frequency increases, loading will change. For example, at 1 kHz, the input resistance will now be closer to 850 kΩ, and at 100 kHz it will be closer to 16 kΩ.

Friday, November 19, 2010

Power Supply Performance when using Remote Sense

In test systems the cabling path impedance between power supply and DUT can be significant. Meters of cabling combined with switch relays can equate to large voltage drop during times of high current consumption by the DUT. Its probably no secret to you that the method to compensate for this is to use the power supply's remote sensing capabilities. But what you may not know is the effect that remote sense has on the output performance of a supply. For most test applications this degradation to performance can be ignored. When it  becomes a factor is when a DUT makes large and fast load changes during operation. An example of this would be communication equipment when it goes into a transmit state.
When in remote sense mode the power supply is a feedback control system designed to control a voltage at the DUT. The figure below represents the voltage control system in block diagram form.

"Vprog" represents the desired voltage at the DUT. The summing amplifier compares the desired output voltage with the voltage at the DUT as measured by the sense amplifier. Any difference is amplified and applied as a correction signal to the power control circuit block. This block adjusts the power supply output until the measured output voltage equals the desired output, compensating for any external voltage drop. As shown in the block diagram the load and lead impedance become part of the voltage control system.
This system provides great control of the average DC voltage at the DUT. However, the accuracy to which the instantaneous voltage can be controlled for sharp load changes depends on several factors, namely:
1. The resistance and inductance of the cable connecting the power supply and DUT.
2. The input impedance of the DUT.
3. The amplitude, rise and fall times of the load change or current pulse.
4. The bandwidth of the power supply control loop.
5. The voltage slew rate and transient response capability of the power supply.
The final result is that while the average voltage may be ideal, the instantaneous value may be
less than ideal. If the instantaneous voltage drops too low it can cause your DUT to go into a reset mode which ruins your test. There are three common ways to compensate for this: minimize path impedance, add filtering to the DUT, and use high performance supplies.
To lower path impedance (both R and L) shorten wire lengths when possible, use larger gauge wire, make good connections, use twisted pair, pay attention to contact resistance, and try to avoid hot switching. A second solution is to place a large electrolytic capacitor across the terminals of the DUT right at the test fixture.When the load goes through a sharp change the capacitor will charge or discharge to maintain a constant voltage giving the power supply time to catch up. This method does have drawbacks though such as longer settling times when a voltage change is made and distorted current measurements during sharp load transients due to the charging and discharging of the cap. Finally the best way is to just buy high performance power supplies with high control loop bandwidth and fast transient response (to see a post on the transient response spec click here). Look for transient response specs < 100 us.

Click here for more info on Agilent N6700 series of high performance power supplies

Monday, November 15, 2010

Creating a Chirped RF Pulse with an Analog Signal Generator and 2 Channel Function Generator

Last week I needed to simulate an chirped RF pulse signal for some testing I was doing. Chirped pulse signals are often used in radar and electronic warfare applications as a pulse compression technique. Not having created a signal like this before I sent a quick email to an RF/Microwave AE colleague of mine asking how they did it. They sent me a quick reply back saying I should use Agilent's VSA software and a vector signal generator. The problem is I did not have a license for the VSA software and I did not have a vector signal generator (vector sig gens can easily be double the price of an analog). With limited time to get the test done I had to figure out a way to do it with my N5183A MXG analog signal generator. Now a chirped RF pulse is a pulse modulated RF carrier, but the carrier is swept (for more info check out this Wikipedia page Chirp). My MXG did have pulse modulation and FM (to sweep the carrier) built-in. I used these functions to create a chirped RF pulse signal. The problem with my chirped RF pulse signal was there was no easy way to sync the pulse modulation and FM so from pulse to pulse the sweep was always starting at a different value. To fix that problem I used the 33522A 2-channel function / arbitrary generator that has the ability to sync its outputs, meaning I could control the phase relationship between the channel 1 waveform and the channel 2 waveform. I set the MXG's pulse modulation source and FM source for external. Channel 1 of the 33522A served as the pulse modulation source. I wanted to linearly sweep the carrier so I set channel 2 of the 33522A, my FM source, for a linear ramp waveform. You can see my setup in the image below:

A positive sloped ramp gives you a low to high frequency sweep and a negative sloped ramp gives you high to low frequency sweep. The sweep speed of the chirp is set by the rate of the slope. The frequency range of the sweep is set by the FM deviation setting on the MXG. You can see the function generator outputs on the scope screen in the image. The start of the pulse modulation on the MXG (bottom left) is controlled by the rising edge of the pulse waveform from channel 1 of the 33522A. If you notice in the image, the ramp waveform is already approximately halfway through its ramp up cycle by the time the pulse is triggered. This was done intentionally in fact you can control where the carrier sweep starts and stops inside the pulse by adjust the phase of the ramp in relation to the pulse output. On the bottom right, the N9020A MXA signal analyzer shows the resulting chirped RF pulse in the frequency domain. Below the 33522A in the image, the 53230A universal counter's microwave channel is also being used to measure the pulse characteristics including: pulse width, pulse repetition rate, and carrier frequency.
The analog signal generator and 2-channel function generator method for creating chirped RF pulse signals filled my testing needs and was much lower cost then turning to a vector signal generator.

Wednesday, November 10, 2010

Solar Array Simulation

When designing a device with max power point tracking (MPPT) capability such as a solar inverter, micro inverter, or power optimizer you are going to need to simulate the output of a PV panel or panels to test your MPPT design.  This is necessary to verify your design and provide an accurate efficiency spec under a wide variety of weather patterns (I-V curves). Now a lot of engineer new to this type of testing decide to take a standard programmable power supply, connect it remotely to a computer (using GPIB, LAN, USB, etc), and create software with adjustable I-V curve look-up tables with the idea of turning their power supply into a PV panel simulator. This whole concept is shown in the figure (click to enlarge).

You can also find power supply companies out there that sell this exact solution. I am here to save you time, frustration, and money by telling you this method will not work or at least it won't work how you think it will work. With this type PV simulator system the I-V curve 3 dB bandwidth will be < 1 Hz. Most MPPT designs are making load changes and measurements much faster than once per second. Here is a quick breakdown  why the bandwidth to this solution is so poor:
  1. The IO latency between the computer and the power supply.
  2. The supply programming time which consists of the time it takes the supply to process a command and the time for the internal analog circuitry to move the output of the supply to where it should be on the curve.
  3. The most limiting factor is that this is a closed loop system, which leads to oscillations in the output. Now to get rid of oscillations digital filtering will need to be added to the software which leads to multiple iterations of back and forth adjustments between the computer and supply to zero in on that point on the I-V curve where the supply output should be.
Because of the overhead just discussed you cannot achieve output bandwidths better than 1 Hz using this solution to simulate the output of a PV panel. For this reason it is not an acceptable test method to verify your MPPT design or spec the efficiency of your MPPT design. Now does this PV simulation method have a place in the MPPT hardware test cycle? Because of its relative low cost it could be used in long term reliability testing where you are just interested in continually feeding power through your design over a long period of time to make sure it doesn't break down.
So what is out there to simulate the output of a PV panel? There are two main ways to do it: take a power supply and put some custom analog circuitry around it or purchase a solar array simulator (SAS). I have heard of many different approaches using custom analog circuitry, below is a link to a paper that presents one way to do it:

Of course building a solution yourself comes with a high overhead of simulation, layout, testing, and support. If you want a finished solution you could purchase an SAS. An SAS is not a standard power supply. It is more comparable to a high powered current source with a low output capacitance (< 100 nF) to give it a high output bandwidth. Of course with these more advanced capabilities it comes with a higher price tag than a standard power supply. There are not too many companies out there that make SASs. Agilent is one of the few and below is a link to Agilent's E4360A SAS:

Thursday, November 4, 2010

Create an Electronic Message Board with an Arb Generator and a Scope

As a fun project I recently created a Matlab program that allows you to use a 2 channel arb and a scope in XY mode as a electronic message board. Below is a link to download the Matlab files if you would like to try it out. It is a great way to display away messages when you are not at your desk / lab station or to show school pride or to brag about your favorite team’s performance. The included figure (click to enlarge) shows my school pride displayed on a scope using an electronic message created with the XY_Text_Writer program. The program works by inputting a message (up to 26 characters including spaces). It takes the message and generates XY amplitudes corresponding to points in vectors. The vectors are moved around and mathematically manipulated to form each character. Using those amplitude vectors it generates an X and Y waveform. The program uses the LAN connection to download and output the waveforms from the 33522A. With the scope in XY mode the two waveforms coming out of channel 1 and 2 of the 33522A are converted to electronic messages on the scope display. 
In the "XY_Text_Writer" download zip file you will find all the Matlab files that you need plus a user guide that explains how to setup the equipment and run the program in Matlab. Below is the link to download:

Click here to download from Matlab Central File Exchange

Now if you do have a 2 channel arb but it is not the 335220A 2 channel arb you can possibly still use the program with a few modifications. Instead of sending the arb points to the 33522A you would send them to a .csv file that you could upload to your arb generator, here is how:

  1. In the "XY_Text_Writer.m" file comment out the  "vec2arbstring()" functions and the "arbs_2_33522A()" function.
  2. put the 'x' and 'y' vectors into a single 2D matrix like so z=[x;y]
  3. Then use the following function to generate a .csv in your current directory: dlmwrite(csv_name, z', 'coffset', 0, 'roffset', 0). Notice the 'z' matrix is transposed.
  4. From there you just have to load the waveform from the .csv file to your two channel arb. Set the output Z of the arb to match the input Z of the scope. Start the output amplitude at about 5Vpp for each arb channel and adjust as needed. Set the arb to run at a sample rate of 300 to 400 KSamples/s.

Friday, October 29, 2010

Graphing on a Universal Counter

I am on the east coast of the US this week visiting engineers who are designing some pretty cool stuff so this is just going to be a quick post. Back on Oct 17th I posted about Agilent's new 53200A series of universal counter / timers. Here I am going to talk about their graphing capabilities. In the past all a universal counter display gave you was a constantly changing long string of digits. Looking at this constantly changing long string of digits you could do a quick calculation in your head to figure out how far off you were from some reference value. Things that you probably could not calculate from the long string of constantly changing digits was how much random error is on my signal, is there multiple sources of random error, and is my systematic error changing with time. The histogram and trend chart capabilities found on the 53200 series of universal counters can give you information like that and more with a quick glance Below are two links to Youtube videos that provide an overview of the 53200 series histogram and trend chart capabilities. The actor that provides the overview in the video is also the designer of the features, enjoy.

Friday, October 22, 2010

Industry Leading Single Shot Resolution Specification

In my last post on Oct 17th I introduced Agilent's new 53200A family of Universal Counter / Timers. In the post I gave a general overview of various features and specs that place the 53200A series as the top universal counters on the market. In this post I am going to go in more depth on the 20 ps single shot resolution (SSR) spec for the 53230A. What SSR resolution represents is how well the counter can resolve an event in time where an event is a threshold on an edge. 20 ps SSR is an industry leading timing spec. Any counter measurement consists of at least two events (except maybe totalizing). To calculate the SSR of two events measurement we use the root sum of squares (RSS) so for the 53230A the SSR for a two edge measurement would be: 
Keep in mind this is the resolution for a single two event measurement, we can achieve even better resolution by averaging multiple measurements together to eliminate random noise. Of course this is at the cost of decreased measurement speed. Now SSR resolution is most often associated with time interval measurements, but every counter measurement basically comes down to timing so the better the SSR of a counter the more digits of resolution you get in a frequency measurement.
I am going to give a quick demo that calculates the SSR of the 53230A prototype sitting at my desk. The setup I use for the demo consists of the 53230A universal counter, 33522A function generator, two BNC cables, and a BNC tee. A continuous squarewave is first fed to channel 1 of the counter and then it passes through the other BNC cable to channel 2, as shown in the figure (sorry for the pic quality it was taken with my phone). Since the counter is measuring the same event (rising edge of squarewave) out of the function generator on both channels we can ignore the jitter on the signal from the function generator. Now we are not interested in the actual time interval measurement of the counter because we don't know the electrical length of the BNC cable between channels 1 and 2. What we are interested in is the standard deviation of the time interval measurement we get using the counter's statistics capability. As shown in the screen shot, we get a standard deviation of 15 ps (circled in red). If we assume all of the time interval measurements are within 3 standard deviations, then the max resolution we are seeing in this two event measurement is about 22.5 ps. Now to get the SSR of the 53230A at my desk we have to use RSS backwards on 22.5 ps. The answer is approximately 16 ps, which means the 53230A at my desk is well within the industry leading SSR spec of 20 ps!

Sunday, October 17, 2010

The Next Generation of Universal Counters

Big announce for the world of GPETE, today Agilent releases the 53200A Series RF / Universal Frequency Counter / Timers!  This family is truly the next generation of universal counters or another way to say it is these are not your parent’s universal counters. I am sure you are thinking “what makes these the next generation of universal counters?” Two reasons, the advanced measurement capability contained inside these marvels and the user interface that makes them easy and fun to use. The 53200A series consists of three models: 53210A, 53220A, and the 53230A. Here is a breakdown of key features:
  •     Up to 12 digits/sec single-shot frequency resolution on a one second gate time
  •     Single-shot time interval measurements can be resolved down to 20 psec
  •     Built-in analysis and graphing capabilities that can be shown on the front panel display
  •     Gapless sampling up to 1 MSamples/s (53230A only)
  •     350 MHz baseband frequency, 6- or 15-GHz optional microwave channels
  •     Optional pulsed RF/microwave measurement capability (53230A only)

The three advance measurement features that make these counters stand out from any other universal counters that are available today are the 20 psec single shot resolution (SSR), gapless sampling up to 1 MS/s, and the pulsed RF/microwave measurement capability. The 20 psec SSR is an industry leading timing spec. Working for Agilent I had the privilege to start testing these marvels out months ago and I can tell you that the typical SSR is about 10 psec (Agilent is always conservative on the specs). Keep in mind light only travels 3 mm in 10 psec! Now SSR resolution is most often associated with time interval measurements, but every counter measurement basically comes down to timing so the better the SSR of a counter the more digits of resolution you get in frequency and any other measurement. Gapless sampling means there is no dead time or re-arm time between gate times, basically there is no gate time. This allows the 53230A to make true Allan deviation measurements. Gapless sampling also gives the user the ability to pull the gapless time stamp measurements from the instrument’s memory and perform modulation domain analysis (MDA). Finally the 53230A has optional pulsed RF/microwave measurement capabilities for measuring pulse width, pulse repetition rate, pulse repetition interval, and carrier frequency. This capability is invaluable for radar and electronic warfare applications.
The 53200A series has a large LCD color display and a user interface similar to that of a scope with a hard-key / soft-key layout. The large display increases the usefulness of the universal counter by displaying more data in more intuitive ways instead of just the traditional long string of numbers. As an example see the histogram and trend chart screen captures from the 53200A series. The large display also makes it easy and quick to navigate through menus for a more user friendly experience when accessing some of the more advanced features of a universal counter. That is all for now but you can count on seeing more posts pertaining to this new counter family in the near future. For more information on the 53200A series check out the link below.

Tuesday, October 12, 2010

2010 Solar Power International

Hey all just wanted to let you know that I am in LA for SPI and if you are here too please come by Agilent's booth (5545) to say hi. I will be showing off our new SMU the N6784A. I will also be giving a presentation entitled "Maximize Efficiency by Properly Testing Your MPPT Algorithms and Hardware." 

Thursday, October 7, 2010

Low-Cost Photovoltaic I-V Curve Measurement System

I have noticed that my posts on photovoltaic test are getting lots of hits so here is another one. Here I am going to present a $3k photovoltaic I-V curve measurement system. The measurement system can be seen in the figure below (click on it to enlarge it).

Here is how it works:

  •  The op amp, FET, and Rsense act like a poor mans programmable electronic load with the PV panel connected across it. The op amp will drive the FET (lower its resistance) until the voltage at the op amps negative input equals the voltage at its positive input. If we can control the voltage at the positive input then we can control the current flowing out of the PV panel into the FET and Rsense just like an eload in consant current mode
  • Setting and stepping the voltage at the positive input of the op amp is done by the 34972A DAQ switch unit. It provides a DAC output from 0 to 15 V. When the DAC is set to 0V the FET is in an open condition and the PV panel is at Voc.
  • Rsense is 100 mOhm precision shunt. If we set the 34972A's DAC output to .5 V the op amp will drive the FET until the voltage drop across Rsense is .5 V. Since Rsense is 100 mOhm we know that 5 A of current is flowing out of the PV panel through Rsense (Ohm's law: .5 V/.1 Ohm = 5 A).
  • The 34972A's built-in 61/2 digit DMM combined with plugin MUX switch cards allow us to measure voltage, current, temperature, and more on a large number of channels. In the figure we use one channel to measure the PV panel's voltage, another channel to measure the panel's current (voltage measurement across Rsense), and two other channels to measure temperature. 
  • Putting it all together the way we get the I-V curve is by stepping the DAC's voltage up from 0 V (panel at Voc) until we reach Isc. At each step we measure the panel's output voltage and current to get our I-V curve. 
  • The way we know we have reached Isc is when further voltage steps from the DAC do not result in the voltage across Rsense increasing. At this point the FET is essentially a short.
This photovoltaic I-V curve measurement system will not work well with low voltage low power PV cells since Rsense and the FET cannot actually become a true short. It is better suited for PV modules and panels. Although the hardware is low cost you do need to wrap some type of software around it to control the DAC and gather the measurements (unless you want to do it manually). The value of Rsense and its power handling capability should be chosen based on your PV device's power range and your measurement accuracy needs. Below is a list of parts I used and their approximate cost. This was just a brief overview of the solution if you need more info just comment or email me.

Part description
Model/part number
Approximate price
Product Web site
DAQ switch measure unit
20-channel MUX module
Multifunction module
10-V power supply
2 x $120
Operational amplifier
0.1-ohm shunt resistor
PCB prototyping board
Click here for more information on the 34972A DAQ switch unit

Monday, October 4, 2010

Agilent Introduces 4-Quadrant General Purpose SMU

Today Agilent introduced a 4-Quadrant General Purpose SMU known as the N6784A. The N6784A is a plug-in module for Agilent's popular N6705B DC Power Analyzer and N6700B Modular Power System. The N6705B mainframe is optimized for bench-top use and the N6700B mainframe is optimized for system use. It is the first 4-quadrant module for these platforms. Features and capabilities include:

  • Two output ranges: +/-20V +/-1A or +/-6V +/-3A
  • Glitch-free operation – change sourcing ranges or measurement ranges without any glitches
  • Four current programming ranges – precisely source current down to μA
  • Stable operation with capacitive loads up to 150 μF
  • High-speed output can slew at 10 V per μs into a resistive load
  • Fast modulation of DC output – create arbitrary waveforms up to 100 kHz (sine) into a resistive load
  • High-speed digitized measurements – capture/view the power consumption of the DUT up to every 5 μs with built-in 200 kHz digitizer
I am a big fan of its emulation modes which improve usability by instantly configuring the SMU for the most common use cases. When one of the emulation modes is selected, the SMU optimizes all of its features and settings for that particular use case. Emulation modes:

  • 4-quadrant power supply
  • 2-quadrant power supply
  • Unipolar power supply (i.e. 1-quadrant)
  • CC load
  • CV load
  • Voltage measure (i.e. voltmeter mode)
  • Current measure (i.e. ammeter mode)

Thursday, September 30, 2010

Two Big Product Announcements in GPETE

Two big GPETE related product releases this week that I need to cover. First, the #3 volume scope provider LeCroy takes the scope bandwidth lead with the Wavemaster 8Zi-A which offers 45 GHz of bandwidth on one channel, 30 GHz on two channels, and 20 GHz on four channels. Back on June 14th in my post entitled "The World's Fastest Real-Time Scope!" I talked about how Agilent's Infiniium 90000 X-Series oscilloscope family took the bandwidth lead on scopes at 32 GHz of true analog bandwidth over Tek. It looks like Agilent only held that lead for 4 months with LeCroy's announcement today. LeCroy achieves the 45 GHz bandwidth by interleaving three sampling channels together into a single 120 GSample/s channel. Follow the link for more info: Wavemaster 8Zi-A Oscilloscopes 

The second product release announcement is Tektronix has just released a family of counters (they refer to them as Timer/Counter/Analyzers). These counters have impressive specs including 12 digits of resolution, 50 ps (FCA3100 Series) or 100 ps (FCA3000 Series) Single-shot Time Resolution, and up to 250 KReadings/s of time stamped measurement data to memory. They can do gapless sampling up to 250 KReadkings/s giving them some modulation domain analysis (MDA) capability. The large display on these counters provide the capability to do histograms and trend charts. I am 99% sure that these new Tektronix counters are OEM'd from Pendulum's CNT-91 and CNT-91R counter family simply because the specs, front panel features, and form factor are pretty much the same. Follow the link for more information on the new Tek counters: FCA3100 and FCA3000 Series

Wednesday, September 29, 2010

Danaher Buys Keithley!

Big acquisition news for test and measurement and GPETE, Danaher buys Keithley Instruments for 341 million! For those of you who do not know who Danaher Corporation is, they have a $26.5 billion market cap and they provide a wide breath of products in the following industries: Professional Instrumentation, Medical Technologies, Industrial Technologies and Tools & Components. The last few years Danaher has been an acquisition machine in the test and measurement world gobbling up well known names such as Tektronix and Fluke. For more information on Danaher click here.

Keithley is a well known name in both test and measurement and GPETE. Some of Keithley's product lines include DMMs, current sources, high speed power supplies, and switch products. For more information on Keithley click here. Danaher has reported that they plan to add Keithley to their Tektronix business. It will be interesting to see what Danaher does with Keithley product lines that overlap with their current product lines, such as 6 1/2 digit bench top DMMs. If I were to speculate I would say in the coming years we will see one of three things happen to Keithley product lines:

  1. Product lines that Keithley is well known for such as SMUs and Parametric test equipment will live on with the Keithley name.
  2. Switching and GPETE product lines that Danaher does not have will probably take on the Tektronix name since it seems Danaher is really trying to extend the Tek name beyond scopes and arbs.
  3. Product lines that Danaher already has and that Keithley OEMs will go away.
Of course all that is pure speculation on my part. Feel free to comment with your thoughts. Below is a link to an LA Times article on the acquisition.

Saturday, September 25, 2010

Innovative Way to Interface a Measurement Instrument with a Computer

In this post I am taking a turn from the usual. Typically when I praise a new product it is an Agilent product. Here I am going to lay some praise on an innovative new NI product, USB-TC01 Thermocouple Measurement Device. The innovation in this device is not the measurement technology but the way it interfaces with a computer allowing you to easily retrieve data. Typically interfacing a measurement instrument with a computer and retrieving measurement data from it involves either installing software (that you may have to pay for) or installing drivers and creating your own software. The clever USB_TC01 requires no drivers and no software! The way it works is by connecting to your computer via USB. To Windows the TC01 appears as a mounted disk drive (so no driver). By navigating to the now mounted TC01 “drive” you can launch software that is stored on the TC01. The software allows you to display readings, log readings, and download readings. It also has extras that make it easy to interface with LabView. Pretty cool functionality for a mere $129 price tag.
The technology is similar to the LXI instrument connection standard (to learn more about LXI click here). An LXI compliant instrument acts like a web server so you can connect it to a computer via LAN. The difference between LXI and the TC01 is LXI does require software (web browser which everyone already has), you need to know the instrument’s IP address or host name, and the LXI standard only requires instrument manufacturers to put network setting control in the web interface and not instrument control. Although most LXI instrument manufacturers, like Agilent, allow you to control and retrieve data from the instrument via the web interface.

Monday, September 20, 2010

Sequencing Multiple Power Supply Outputs

Today’s high performance power supplies continue to add more and more capabilities to make the test engineer’s job easier. The capability that I am going to talk about here is sequencing on or off multiple power outputs. This capability allows you to set the order and timing that each power supply output powers on or off. This is useful for designing and testing embedded system designs. Embedded systems can be made up of any combination of microcontrollers, FPGAs, ASICs and memory chips. These individual integrated circuits often have multiple power input requirements that must be properly sequenced on and off to prevent latch-up. Latch-up may cause a wasteful initial surge of current at turn on, or it may be severe enough to inflict permanent damage to the semiconductor device. Ultimately these devices will have a power distribution system with regulators that will ensure the proper sequencing and timing for each power supply turn on and turn off, but during initial design and testing the power distribution system is often not in place yet so test and measurement equipment is used in its place to simulate the proper turn on and off conditions of the design.
In the past power sequencing was typically done in one of two ways: using programmable power supplies with software or using supplies with custom switches. The first way uses programmable supplies and then in software the supplies are properly sequenced on or off. The drawback to this method was unless you were running a real time operating system (windows is not a real time operating system) there was no way to accurately guarantee the timing from one supply output to the next with better than 30 ms of precision. The other method required additional hardware in the form of switch cards and control circuits. A hardware timed control circuit would provide the precision timing that a computer operating system could not. The control circuit would then control the sequencing of the supply output on or off using switches. If you wanted to avoid switch bounce you had to use solid state or mercury switches versus traditional mechanical switches. The problem with this method is complexity it adds to the testing process.
With the capability built into the supply you avoid the complexity of dealing with multiple pieces of hardware and since the timing is done inside the hardware of the supply it is highly accurate. Agilent’s N6700B series of supplies and the N6705B DC Power Analyzer are examples of supplies with built-in power output on / off sequencing. Each one has up to four power supply outputs per mainframe that can be sequenced on or off. Sequencing from one mainframe to another is also possible for applications that require sequencing of more than four power supply outputs. The figure shows a picture of the N6705B DC Power Analyzer being used to test an embedded design that requires turn on power sequencing.

Monday, September 13, 2010

Largest Modular Instrument Introduction Ever

Today at Autotestcon Agilent is launching the largest modular instrument introduction in test and measurement history! Agilent is introducing 47 products in both PXI and AXIe (more on AXIe form factor click here), with the bulk being in PXI. The products range from RF switches to pulse pattern generators to DMMs. If we rewind back a couple decades, Agilent was the pioneer in modular instrumentation with their extensive VXI portfolio. While still staying in the modular instrumentation arena, over the last decade they focused more on high accuracy ‘box’ instruments. With this introduction it is clear that Agilent plans on playing a bigger role in the modular instrumentation industry. You can see a list of Agilent’s modular instrument introductions in the top figure (click on it to enlarge).
Since this is a GPETE blog I wanted to fill you in on the details on Agilent’s new PXI DMM family. The family consists of the M9182A (shown in lower figure) and the M9183A. Both are 6.5 digit DMMs, the M9183A provides higher speed performance and added features. Here is a quick rundown on specs and features:

  •      6.5 digit resolution
  •      4.5 digits at 4,500 rdg/s (82) and 20,000 rdg/s (83)
  •     Measurement capability: dcV/I, acV/I, 2/4-wire Ω, Freq, °C/F, and capacitance (83 only)
  •     DCV basic 1 year accuracy: 30 ppm
  •     Compatibility: cPCI, PXI-H, PXI-1
Surprisingly there are not really many PXI DMMs out there. National Instruments is by far the leader in this space with about 90% of the PXI DMM market so Agilent has an uphill battle. If you’re a consumer of PXI DMMS this is good news! Fierce competition means lower prices, more promotions, and more innovation for you as the end user. For more info on Agilent’s new PXI DMMs see the links below.