Friday, July 23, 2010

Measuring Propagation Delay with a Universal Counter

A universal counter’s precision timing makes it a great low cost solution for measuring the propagation delay of high speed digital signals through cables, solder runs, or digital logic circuits. Propagation delay measurements are critical for designs that have tight timing specifications. You can measure propagation delay using a counter’s time interval measurement feature. A time interval measurement is an elapsed time measurement between a start event and a stop event. It is comparable to using a stopwatch to time a runner, where the start event is the signal telling the runner to go and the stop event is the runner crossing the finish line. For a counter making a time interval measurement, the start event is an electrical edge and the stop event is an electrical edge occurring later in time.
Why not just use a 2-channel scope that gives a nice visual picture of the edges to determine propagation delay? The reason is simple, for timing measurements a counter is much higher in accuracy and resolution. A typical scope only give you about 5 to 9 digits of useful resolution (unless you are buying a +$80k high end scope) where a counter can easily deliver 10 to 12 digits of useful resolution. For instance Agilent’s 53131A and 53132A universal counters provide single shot measurement resolution down to 750 ps and 350 ps respectively. When I say “single shot resolution” I mean a measurement that only consists of a single electrical edge event. Things get a lot better when we can measure the propagation delay of a medium, like a cable, with a continuous signal. When we do a time interval measurement with a continuous signal we can use averaging to eliminate the affects of random noise. If we do a lot of averaging (done inside the counter) we can resolve down to the noise floor, for the 53132A universal counter that is about 10 ps. Light only travels about 3.3 cm in 10 ps!

Wednesday, July 21, 2010

Photovoltaic I-V Curve Characterization using a DC Electronic Load

This is a follow-up to my post on 6/11/10 that covered performing MPPT using a DC electronic load (eload). MPPT with an eload is done for design verification and durability testing of Photovoltaic (PV) devices like PV panels and concentrated PV. Besides just performing MPPT related testing, eloads make great solutions for I-V curve characterization for manufacturing and R&D test. They are great for testing high current PV devices like large area cells, modules, panels, and concentrated PV since eloads can sink and measure high current for a low cost. Below is a link to a video on youtube starring yours truly that covers using an eload for characterizing I-V curves.

Video link: Testing High Power Solar Cells and Modules Using Agilent's Electronic Load

Friday, July 16, 2010

Calculating DMM Accuracy and Resolution

The figure below shows the accuracy specifications for Agilent’s 34410A 6 ½ digit DMM from the data sheet. Let’s assume we are using the 34410A in the 10 V range, it was calibrated within the last 24 hrs, and we are measuring an exact 5 VDC source. We would use the parameters circled in red in the figure and the following formula to calculate the accuracy and useful resolution of our measurement: +/- (% of reading + % of range). For our hypothetical measurement the 34410A would measure: +/- ((5.0 x .000015) + (10.0 x .000004))--> 5.000115 V to 4.999885 V. Since the last digit does not give use any useful information the 34410A provides 6 ½ digits of resolution and the display would read between 5.00012 V and 4.99989 V.

Now that was pretty easy right? One important variable we need to add to the mix is measurement sample time. Sample time is how long the DMM’s internal ADC samples the voltage before integrating the measurement and displaying the result. In the accuracy calculation that we just did, it was assumed we were using the max sample time of 10 PLCs. PLC stands for power line cycle. In the US we use 60 Hz for AC power so 1 PLC is 16.667 ms (1/60). In other parts of the world 50 Hz is sometimes used so 1 PLC would be 20 ms. The DMM’s sample time units are typically based on PLCs. The more PLCs you sample over the better the accuracy you get (To a point). Of course longer sample times equal less throughput so as always there is a tradeoff. Let’s take the same measurement scenario using the 34410A we used above but this time we want to be able to make 10,000 measurements in 1 second. To do this we must set the DMM’s sample rate to 0.006 PLCs. Since our measurement sample time is so much lower we add another uncertainty part to our accuracy calculation. This is due to power line noise and our integration time is smaller so less random noise is being cancelled out. For the 34410A we add the RMS Noise Adder shown in the figure below circled in red.

Accuracy at the 10 V range for a 5 V measurement at 10,000 meas/s: ((5.0 x .000015) + (10.0 x .000004) + (10.0 x .000012))--> 5.00024 V to 4.99976 V. Once again the last digit in this calculation provides no information so there are only 5 digits of useful resolution. The 34410A’s display would read between 5.0002 V and 4.9998 V.
Be careful when purchasing a DMM that is advertised as having high resolution at high measurement speeds. You want to look at its specs and calculate its accuracy at that speed to ensure the advertised resolution is actually “useful” resolution if it isn’t your just paying for extra digits of random noise.
For more information on understanding DMM accuracy and resolution click here

Friday, July 9, 2010

Easy Instrument Connectivity with Matlab

Matlab is my favorite tool for creating complex arbitrary waveforms since it has a wealth of built-in mathematical capabilities and makes it easy to create and manipulate long floating point matrices. But often when I suggest matlab to colleagues as a tool for creating arbitrary waveforms for test instrumentation they either don't know matlab can connect to instruments or they think they have to sit down and learn how to use some complex driver. This could not be farther from the truth! Matlab makes it easy to connect to instrumentation by providing a graphical user interface instrument control toolbox. Here is a quick summary on how to access and use matlab's instrument control toolbox.
1. Go to matlab's 'Start' menu, select 'Toolboxes', select 'Instrument Control', and 'Test and Measurement Tool' as shown in the top figure.
2. Follow the instructions to establish a connection with the instrument as shown in the middle figure.
3. Go to the 'Session Log' tab and copy the needed connection code from the window and paste it in your script or function.
Now isn't that easy?
For any Matlab experts out there, when I try to build strings that are 1.2 MB long Matlab lab seems to lock up on me (I give it a while to run). Is there a special way to handle strings that long or is that out of matlab's capabilities (which I doubt)?

For more information on Matlab's instrument control toolbox click here

Wednesday, July 7, 2010

Rohde & Schwartz Jump into the Scope Market

At the end of June Rohde & Schwartz announced they are entering the oscilloscope market by introducing two new families of scopes: RTO family and RTM family. The RTO family is for high speed applications and provides bandwidths of 1 and 2 GHz. The RTM is more for the general electronics market providing bandwidths up to 500 MHz.
In case you don't know R&S is a German based company who makes mainly RF/Microwave test and measurement equipment. They are a big player in the wireless and aerospace defense test market.
The scope business is a highly competitive market with Tek out in front, Agilent in second, and LeCroy a distant third. Good luck!

For the official press release click here

Saturday, July 3, 2010

Repeatability Specification in RF / Microwave Switches

High quality RF/microwave switches often have a “repeatability” specification. Repeatability is typically given in dB and is often confused with the insertion loss specification (also given in dB). Insertion loss is a well known spec and can be defined as the loss of signal power due to the “insertion” of the switch into the signal path. Repeatability specification is a little trickier to understand, it can be defined as the variation in insertion loss from one switch closure to the next. You may wonder why does the loss through the switch change at all since it is just two high quality flat conductors making contact? Remember at RF/microwave frequencies we are dealing with wavelengths that about the size of the signal path or shorter. This means the impedance that the signal path presents to the signal is affected by small changes in the signal paths dimensions. When the switch is cycled it never closes exactly the same way it did on the previous closure which results in a slight variation in impedance. This variation in impedance from one closure to the next is represented by the repeatability spec. The repeatability spec is only a small fraction of the switches overall insertion loss, for instance Agilent RF/microwave switches have a repeatability spec of 0.03 dB.

So why do we care about the repeatability spec? The reason we care about the repeatability of a switch is because it is a random error not a systematic error (like insertion loss) and therefore cannot be calibrated out or compensated for in measurements. If you have an RF / microwave signal path that you characterize with a network analyzer and that same path has a switch in it once you cycle that switch the insertion loss or S-parameters you measured with the network analyzer has now changed. So switches add measurement uncertainty to your characterized or calibrated signal paths and that is why it is critical to keep the number of switches in a signal path as low as possible.

Let’s say we have an RF / microwave signal path that we have characterized with two switches in it and these switches have a repeatability spec of 0.03 dB. Does that mean we would spec the signal path with a 0.06 dB uncertainty? The answer is no and the reason is because our repeatability error is random so it takes on a Gaussian distribution like the one shown in the figure.

Since repeatability error takes on this type of distribution we can use the root sum or squares (RSS) method to calculate the measurement uncertainty of the signal path. RSS is calculated using the following formula: sqrt(x1^2+x2^2+....+xn^2). Using RSS means the measurement uncertainty of our signal path with two switches in it is 0.0424 dB instead of 0.06 dB. RSS works because the repeatability is typically calculated out from the Gaussian distribution by 3 sigmas or standard deviations (see figure) so the probability that both switches are at .03 dB deviation (both on the same side of the curve) from the calibrated value is very unlikely (about 1 in 274,000).

For more info on calculating measurement uncertainty check out the NIST page on it here

Now if you’re making high precision measurements in applications where phase accuracy is critical, such as radar test, you may be wondering where the phase repeatability spec is. Unfortunately phase repeatability is not spec’d by RF / microwave switch manufacturers (or at least I have not ever seen it spec’d). This leaves you with two choices if you need to establish phase repeatability for your switches: contact the manufacturer to see if they have data on phase repeatability or calculate it yourself using Gaussian distributions and the principals of probability.