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)