Counters

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7/24/12
Free Program for Analyzing Clocks and Oscillators


In this blog post we will look at a free MATLAB program that I created and posted on MATLAB Central for download. The program is called Stability Analyzer 53230A and it provides stability analysis capabilities of clock and oscillator measurements, including Allan and Hadamard Deviation calculations. The "new" program is actually an update to a previous version of Stability Analyzer 53230A that was created over a year ago, but the new version has much more capability. The following is a summary of the program and its capabilities.

Description:
The Stability Analyzer 53230A (2.0) is a free MATLAB program that allows you to analyze the stability of clocks, oscillators, and other signal sources using frequency measurements. The program either inputs / uploads frequency measurements from Agilent’s 53230A universal counter or stored measurements on a CSV file. The program provides the user with a choice of two stability calculations, Allan deviation or Hadamard deviation (both overlapping). The program outputs three plots:
  • Allan or Hadamard deviation plot with optional confidence intervals. 
  • Frequency vs time plot of all measurements 
  • Histogram frequency plot 
All plots provide zoom and pan capabilities as well as the ability to identify a particular plot point. The program is setup and run via a graphical user interface (GUI) and is started from the MATLAB command line. If the 53230A universal counter is used with the program for making measurements then the Instrument Control toolbox is required in your MATLAB package.

Example run:
Below is an example analysis made with the Stability Analyzer 53230A program on a 69.251 MHz oscillator. The gap-free frequency measurements were made with the 53230A universal counter. The measurements were made at a gate time of 10 ms and a total of 5,000 measurements were made for a total measurement time of 50 seconds. The result can be seen below (click to enlarge).


In the Time vs Frequency plot you can see at the measurement gate time (10 ms or 100 Hz) that the dominate noise types seem to be White FM and Flicker FM. From the Overlapping Allan Deviation plot we can see from the slope that from the Tau value of 50 ms to the Tau value of 5 s the oscillator signal is dominated by Random Walk FM noise.

Click here to download the Stability Analyzer 53230A from MATLAB Central

If you are new to stability analysis I recommend the NIST Handbook of Frequency Stability Analysis

Click here for more info on Agilent's 53230A universal counter



5/14/12
Jitter Measurements with a Counter

In this post we will look at using a counter's high timing measurement resolution for making Jitter measurements on a clock signal. Jitter is defined as the deviation of a signal edge transition from its ideal time. The most common instrument used for measuring Jitter is a high performance scope, so that begs the question why use a counter? High performance scopes are very expensive, ranging from tens of thousands of dollars to hundreds of thousands of dollars. The counter offers a much lower cost alternative with measurement resolution specs that match or come close to those found on a high performance scope. There are various ways to quantify Jitter, the three most common are: Period Jitter, Cycle to Cycle Period Jitter, and Time Interval Error (TIE). A high performance scope can quantify all three types. A counter typically can only quantify Period Jitter. For more on Jitter fundamentals and Jitter types click here.

Period Jitter is an RMS calculation of the difference of each period from a waveform average. It can be calculated by making a large amount of period measurements on a signal and calculating the standard deviation of the period measurements. To measure period Jitter on a counter we cannot simple use the "Period" or "Frequency" measurement function, because these are integration measurements that average multiple period or frequency measurements together. We need a single shot period measurements for calculating Period Jitter. To do this we would use the "Time Interval" measurement menu and in the Time Interval menu you will find a measurement called a "Single Period" measurement that is ideal for calculating Period Jitter.

When measuring Period Jitter with a counter the main counter spec that you need to be aware of is its maximum resolution. Resolution can be spec'd in digits or as a time value, which is often referred to as the time interval resolution. The time value resolution is the spec we are interested in for calculating the counter's Jitter measurement floor. This time represents the standard deviation of the random measurement error associated with a time interval measurement. Since the standard deviation in an edge to edge timing measurement is the same thing as RMS Period Jitter, a counter's time interval resolution spec is its jitter measurement floor. Another way to look at it is the counter's time interval resolution is its internal RMS jitter error in its measurement. For example Agilent's 53230A counter has a time interval resolution spec of 20 ps so its Period Jitter measurement floor is 20 ps.

Let's look at a Period Jitter measurement example using the 53230A counter. The 53230A is setup to make single period measurements on a 10 MHz digital clock. Using the statistics function the counter computes the  standard deviation after each successive measurement. As mentioned early the standard deviation calculation of the period measurements is the RMS Jitter of the signal. The below screen shot circled in red shows the measured RMS Jitter of the digital clock. From the statistics we can also see the peak to peak Jitter and we can see the mean value of the period is right around 10 MHz.


Modern counters like the 53230A also provide some basic plotting features, which can be a helpful tool for analyzing Jitter. The below screen shot shows a histogram plot of the measurements made on the digital clock signal. In the histogram plot we can see two separate Gaussian distributions, which means that besides random Jitter we also have deterministic Jitter in the signal. The deterministic Jitter jumps between 10.01 MHz and 9.99 MHz. Even though our mean came out to be about 10 MHz we can see from the histogram that the signal is typically 10 KHz off of the ideal frequency of 10 MHz. 


As a comparison, the same signal was measured with a high performance scope. A screen shot of the measurement using the scope can be seen below. The scope screen shot is zoomed in on the signal's rising edge and the scope's persistence setting is on. To measure the Period Jitter of the signal the histogram feature of the scope is on (histogram shown at bottom of screen shot in light blue). Notice that the scope's measured RMS Jitter value (circled in red) matches that of the counter. Also notice that the histograms match as well.


In this post we looked at measuring and analyzing Jitter with a frequency counter. Because of their high resolution and statistical features, counter's make a good low cost tool for measuring Period Jitter compared compared to a high performance scope. Modern counter's offer basic plotting features, like trend charts and histograms, which allow you to identify Jitter patterns and deterministic Jitter for deeper analysis. If you have any questions on this post feel free to email me. If you have any personal experience or incite to add to this post please use the comments section below.

For more on the 53230A counter click here



11/4/11
How to do an Accuracy Calculation on Agilent's 53200 Series Counters

Calculating and understanding the accuracy of Agilent’s 53200A series of universal counters is not easy and can be a source of frustration. In this post I will walk you through an example frequency accuracy calculation using Agilent’s 53230A universal counter. Each step will include an explanation of the various terms so in the future the reader can perform an accuracy calculation on any of the 53200A series of counters with any input signal. I will also refer to data sheet pages where calculations, specs, and notes can be obtained. Let’s start by setting the initial conditions of the signal we are measuring and the counter we are using.

Signal being measured: 1 MHz sine wave with 1 Vpp amplitude and 1 mV of RMS noise

Counter hardware and settings: 53230A with TCXO timebase set for frequency measurements in ‘Auto’ mode (default) with a 1 s gate time. The input amplitude range is 5 V with a trigger level of 0 V on a positive edge event. The counter was calibrated at the factory upon purchase and it has been ~90 days since the initial 30 day warm-up period (page 15 note 1 of data sheet).


The basic accuracy calculation is as follows (page 16 datasheet):
Accuracy = +/- [(k*Random Uncertainty) + Systematic Uncertainty + Timebase Uncertainty]

For a definition of random, systematic, and timebase uncertainty refer to page 16 of the data sheet.
Let’s start by calculating the random uncertainty (RU) error which is the hardest of the three. The variable ‘k’ is the standard deviation or sigma multiplier for establishing the confidence interval of a Gaussian or normal distribution, RU has a Gaussian distribution. For our calculation we will choose a ‘k’ of 3 which gives us a confidence interval of 99.7%. The equation for calculating the RU is (data sheet page 16):
RU = [1.4*(TSS^2 + TE^2)^1/2] / RE*gate

TSS --> Single-shot time resolution spec found on page 19 of data sheet. For the 53230A this spec is 20 ps.

T--> Threshold error (page 19) is the amplitude noise on the signal being measured that causes error at the trigger level point on the signal edge. The equation to calculate TE for 5 V input range is: (500μV^2 + EN^2 + VX^2)^1/2 / SR
  •  The ‘500μV’ term is amplitude noise added by the counter
  • The ‘EN’ term is the amplitude noise on the signal being measured (page 20). In our initial conditions we said this was 1 mV. EN is sometimes hard to calculate so if you are working with a clean signal it is safe to assume EN is zero.
  • The ‘SR’ is the slew rate of the signal at the counter’s set trigger point. The slew rate or the rate of change at any point on a given waveform is found by taking the derivative of the waveform function and then solving for the time when the set trigger amplitude occurs. Fortunately on page 20 the data sheet provides an SR formula for a sine wave and a square wave. The formulas are the result of solving for the derivative of each waveform and using the point of max SR as the trigger point. Choosing a trigger point on the waveform that has the highest slew rate leads to better measurement accuracy. In our example we have a sine wave which has max SR at the 50% amplitude point. A sine wave with no DC offset will then have its max slew rate at the 0V crossing and that is why we choose 0 V trigger point for our initial conditions. So we can use data sheet calculation for our example: SR = 2*pi*F*V0 to p
    • ‘F’ is the frequency of the signal being measured.
    • ‘V0 to pk’ is the delta amplitude from 0 to peak. For our signal this is 0.5 V (half of 1 Vpp).

RE --> When the 53230A and 53220A are in ‘Recip’ mode they return the average of all the measurements within the set gate time as the measurement result. When they are in ‘Auto’ mode, which is the default mode, they use a proprietary algorithm to achieve better resolution from a set of measurements in a specific gate time compared to just simply averaging the measurements. ‘RE’ stands for Resolution Enhancement. When in ‘Auto’ mode the RE factor can be more than 1 because the resolution enhancement algorithm is being implemented. When using the 53210A, 53220A, and the 53230A in ‘Recip’ or ‘TST’ modes RE = 1.   
Let’s calculate SR, TE, and RE
SR = 2*pi*F*V0 to pk = 2*3.1416*1e6*0.5 = 3.1416e6
T= (500μV^2 + EN^2 + VX^2)^1/2 / SR = ((500e-6^2 + 1e-3^2)^1/2) / 3.1416e6 = 3.5588e-10

The data sheet on page 19 gives clear instructions on what to do for RE when Tss >> TE, we simply use the following equation RE = √(FIN * Gate/16) and check it against the max value RE table shown below:
Gate time >= 1 s, RE max of 6
Gate time 100 ms, RE max of 4
Gate time 10 ms, RE max of 2
Gate time =< 1 ms, RE = 1

If your gate time falls somewhere in between the gate time values in the table just use an RE value that falls between the table values, for instance if your gate time is 600 ms use an RE value of 5. Now here is where things get confusing, since the data sheet does not really provide much guidance on what to do for cases when Tss > TE, Tss < TE, and Tss << TE (which is the case we have for our example). The safe thing to do here is to just use the same procedure that was used for Tss >> TE. Since the resolution enhancement algorithm results in increased resolution the higher Tis, using the above calculation method ensures a safe result every time. Hopefully a future version of the data sheet will provide a better guidance for calculating RE. Let’s calculate the RE value for our example measurement:

 RE = √(FIN * Gate/16) = √(1e6 * 1/16) = 250, since the result of the equation is higher than the table value we use the table value so RE = 6

Now we have the variables we need to determine the random uncertainty in our accuracy calculation.
RU = [1.4*(TSS^2 + TE^2)^1/2] / RE*gate = [1.4*(20e-12^2 + 3.5588e-10^2)^1/2] / 6*1
RU = 8.3170e-011

Notice above, since TSS and TE are random error components we apply the root sum of squares (RSS) method to them. Also since they are random the higher the gate time the lower the random uncertainty is due to more averaging.
From page 16 of the data sheet the SU is determined as follows:
If RE ≥ 2: 10 ps / gate (max), 2 ps / gate (typ)
If RE < 2 or REC mode (RE = 1): 100 ps / gate

We can see SU is based off of the gate time and the calculated RE value. For each range of RE values there are two different ways to calculate SU, ‘max’ or maximum and ‘typ’ or typical. The typical spec is the performance of most of the units tested. The maximum is the warranty spec of the counter. For our calculation we will use the maximum:
SU = 10 ps / gate = 10e-12 / 1 = 10e-12

The last term we need to calculate is the Timebase Uncertainty (TU). On page 15 you will find the TU equation, the various time base specs, and four important notes. Understanding time base specs can be a little tricky. It is important to note that crystal oscillators are as much a mechanical device as they are an electrical device. That is why factors such as temperature, loss of power, and movement have such a dramatic effect on time base uncertainty. The TU calculation is:
TU = ( Aging + Temperature + Calibration Uncertainty )

The following is an overview of each of the three parts that make up the TU calculation:
Aging --> The 53200A series of counters must be on for 30 days after you receive it from the factory before the time base aging specs take effect. This is a settling time that is required by the physics of the crystal oscillator. The aging specs apply from the date of the counter’s last calibration. Since the 53230A for this example has been around 90 days after the initial settling time and we have the TCXO timebase our aging spec is +/- 0.6 ppm. This was calculated by multiplying the 30-day aging spec by 3 (90 days). Use the 30-day aging spec times the number of months since calibration for about 5 months at which time you will want to switch to the 1-year aging spec since it will be the lesser of the two. Notice the second half of note one on page 15, after the first year you use half of the 1-year and 30-day aging specs.

      Temperature --> the first spec “0 °C to 55 °C relative to 25 °C” is used if the counter was operated at temperatures more than +/- 5 °C from the temperature it was calibrated at (25 °C ideally). Since we are not sure what temperatures our example counter experienced during shipping we will use this in our calculation. The TCAL spec can be ignored for our accuracy calculation since it is included in the aging spec.

           Calibration Uncertainty --> This term is only used if the counter still has the factory calibration. It was added because after the counter is shipped from the factory we do not know what kind of handling the counter may experience during shipping. For instance the package may be dropped which can affect the calibration. Once the counter has been calibrated again this term can be ignored. Since in our example the factory was the last place the 53230A was calibrated we will use this spec. The 53200A series of counters should be calibrated onsite for best performance. If you send the 53200A counter off site for calibration you should add the “Initial factory calibration” spec into any accuracy calculation.
      
      We can now calculate the TU:

      TU = (Aging + Temp + CU) = 0.6 ppm + 1 ppm + 0.5 ppm = 6e-7 + 1e-6 + 5e-7 = 2.1 e-6
      
      Finally we now have all the information and data needed to calculate the basic accuracy of our example 1 MHz signal:

Accuracy = +/- [(k*RU) + SU + TU] = (3 * 8.317e-011) + 10e-12 + 2.1e-6 = 2.10025951e-6 or in parts and rounded 2.1003 ppm

Since the error calculation is in parts the result 2.10026 ppm means that if our signal was exactly 1 MHz the counter would output a reading between 1,000,002.10026  and  999,997.899740 as the measured frequency.

Important Accuracy Notes:
Notice in for our example that the timebase error is multiple orders of magnitude higher than the other error components. Even with the OCXO option this will still be true. That means in the future, if you are not using a highly stable external time base reference, you can just use the TU as a very close approximation of the counter’s measurement accuracy.

The accuracy of any of the 53200A counters can be increased by multiple orders of magnitude by using a rubidium or GPS based external frequency standard.

Better measurement accuracy is achieved by calibrating the 53200A series counter onsite after the initial 30 day warm-up / settling period and all future calibrations are carried out onsite.

Triggering at the amplitude point of maximum slew rate on the signal you are measuring provides better measurement accuracy.


8/2/11

Getting Up Close And Personal With Your Local Oscillator

Recently I had an article published that discusses using a counter with gap-free measurement capability to perform signal analysis on local oscillators in communication, radar, and electronic warfare designs. Below you will find the intro to the article and a link to the article in its entirety. The article appears in Electronic Military & Defense.

A local oscillator (LO) is the heart of any modern radar, electronic warfare or communication system. LOs produce carrier signals for our transmitters and reference signals for our receivers. With more and more data being squeezed into small bandwidths and the continual need to resolve targets accurately in crowded areas,  ensuring an LO has high stability and spectral quality is a critical part of modern transmitter and receiver design.

To verify stability and spectral quality, instruments like signal analyzers provide a great wide-area view of the noise and spectral content around the LO carrier signal. Where they fall short is providing a clear picture of the phase noise close to the carrier. Modern frequency counters can pick up where instruments like signal analyzers leave off. This article discusses how the high resolution and gap-free measurement capability found in modern counters can be used to get you closer to your LO signal.








5/7/11

Free Matlab Program for making Allan Deviation Measurements with the 53230A Universal Counter

Recently I created a Matlab program for making Allan deviation measurements using Agilent's 53230A universal counter called "Stability Analyzer 53230A." The program performs true Allan Deviation measurements using the 53230A's gap-free sampling capability. The program provides an "All Tau Analysis" plot of the Allan deviation calculations. The Allan deviation calculations made are based on an array of Tau values you as the user provide. A second plot is also done of the measurement data that features time on the x axis and frequency on the y axis. All of the measurements and Allan deviation calculations can be accessed from the Matlab's command line after the program runs for further analysis. It is a great easy to use program for doing general frequency stability measurements on oscillator, clocks, and amplifiers.

The program uses LAN to connect and control the 53230A. All you need is the 53230A's IP address to connect. The program requires Matlab's Instrument Control Toolbox package to be installed. You can make the measurements on any of the 53230A's channels including channel 3, the optional microwave channel. This give you the ability to make stability measurements all the way up to 15 GHz. The "Stability Analyzer 53230A" program is free to download from Matlab Central (link below). In the downloaded program folder there is a Word document with operating instructions.

Below are two plots from an example run of Stability Analyzer 53230A (double click to enlarge). The example signal used was a 10 MHz signal with 0.1 Hz of frequency modulation added to simulate a cyclic disturbance such as temperature cycling. We can easily deduce the frequency of the cyclic noise in either plot. In the Allan deviation this is clear because we see dips at tau value 10s which is equal to the period of the noise and 20s which is an integer multiple of the noise's period. In the second plot we can see the time domain shape of the noise (sine) and its period.









4/18/11
53131A, 53132A, and 53181A Universal Counter Discontinuance Announcement


Today (April 18th 2011) Agilent is announcing the discontinuance of the popular 53131A, 53132A, and 53181A universal counters. A custom component used in this counter family is no longer available and is forcing this unplanned product discontinuance. The last order date for this counter family is November 1, 2011, subject to availability.

Compatible replacements exist. The new 53200 Series of Counters are functional equivalents to the previous generation counter family with vastly improved performance, features, and usability. They include “53100 Emulation Mode” enabling 53100 Series Standard Commands for Programmable Instruments (SCPI).

The 53200 Series Counters provide more speed, resolution and accuracy at the same price or better. Features of the 53200 series include:
  • 350 MHz standard inputs, with timing resolution down to 20 ps (up to 12 digits of resolution on 1 second gate time)
  • Up to 75,000 frequency measurements per second 
  • Large color graphical display for data logging, trending, histograms and more 
  •  LXI/LAN, USB and GPIB connectivity
  • Gap-free measurements for modulation domain analysis and true Allan Variance / Deviation calculations (53230A only)
  • Optional pulsed RF measurement capability (53230A only)
You can also read the Agilent RF & Universal Frequency Counter/Timers Programming Comparison Guide for a list of those areas where 531xxA Series users might find differences in operation when using a 53200 Series counter. These differences are few, but documented in order to make it easier to verify programs.

When 531xxA Series compatibility mode is selected, all programming is performed through one of the 53200 Series’ remote interface (LAN, USB, GPIB). The counter display responds according to the remote commands received. Pressing any front panel key while in 531xxA Series compatibility mode returns the counter to 53200 Series mode as prompted.


2/27/11

Low Cost Solution for Measuring Frequency Hopping

Frequency hopping is a method of switching or "hopping" a carrier signal among many frequency channels, using a pseudo-random sequence known to both transmitter and receiver. Frequency hopping is more widely known in wireless communication, but it is also used in radar where it is sometimes referred to as an agile signal or agile carrier. In wireless communication frequency hopping is a way to lower signal interference and share a small section of bandwidth. It also serves as a method to protect a signal against ease-droppers. In radar frequency hopping is used to guard against jamming and deception or as a way to reduce signal interference. 
To measure the frequency and modulation content on a comm or radar signal the signal analyzer or similiar instrument is typically the solution of choice. But lets say you wanted to measure or capture a long record of frequency hop data to check long term accuracy or validate an algorithm or verify frequency channel transitions, how would you do it? You could use a high speed digitizer or mixer digitizer combo with a lot of memory. You then would have to post process the digitized data to create an easy to read frequency plot. 
An easier low cost way to do it is with a modern universal counter that has gap-free measurement capabilities. A counter works by making high accuracy and high resolution timing measurements between signal edge events. A counter with gap-free measurement capability can make timing measurements without any rests or gaps in between up to a certain sample rate. For instance the Agilent 53230A counter has a gap-free sample rate of 1 MS/s. What that means is if you have a carrier at 2.4 GHz the 53230A will pre-scale or divide down the signal to fit the sampling rate. You can than use the pre-scale value and the resulting timing measurement to calculate frequency on a continuous bases. Since a counter is just making edge event timing measurements and not digitizing you can make a lot of high resolution frequency measurements without using much memory at all.
As an example, below are two plots of 32,000 frequency measurements made on a frequency hopping carrier signal using the 53230A. The plot has frequency on the Y axis and time on the X axis (click on the figure to enlarge).


The frequency range of the signal is about 2.48 to 2.58 GHz. The top view shows all of the measurements and the bottom plot is a zoomed in picture of about 850 measurement points at one of the carrier hop levels. The bottom plot shows that even though we are capturing a large time slice of frequency hop measurements we still have ample measurement resolution to zoom in and see noise on the carrier. 
The 32,000 measurements made at a 1 MS/s rate only took up about 3% of the 53230A's 1 million measurement reading memory. Imagine the price tag on a digitizing based solution that could capture that much high resolution frequency data! Besides just capturing long periods of frequency hopping data the gap-free measurement capability found in modern universal counters also can be used as a low cost modulation domain analysis and close-in noise analysis tool on carrier signals.





1/11/11
A Low Cost Way to Capture Agile PRFs and PWs in Radar Test


In military radar applications the pulse repetition frequency (PRF) and sometimes pulse width (PW) parameters of a radar's pulsed RF/microwave signal may vary during operation. There is typically two reasons why PRF and PW may be varied:

  1. PRF and PW are tied to a measured target's range and resolution as well as the max range of the radar. A sophisticated radar system that can change modes from searching to tracking or from one search range to another must change its PRF and PW to match the radar's current mode. This is often referred to as mode changes.
  2. Military radars often employ PRF and PW that are constantly changing to help prevent an enemy from "spooking" the radar (creating a false target). These radar's employ complex highly secret algorithms for constantly varying the PRF and PW. This is often referred to as an agile pulse.
In this post I wanted to cover cover a fairly low cost solution for continuously capturing these parameters for verifying an agile PRF/PW algorithm, analyzing pulse noise from one mode to the next, or verifying tolerances from one pulse signal change to another. The solution is made up of a simple RF power detector and one or two gap-free sampling universal counters like the 53230A. The power detector is used to strip off the RF carrier and just output the pulse signal. The gap-free sampling counter or counters are then used to make the needed continuous timing measurements on the pulse signal. Why gap-free sampling counter?
  1. Counters provide high accuracy and high resolution timing measurements.
  2. Gap-free means no pulses will be skipped so you can get a complete and continuous picture of a group of pulses.
  3. Since counters are just making timing measurements with the edge event of a signal, you can capture a large amount of continuous pulse data with less memory and simpler post processing compared to an instrument that is digitizing the entire pulse signal. For instance the 53230A can store up to 1 million readings. There is not many scopes out there that can store 1 million digitized pulses in memory.
  4. Finally counters are low cost compared to RF / high speed digitizing instruments
As I mentioned above you may want to use one or two counters. You only need one if you are just interested in capturing one of the pulse parameters at once. The one counter based solution can be seen below.
The counter in the above figure is set for timestamped measurements which returns the time from positive signal edge to positive signal edge or negative signal edge to negative signal edge. Using the counter for timestamped measurements in this application returns the pulse repetition interval (PRF) from pulse to pulse. The PRF can them be obtained by inverting each reading. Below is an example measurement of an agile PRF signal using the setup above.
The below setup uses two counters to capture both PRF and PW simultaneously. One counter is used in timestamp mode for capturing the PRI and the other is making PW measurements. 
In the above setup, after the pulse parameter measurements are made on the desired set of pulses some simple software can be used to post process and combine the timing measurement data to give you a PRF, PRI, PW, and the duty cycle of each pulse in the set. 
Depending on the gap-free sampling counter you use, it may not measure the first couple of leading pulses from the set. This is because the counter may need to set the right input conditioning and edge leveling before the timing measurements can start. Also if you are using two different measurement modes in the counter, such as timestamps and PW, each mode may skip a different amount of leading pulses which you will need to know when using two counters in parallel to properly align your measurement. Finally, you want to be sure you use a high quality RF power detector for this type of test. Using a low quality detector can lead to carrier noise on the output pulse signal, which can really lower your measurement accuracy. 





1/3/11
Analyzing Close In Noise with a Gap-Free Sampling Counter





I wanted to expand a bit on the MDA capture portion of the video in case it was not clear. Like I mentioned in the video the Y axis was frequency and the X axis was time. 10 MHz is located at the center of the Y axis. The span of the Y axis was approximately 8 Hz. Please add a comment or email me if you have any questions.




10/29/10
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.



10/22/10
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!




10/17/10
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.





9/30/10
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



7/23/10
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!