When is 64,000 points too many?

Hi everybody!

This month's blog post is based off a customer question that we received this month.  The question was around arbitrary waveforms (arbs), the number of points for the arb, and waveform fidelity.  I have spoken about arbs in the past: click me for Matt's old blog post.  Just to quickly reiterate, there are two options for arbs on the N6705 DC Power Analyzer and the N7900 Advanced power System. There is the Constant Dwell (CD) Arb that allows up to 64,000 point with a minimum Dwell time of 10 us per point and there is the standard List Arb that allows up to 512 points with a dwell as low as 1 us per point.

The question that we are trying to answer today is: When is 512 points more than 64,000 points?  It is an interesting question to think about.  It is definitely not true in cases where you have a non-repeating waveform.  The CD Arb will always be the preferred method there and will give you the best fidelity (smallest dwell times).

The answer is when you have long DC levels in your waveform.  Let's look at the proposed waveform below (please pardon the picture, I hand drew this on my tablet; also note that it is not to scale):

If you look at this waveform, the total time is 11.5 s.  It's a pretty simple waveform that goes from 4 V to 6 V with a 0.05 s ramp between the two values.  We need to pay attention to those times.

Lets with the math behind programming a CD Arb.  With a CD arb, there is a single dwell time so you basically sample the waveform 64000 times.  Lets use that to calculate a dwell time:

11.05 s/64000 = 172.66 us

This means that every point is going to last 172.66 us, no matter if it is in the constantly changing ramp or at a DC level.  This means that when the waveform is at 6 V for 10 s, you will use 57,918 points. That is 90% of your points just sitting at 6V!  For the 0.05 s ramp, you will only be using 290 points.  The ramp is where the waveform is actually changing but due to the nature of how the CD Arb works, you cannot increase the number of points allocated to the ramp.

Let's take a look at the 512 point list now.  We know that the first point of the list will be 4 V for 1 s and that the last point of the list will be 6 V for 10 s.  That leaves us with 510 points to do the 0.05 s ramp which results in s dwell time of 98 us.  This will give us more points in the ramp area and a better looking waveform overall.

That is all I have for this month.  Please feel free to use the comments if you'd like to get in touch with us.

Watt's Up with Datalogging and Digitizing?

All of our power supplies offer the ability to take an average measurement using either the front panel or the MEAS SCPI commands.  Some of our newer power supplies have some more advanced measurement capabilities.   The two capabilities that we are going to look at today are digitized measurements and datalogging.   Let’s take a short look at each one and then talk about when to use each one.

The digitizer has been in our products for a while now.  With the digitizer, you define three parameters and the measurement uses these parameters to return an array of measurements back to you.  The three parameters are: the number of points, the time interval, and the points offset.  The number of points is pretty simple.  It is the number of measurements that you want to take as well as the size of the array that you are going to read back.  The time interval is the pace of the measurements.  This is also the time between the points in the array.  The points offset is a way that you vary the starting point of the array.  This offset can be negative to return measured points before the trigger or positive to delay the start of the measurement.  The most points that we can measure and the fastest time interval is with our N678xA SMU modules.  These modules have a time interval of 5.12 us and a total number of points of 512 Kpoints (keep in mind that 1 Kpoint is 1,024 points).  This yields a total time of 5.12 us x 512 x 1,024 which yields a result of 2.68 seconds.  So the longest measurement that you can make is 2.68 seconds.  The largest time interval that we can measure is 40,000 seconds.  Setting this with the highest number of points would yield 40,000 s x 512 x 1,024 yields a total acquisition of 20,971,520,000 seconds.  That is 666.83 years! 

The other advanced measurement capability that we are going to talk about is our datalogger.  With the datalogger, you set a total acquisition time and an integration time.  The integration time is the amount of time that the power supply will average measurements.  The measurement system is still running at its maximum digitizing rate but it is averaging those measurements and returning that averaged measurement.  The digitizer on the N6705B DC Power Analyzer also will return the maximum measured value and the minimum measured value of each integration period.  The quickest integration time on the N6705B is 20.48 us.  The only limitation in the amount of data that you can log with the internal datalogger is the file size (the maximum file size is somewhere near 2 gB).  If you want to datalog huge files, you can use the external datalog feature (I wrote another blog post about this) or use our 14585A software where the only limitation is the free space on your hard drive.  The catch on the external datalogger is that that the quickest integration time is 102 us.

So when do you use one over the other?  It is pretty simple.  When you want to make a long term measurement (days, weeks, etc.) at a fast rate you should use the datalogger.  You would use this when you are looking to measure something like long term battery drain.  If you are looking for a more short term, faster measurement you would use the digitizer.  You would use the digitizer to measure something like inrush current. 

These are a few of the great features available in our power supplies.  Please let us know if you have any questions on these features or any of the features of our power supplies.          

Arbs! Arbs! Arbs!

Hi everybody,

We have a new intern here and we have recently been talking about the arbitrary waveform capabilities (from now on I will refer to this as arbs)  of our power supplies and I thought that this would make an interesting blog post.  This is a really cool feature that we offer in our products as it give you the ability to create an alternating signal using our DC power supplies.  The two types of arbs are the LIST system and the constant dwell arb.

The LIST arb is a feature that we have in quite a few of our products.  The N6700 family, the N7900 family, and even some of our older power supplies have this feature.  The "Arb" system in the N6705 DC Power Analyzer is similar to the LIST.  These LISTs can contain as many as 512 different points with a timing resolution as low as 1 us.  Each point consists of a voltage or current setting and a time.  The times can be different for each point.  A short example of a programmed LIST is:

VOLT:MODE LIST
LIST:VOLT 10,25,5
LIST:DWEL 5,1,4

In the example above, the voltage will start out at 10 V and stay for 5 seconds, then transition to 25 V for 1 s and then go to 5 V for 4 s.  As you can see there are 3 voltage values with 3 corresponding dwell times.

The second mode for arbs that is only available on the N6705B DC Power Analyzer and the N7900 APS is call the Constant Dwell Arb (CD Arb).  In this mode, you can program as many as 64K points but all of the defined points have the same dwell time.  If we want to do the same waveform as above, we need to choose what will be our dwell time.  Since the smallest dwell I used in my example is 1 s, I will choose that.  Here is what a small part of the code would look like:

VOLT:MODE ARB
ARB:VOLT:CDW:DWEL 1
ARB:VOLT:CDW 10,10,10,10,10,25,5,5,5,5

The code above will produce the same waveform as the LIST example.  CD Arbs can get pretty unwieldy when you have a ton of points but we do offer some tools in our 14585A Control and Analysis software that allow you to import and export csv files to make life a bit easier.

There are advantages and disadvantages to both.  As you can see, in some cases it is easier to program a list since it requires less dwell points and gives you more flexibility with what your dwell can be.  If your waveform has a lot of DC levels in it, then the standard list might work for you.  If you have a long, complex waveform the 64 Kpoints offered in an arb will most likely offer you the best option to replicate your waveform.

Whichever arb you pick, this is a very powerful tool.  I am thinking that I will follow this up at a future date with more information about arbs.  If you have any questions, feel free to leave us some comments.

Use the FETCH Command to Minimize Your Measurement Time

Hi everyone,

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

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

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

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

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

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

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

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

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