Some pre-purchase questions re non-standard sampling and memory

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18 Sep 2009
Posts: 1

I am currently looking at purchasing a DAQ unit for a client wishing to record sample waveform at around 150kHz sampling rate. There are however a couple of potential issues, to which I haven’t managed to find the answers in the manual, so here are a couple of questions:

1. It appears to me as if you cannot fill the complete memory buffer with a single triggered sample. The manual on page 27 is somewhat confusing, as it indicates you can fill the complete buffer with a single sample, but reading on it says you can only fill one frame, and it seems you need at least two frames. Can you please clarify.

2. I understand you can only actively vary the sampling frequency by applying and external signal. Could this come from the built in signal generator if you chose that option? Do you sacrifice any other options if you chose this option? How does the signal get put in, and what requirements are there for this signal?

3. In the manual you list an Analogue Output module, which can play an arbitrary waveforms, could this be used to play back a captured waveform? If so, could you also play back a waveform that you have stored in a PC and restored to the scope memory? There doesn’t seem to be any info around for this module.

4. Can you have a unit that has all options of SigGen, Analogue output and External sampler?

5. It is unclear to me what the logging function does. I tried to simulate this with the SigGen Simulator of the Demo Software but I couldn’t in fact determine whether it was supposed to log signal or just trigger events. I was hoping that there was an ability of automatically transferring a block of captured data to the PC on a regular basis but I couldn’t make this work.

6. I tried to capture a full memory buffer, but managed only 200,000 samples, is this a restriction of the demo software?

Thanks for your looking into this.

21 Sep 2009
Posts: 401

Hello Stefan,
Answers are arrowed below:
1. It appears to me as if you cannot fill the complete memory buffer
with a single triggered sample. The manual on page 27 is somewhat confusing,
as it indicates you can fill the complete buffer with a single sample, but
reading on it says you can only fill one frame, and it seems you need at
least two frames. Can you please clarify.

-----> Currently this is true - you must have at least two frames. We do plan
to change the software to allow single shots to use the whole memory. But we
have a lot on, and so that will be somewhat later this year. In the
meantime, we are about to release a new continuous streaming mode (I attach
it being used), which can stream to hard disk at up to 1 MSa/sec (2A+8D).
This may be useful if you want long time records.

2. I understand you can only actively vary the sampling frequency by
applying and external signal. Could this come from the built in signal
generator if you chose that option? Do you sacrifice any other options if
you chose this option? How does the signal get put in, and what requirements
are there for this signal?

------> We have two options to sampling frequency. These are to use the
inbuilt 100 MHz clock, and decimate the waveform in real time to whatever
sample rate you want. YOu can decimate wither sampled, peak captured, or averaged.
This is the preferred option because you can average
the oversampled signal to reduce noise. With a 1.28us moving average
filter, you get 14.6 bits ENOB with 1 MSa/sec decimation. You could probably
get more with 150 kHz decimation. The decimator is set before sampling, so
you get the full memory at the sample rate you want. You can choose the
sample rate in 10ns steps. 150 khz is 6.667us period, so you would need to
sample at 6.670 us interval (149.925 kHz). Secondly you can use the external
sampling clock in the range 1 MHz to 50 Mhz. I see no need for this in your
application.

3. In the manual you list an Analogue Output module, which can play an
arbitrary waveforms, could this be used to play back a captured waveform? If
so, could you also play back a waveform that you have stored in a PC and
restored to the scope memory? There doesn't seem to be any info around for
this module.

------> This module is under development. We must change the manual. I expect it
early next year. The current signal generator does have a slow data rate
(100 kHz update rate) arbitrary capability.

4. Can you have a unit that has all options of SigGen, Analogue output
and External sampler?

------> Yes, but I would not use external sampler, as above, because you get better
noise performance if you over-sample, and then moving average filter.

5. It is unclear to me what the logging function does. I tried to
simulate this with the SigGen Simulator of the Demo Software but I couldn't
in fact determine whether it was supposed to log signal or just trigger
events. I was hoping that there was an ability of automatically transferring
a block of captured data to the PC on a regular basis but I couldn't make
this work.

------> We have several methods of doing this. First we are about to (ie 2 weeks)
release continuous save to disk at between 1 Sa/sec and 1 MSa/sec for up to
100 G Sample. Second, you can use Information graph logging (choose Show
logging) to log at 20 samples/sec, third you can save the capture frame to
disk on trigger, or use periodic save, automatically.

6. I tried to capture a full memory buffer, but managed only 200,000
samples, is this a restriction of the demo software?

------> Yes, this is a demo software limit. The real system with 8MSa captures up to
4Msa/frame
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