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Modulating a 5644R VST with a BERT


Looks like it's not straight-forward to do. Here's the documentation on the external digital interface of the 5644. Looks like the PFI0 line is LVTTL and so is incompatible with the generator that you have. The intention with the VST is to use the software from RFmx or the modulation toolkits to generate your BPSK signal and measure the received error rate using the VST by itself. You can use the modulation toolkit to create the waveform, play it through RFSG, and then analyze the BER in RFmx. Even better would be to use the RFmx Waveform Creator to generate that signal.


If you did want to give it a shot, I think this is the path you'd take: You either use your own level shifter or use a 6672 or 6674T to bring in the TTL signal from your generator and route it to the VST over the PXI backplane. You could then use the simple VSG sample project that comes with the VST Instrument Design Libraries to use that signal. You can modify the FPGA design to adjust the frequency shift block based on the input from the backplane to accomplish the shifting. Since this is BPSK, that should be pretty simple.



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Message 11 of 19

OK, I get it.  The VST is not meant to be a general-purpose instrument that can be modulated via hardware.  Makes sense, it's a unique thing for unique purposes.  


This just went well beyond the scope of what I wanted to do, which was just replace a bench instrument.  If I had copious free time I could build a BERT from LV or the FPGA and have fun doing it.  Maybe in retirement.  


Thanks, guys.  




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Message 12 of 19

Hi Ed, you're right--modulating the VST from another instrument is not supported. But just to be clear (maybe for someone reading this thread in the future), the VST could replace both instruments using the software from RFmx. Instead of creating the modulation from one instrument to another and back again, you create the waveform (either through the Modulation Toolkit or RFmx Waveform Creator), send and receive through the VST, and read the BER from RFmx. This would be some effort to move your test setup and get things working, but then you could free up two instruments instead of one.

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Message 13 of 19

I hate to burst everyone's bubbles at once but everyone is wrong and everyone is right.

By default the VST is a general purpose thing to do general purpose stuff.

Commonly, the general purpose RF generation stuff that it does well, is taking a waveform that has been synthesized already via some other source(LabVIEW DSP, Matlab, python, etc) and generating that waveform out. The VST by default does not do any modulation alone, think of it as analog output for RF. As the old NI tag-line goes "the software is the instrument"


However taking an external digital signal and supplying it to a VSG to do on the fly modulation of those bits I would consider non-general RF stuff, but its still VERY possible.


As for generating waveforms on the fly based off of a digital pattern in, you could theoretically accomplish this 1 of 2 ways.

  1. Use a digital card and as you get the bits from that, feed them in a stream to some DSP (MT or anything else) to modulate the waveform on the fly to feed the raw complex samples to the VST for generation.
    There will be a delay (as in any system) and you'll need to make sure your modulators are efficient to keep up with the data rate, but this can work.
  2. If you're feeling very audacious, you could stream the data into the FPGA through the front panel digital connector, and then create your modulator on the FPGA itself.
    Presuming your digital rates and levels are compatible with the DIO read rates on the FPGA, you'd be set (shy of a lot of work on the FPGA to create a modulator).
    This would be challenging but indeed fun.

So in short you can use another instruments digital information to modulate a waveform on the VST. In your case, it's just a single line of serial digital so that would be even easier. I'm not sure if PFI0 can handle the data rate that you're trying to do or not, but there is the other digital connector on the front that you could leverage w/ the right cable.


If you've got the HW and the ability to spend time working with learning some LV FPGA, and indeed the signal is going to stay just BPSK, stitching together the complex samples could be very easy.

what is the pulse shame filter that you're currently using for the BPSK signal?

Do you expect you're needs will stay as just generating a BPSK signal? 

If it's unfiltered, even something stupid and simple like this could get you by to prove it out (done on USRP FPGA but same concepts apply)LabVIEW_97liAkHn77.png

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Message 14 of 19

It would be fun and a learning experience but there's no way I have time for it.  I have a bench full of instruments now that works.  


Here is what throws me, and why I started this thread:  The block diagram for the 5644 includes the IQ modulator:



The inputs appear to be driven by hardware DACs.  I was hoping the DACs were exposed on the digital connector and I could drive them directly, by routing through the FPGA.  Or are they unavailable to mere mortals?  Or am I reading this wrong (likely)?  

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Message 15 of 19

I was logged in under an old account, so the name on the posting was different.  Here I am again as AndyTailored.  

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Message 16 of 19

Those DAC are not exposed directly to the digital connector on the front. You are misinterpreting the diagram.

Those DAC are driven through the FPGA, that's how we make any signals out of the DAC of course.


However those DAC are connected to the FPGA and the FPGA is also connected to the front panel digital connector.

It's just up to someone to create an FPGA image that "connects" those inputs to drive the DAC output.

that's what my previous diagram was showing, taking a single digital input line (which is what it sounded like you were wanting) and driving the RF output from that digital line.


Now my above diagram is WAAAY too simplified, however, I do believe that you would see a signal.
quick correction on the diagram btw, for BPSK, it shouldn't go to 1+i0 and 0+i0 but rather 1+i0 and -1+i0.


If you're looking for a way that requires no FPGA programming on your part you have 2 options

  1. give up all hope ye who walk this path. Thou hast been smote with the sword of "this don't exist"
  2. attempt to tantalize some creative and bored engineer (especially during this downturn in industry) to create a simplified modified FPGA image that starts with the concepts mentioned above.

Just to clarify, what is the digital data rate that you want?

Does the BPSK signal need to be filtered?

Can you attach a waveform that you'd like the output to look like?


Most sincerely,

Your friend Prometheus


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Message 17 of 19

Careful, you're playing with fire....  


I didn't understand the code, I have no experience with the FPGA stuff.  I thought you meant instantiate an IQ modulator in the FPGA.  Scary.  


It's a 2Mbps max signal.  BFSK, not BPSK, that was twisted around earlier.  


The E4433B is doing a rectangle filter, which I assume is Agilent's name for a sinc filter.  


I would really like to buy the FPGA toolkit and learn something new, but not in the cards.  

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Message 18 of 19

Modifying the FPGA code in Labview is not an easy thing. Maybe using modulation toolkit mod and demod the BPSK signal by CPU(host), VST as one VSG and one VSA looks feasible.

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Message 19 of 19