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Generating CW Tones with Pluto in Matlab

Question asked by MarcusW on Oct 26, 2017
Latest reply on Oct 27, 2017 by MarcusW

I'm wondering if anyone can help me understand some apparent anomalous behavior of the Pluto SDR when used in conjunction with Matlab.  First of all, let me say that my device seems to work just fine, and it is probably just me that doesn't know what I'm doing.  I've been able to run several examples with the SDR that have worked fine, but I'm looking for some help understanding what is going on now that I'm trying to transmit.  


Hardware Setup:  I currently have a splitter on the Pluto TX port - one output is a loopback straight into the Pluto RX port, and the other splitter output goes into a spectrum analyzer.  


ATTEMPT #1: Success

My first attempt was to generate a constant tone at 10 KHz above a carrier frequency of 1 GHz.  I created the following Simulink Model


Single Tone SLX 

with the settings below

Single Tone SLX Settings


I ran it and viewed the Matlab Scope output as seen below:

Single Tone Scope


And saw the following on an actual spectrum analyzer

Single Tone SA


I got exactly what I was expecting.  The peak of the tone is a little bit off from the 10 KHz signal I was trying to generate, but this is due to both error in terms of the Pluto carrier frequency accuracy, as well as the fact that the particular scope I'm using here is also not calibrated correctly (it is off by about 3 KHz).  All in all, this looks good to me.


ATTEMPT #2: Anomalous Behavior

Now, I tried to replicate the same thing I did above, but with two tones, one at 10 KHz and one at 20 KHz off a 1 GHz carrier.  So I added a second sine wave at 20 KHz.


Two Tone SLX

Again, nothing unusual about the settings

Two Tone Settings


But when I ran the example, the Matlab scope went crazy!

Two Tone Scope


At first I thought there was something wrong with my Scope settings, but when I checked the spectrum analyzer, I faced the same strange result.

Two Tone SA


QUESTION: What is going on here?  What am I doing wrong when trying to generate two tones using the Pluto SDR.  I can easily generate two tones and display it in Matlab, but why doesn't the Pluto play along?  Given that my single tone was successful and produced -7 dBm signal, I was expecting two tones offset by 10 KHz at about -10 dBm since everything goes out the same power amplifier.  What am I missing here?


Thank You.