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Message ID: 2608     Entry time: Thu Feb 11 18:01:39 2021
Author: Anchal 
Type: DailyProgress 
Category: InstrumentCharacterization 
Subject: SR560 Intermodulation Test 

I added script SRIMD.py in 40m/labutils/netgpibdata which allows one to measure second order intermodulation product while sweeping modulation strength, modulation frequency or the intermodulation frequency. I used this to measure the non-linearity of SR560 in DC coupling mode with gain of 1 (so just a buffer).

IP2 Characterization

  • Generally the second order intercept product increases in strength proportional to the strength of modulation frequency with some power between 1 and 2.
  • The modulation frequency strength where the intermodulation product is as strong as the original modulation frequency signal is known as intercept point 2 or IP2.
  • For SR560 characterization, I sent modulation signal at 50 kHz and set intermodulation frequency to 96 Hz.
  • The script sends two tones at 50 kHz and 50khz -96 Hz at increasing amplitudes and measured the FFT bin around 96  Hz with dinwidth set by user. I used 32 Hz bin width.
  • In attachment 1, you could see that beyond 0.1 V amplitude of modulation signal, the intermodulation product rises above the instrument noise floor.
  • But it weirdly dips near 0.8 V value, which I'm not sure why?
  • Maybe the modulation signal itself is too fast at this amplitude and causes some slew rate limitation at the input stage of SR560, reducing the non-linear effect downstream.
  • Usually one sees a straight curve otherwise and use that to calculate the IP2 which I have not done here.

IMD2TF Characterization

  • First of all, this is a made up name as I couldn't think of what else to call it.
  • Here, we keep the amplitude constant to some known value for which intermodulation signal is observable above the noise floor.
  • Then we sweep the modulation frequency and intermodulation frequency both, to get a 2-dimensional "transfer function" of signal/noise from higher frequencies to lower frequencies.
  • Here I kept the source amplitude to 0.4V and swept the modulation frequency from 10kHz to 100kHz and swept the intermodulation frequency from 96 Hz to 1408 Hz, with integration bandwidth set to 32 Hz.
  • I'm not completely sure how to utilize this information right now, but it gives us an idea of how much noise from a higher frequency band can jump to a lower frequency band due to the 2nd order intermodulation effect.


Edit Wed Feb 17 15:34:40 2021:

Adding self-measurement of SR785 for self-induced intermodulation in Attachment 3 and Attachment 4. From these measurements at least, it doesn't seem like SR785 overloaded the intermodulation presented by SR560 anywhere.

Attachment 1: IP2SR560_11-02-2021_175029.pdf  17 kB  | Hide | Hide all
Attachment 2: IMD2TFSR560s_11-02-2021_180005.pdf  44 kB  Uploaded Fri Feb 12 10:08:10 2021  | Hide | Hide all
Attachment 3: SR785_SelfIP2_12-02-2021_145140.pdf  25 kB  Uploaded Wed Feb 17 15:52:31 2021  | Hide | Hide all
Attachment 4: SR785_SelfIMD2TF_12-02-2021_145733.pdf  55 kB  Uploaded Wed Feb 17 15:52:46 2021  | Hide | Hide all
Attachment 5: SR560.zip  146 kB  Uploaded Wed Feb 17 15:53:07 2021
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