"Non Linear" AM Modulation
Kevin Updegraff
Principal Electrical Engineer @ Northrop Grumman | High Power RF Test
I was measuring AM modulation from a Synthesized Signal Generator and noticed a loss of symmetry.
I noticed that the side bands on the left side are lower than those on the right.
Notice the 2 yellow markers on the left and right side bands. The reference marker is on the right and the "delta" marker is on the left. The reading of -7.59dBc on the left is the difference from the amplitude of the marker on the right. The other side bands on the left all seem to be lower.
Troubleshoot: At first I was using the built in Audio Modulation source that comes with the Signal Generator. Thinking that was the source of un-even side bands, I switched to a low distortion ( <90 dB THD ) external source and found the same results.
Setup: The input modulating signal in both cases is a simple 3kHz tone at 1Vrms and the AM modulation is set to 80%. Changing the modulation percentage or tone amplitude does not seem to make a difference.
Edit: Additional information: The source generator is a HP 8664A just back from Cal.
Has anyone ever seen this?
R&D S&E, Computer Engineering at Sandia National Laboratories.
5 年If it's a step sweep, see if the levels change with sweep parameters. Wide sweeps are notoriously inaccurate due to RBW/VBW filtering and A/D quantization. Zoom in and try a quasi-peak or zero span measurement. Check signal quality with demod, SNR, and look at phase noise next. Then get tacos and beverages while considering how to resolve the instrument issue so you can exit maintenance and get back to the plan. Sometimes you just have a unique instrument problem to work around with a rented or borrowed substitute. Always document a bad instrument somehow, so the next user is aware.
Discoverer of the true cause of harmonic distortion
5 年Playing with QAM I've seen it. Here is a tight view of a QAM spectrum with a lot of noise. Apparently it's easy to do, just send two different values to each sideband through I and Q and ta-da.
R&D S&E, Computer Engineering at Sandia National Laboratories.
5 年Was it adjusted to center of specs at cal, or just "Meets specs, time is money, sticker and return"? I once had a faulty Tek492 calibrated and returned with the fault. Investigation revealed it was "calibrated" to an HP856x spec sheet because the cal house didn't have a Tek492 calibration manual. Is proper function of your source and analyzer an assumption?
Security Supervisor ( Physical and IT ) at G4S
5 年What is the percent modulation ?
Senior Engineer at Siemens
5 年I have worked with many RF projects and I have seen this.? In your case, since it is only a single tone modulation, the sideband filter attenuates the other modulation components as they are were not needed.? In your case, they are needed.? Very interesting ....huummm