I am developing a non-regenerative RF repeater and am getting Spectrum Analyzer readings I don't believe. I have filters that should be rejecting the harmonics of my in band CW test signal.
For example: My 70 MHz bandpass IF 8 KHz BW is rejecting the 140 MHz harmonic by 15 dB??? I checked the filter response on the network analyzer and it should be rejecting the 140 MHz signal by -80 dB.
I see similiar things after other bandpass filters in the system.
I did a test and used a 302.75 MHz 6 pole 9 MHz wide bandpass filter connected between the sig gen and the spectrum analyzer. I set the sig gen fundamental to 303 MHz and the sig gen has a harmonic at 606 MHz. I combined a separate 607 MHz signal from a 2nd sig gen and then looked at the spectrum analyzer.
I tried two different spectrum analyzers and they are showing almost the same thing. I am keeping the Spectrum Analyzer reference level above the fundamental level so as to not saturate the device.
Here is what I see:
The 303 MHz signal got attenuated by 2 dB (the insertion loss of the filter...good)
The 606 MHz harmonic got attenuated by 12 dB What the...?!?!?!
The 607 MHz signal got attenuated by -95 dB (expected due to rejection of BP filter)
I put a 10 dB attenuator in and the 606 MHz harmonic got attenuated by DOUBLE or 20 dB. The 607 MHz signal got attenuated by 10 dB.
What do I think: The harmonics level is bogus. The impedance mismatch caused by the filter outside its pass band is creating havoc with the spectrum analyzer and/or sig gen and the harmonics are being re-generated. The double attenuation is because of reflections.
Solution: I need to filter out the fundamental to measure the true harmonics accurately
What do you think? Thanks
Tracy
For example: My 70 MHz bandpass IF 8 KHz BW is rejecting the 140 MHz harmonic by 15 dB??? I checked the filter response on the network analyzer and it should be rejecting the 140 MHz signal by -80 dB.
I see similiar things after other bandpass filters in the system.
I did a test and used a 302.75 MHz 6 pole 9 MHz wide bandpass filter connected between the sig gen and the spectrum analyzer. I set the sig gen fundamental to 303 MHz and the sig gen has a harmonic at 606 MHz. I combined a separate 607 MHz signal from a 2nd sig gen and then looked at the spectrum analyzer.
I tried two different spectrum analyzers and they are showing almost the same thing. I am keeping the Spectrum Analyzer reference level above the fundamental level so as to not saturate the device.
Here is what I see:
The 303 MHz signal got attenuated by 2 dB (the insertion loss of the filter...good)
The 606 MHz harmonic got attenuated by 12 dB What the...?!?!?!
The 607 MHz signal got attenuated by -95 dB (expected due to rejection of BP filter)
I put a 10 dB attenuator in and the 606 MHz harmonic got attenuated by DOUBLE or 20 dB. The 607 MHz signal got attenuated by 10 dB.
What do I think: The harmonics level is bogus. The impedance mismatch caused by the filter outside its pass band is creating havoc with the spectrum analyzer and/or sig gen and the harmonics are being re-generated. The double attenuation is because of reflections.
Solution: I need to filter out the fundamental to measure the true harmonics accurately
What do you think? Thanks
Tracy
