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Old 1st Dec 2022, 1:18 am   #313
G0HZU_JMR
Dekatron
 
Join Date: Sep 2010
Location: Cheltenham, Gloucestershire, UK.
Posts: 3,077
Default Re: 6-gang FM stereo tuner heads

A low-cost way to measure close-in phase noise with your Siglent analyser would be to make a notch filter using crystals. I've done this in the past with very good results. The downside is that the phase noise can only be directly measured at the frequency of the notch filter. However, if a low noise LO and a mixer is used to convert the DUT to the frequency of the notch the other DUT frequencies can be measured using the same notch filter.

The notch below was made using crystals that cost about 20p each. The span is only 10kHz and the notch is 70dB deep. You can see that the notch is very sharp.

This effectively nulls the carrier from the DUT, and this filters away the contribution from the analyser phase noise. The VNA plot below shows the filter response. The other plot shows it in action where three signal sources are compared for close in phase noise. The carrier power is 0dBm, so the marker reading is in dBc/Hz. You can see that the cleanest source was a crystal oscillator made using one of the 20p crystals. It managed -163dBc/Hz at about 5kHz offset. The other two sources were an Agilent vector sig gen (noisy) and an old Marconi 2019 sig gen.

Your Siglent analyser would probably be able to measure down to about -155dBc/Hz with this setup if the preamp was enabled. This is over 50dB better than the analyser can manage without the notch filter.
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Regards, Jeremy G0HZU
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