Spike on 120.000 mHz

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Paul
Posts: 288
Joined: Fri Feb 17, 2017 9:52 am

Spike on 120.000 mHz

Postby Paul » Tue Feb 13, 2018 7:59 pm

Not a real problem, but just curiosity really.
On both SDR's that I'm currently using - An SDRplay RSP2 and a Chinese RTL, there is a permanent , strong carrier centered on 120.000 mHz.
I'm getting old, but am I remebering correctly that this was produced internally on "old-fashioned" receivers as well? - or m I, as they say "just blowing smoke?"
This may well seem a stipod question, but did it have something to do with the local oscilator?
Another display of ignorance...

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g1hbe
Posts: 425
Joined: Sat Jan 17, 2015 3:28 pm
Location: Cheshire, UK

Re: Spike on 120.000 mHz

Postby g1hbe » Tue Feb 13, 2018 10:33 pm

One of the RSP's converter oscillators runs at 120 MHz, so it may be that. Not sure about the RTL stick - probably something similar, maybe a clock oscillator harmonic.

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Andy

Tech_Support
Posts: 438
Joined: Mon Jun 01, 2015 7:00 pm

Re: Spike on 120.000 mHz

Postby Tech_Support » Tue Feb 13, 2018 11:05 pm

All RSPs use a 24 MHz clock, so 120 MHz is the 5th harmonic of the reference clock. You will always see spurs at harmonics of the clock and odder order harmonics tend to be stronger than even order harmonics. All radios will exhibit this effect due to finite cross-talk isolation. The Xtal oscillator runs at 3v peak to peak and produces an approximate square wave output. harmonics roll off as 1/x and so the 5th harmonic will be 1/5th or -14 dBc. With 3v p-p if you can achieve 100 dB of isolation, you would still see a spur at around -100 dBm at 120 MHz, which would be very easy to see at VHF.

Sincerely

SDRplay Tech_Support

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Paul
Posts: 288
Joined: Fri Feb 17, 2017 9:52 am

Re: Spike on 120.000 mHz

Postby Paul » Wed Feb 14, 2018 7:19 am

Great Scott!
Thats what you call an answer!
Thanks Support, for your time and trouble - at least memory partially vindicated - I think...
Sorry abou the typos

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