Spurious signals every 12 MHz

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I2SVA
Posts: 18
Joined: Wed Jul 25, 2018 7:27 am

Spurious signals every 12 MHz

Post by I2SVA » Wed Oct 31, 2018 10:17 am

Hi support, all,
I noticed spurious signals on my RSPDuo, even when the antenna is disconnected and the antenna connector is closed on 50 Ohm termination. I used a PC battery powered and shut down all the other rigs in my home in order to avoid external disturbances.
The spurious signals seems to be multiple of 12 and 24. Multiple of 24 MHz are much stronger (-80 to -100dbm) and rage from 24MHz to the upper edge. In particular the ones at 432MHz and 1296 MHz are very annoying when using RSPDuo in he ham bands for low signals. No change with low IF or zero IF.
Is this a common issue? I did not find any way to mitigate it.
Thanks,

Alessandro i2sva

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g1hbe
Posts: 494
Joined: Sat Jan 17, 2015 3:28 pm
Location: Cheshire, UK

Re: Spurious signals every 12 MHz

Post by g1hbe » Wed Oct 31, 2018 11:25 am

The reference frequency is 24 MHz, so that's probably leaking around the circuitry a bit.

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Andy

I2SVA
Posts: 18
Joined: Wed Jul 25, 2018 7:27 am

Re: Spurious signals every 12 MHz

Post by I2SVA » Wed Oct 31, 2018 12:22 pm

Hi Andy,
I thought about 24MHz as a ref frequency; what surprise me is that, in my device, ALL the harmonics of 24MHz are equally strong (very strong S9 / S9+) from 24MHz up to 1992 MHz. In addition I have also the harmonics of 12MHz at a variable level from NIL to S9/S9+. IMHO this is a bit more than "leaking around the circuitry a bit" ;-)
I just wonder if this issue is common to all the devices or if there could be an issue with mine that could be investigated further.
Thanks, 73
Alessandro i2sva

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g1hbe
Posts: 494
Joined: Sat Jan 17, 2015 3:28 pm
Location: Cheshire, UK

Re: Spurious signals every 12 MHz

Post by g1hbe » Wed Oct 31, 2018 3:07 pm

OK I've just had a look. Ant A terminated with 50r. Most (if not all) 24 MHz harmonics are at ~100 and most 12 MHz ones are around -130. This is with the RF gain slider at its usual gain setting of 75% up (my slider is set for gain, not reduction).
I've probably not noticed this too much as I rarely listen on exact MHz settings. VHF/UHF calling freqs are usually on .200 or 300.
I suppose it must be difficult to get rid of this kind of thing when most of the 'works' are contained in one or two chips and the entire receiver is on a PCB only a few inches square. No doubt the signals could be better suppressed but the screening, earth-pathing etc would lead to much greater development and production costs.

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Andy

Tech_Support
Posts: 499
Joined: Mon Jun 01, 2015 7:00 pm

Re: Spurious signals every 12 MHz

Post by Tech_Support » Wed Oct 31, 2018 6:42 pm

All RSPs operate off a 24 MHz master clock. All receivers suffer from a degree of spurious coupling of clock harmonics. Please consider the following:

Capacitive coupling increases with frequency. Clock harmonics decrease at a rate of 1/x where x is the harmonic number. As a consequence, it is not surprising that clock harmonics appear at approximately the same level. Even if the coupling mechanism was constant and not frequency dependent, the 20th harmonic would only be attenuated by 26 dB.

-100 dBm of spur level implies approximately 100 dB of crosstalk isolation from the master clock at 24 MHz, which as has already been implied is not easy to achieve in such a small physical space.

Sincerely

Tech_Support

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I2SVA
Posts: 18
Joined: Wed Jul 25, 2018 7:27 am

Re: Spurious signals every 12 MHz

Post by I2SVA » Mon Nov 05, 2018 5:32 pm

Hi Andy and tech_support,

thanks for the explanation and for the clarification; I agree that this is not a very big issue and overall the RSPDuo is a very nice product, with excellent price/performance ratio.
Thanks again for your courtesy and best 73

Alessandro i2sva

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