Bothersome spurs

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kc6ete
Posts: 11
Joined: Wed Apr 26, 2017 7:37 pm

Bothersome spurs

Postby kc6ete » Thu May 04, 2017 8:38 pm

I'm seeing intermittently, the appearance of spurs all over the band.
These happen with no antenna, but generally don't happen if I select the High Z antenna.

They are at regular intervals, and I can usually ignore them, but at the moment one is sitting right on top of a signal that I need to see.
They are peaking at -40dBm, with broadband noise floor at -72dBm
Settings for bandwidth and frequency don't appear to change anything, except once in a while when all the spurs will disappear on a bandwidth change.

I'm using SDRPlay RSP2 V3 build 696

What's going on, and how can I make it stop?

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kc6ete
Posts: 11
Joined: Wed Apr 26, 2017 7:37 pm

Re: Bothersome spurs

Postby kc6ete » Thu May 04, 2017 10:11 pm

At the moment, changing bandwidths isn't helping, but the entire charachter of the spurs changes if I stop and restart the receiver without changing anything else. Seems like it's sort of random.

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octave9
Posts: 45
Joined: Sun Feb 19, 2017 7:54 pm

Re: Bothersome spurs

Postby octave9 » Fri May 05, 2017 10:59 am

What strength are they (and what noise floor) when antennas are disconnected? And do they extend right up to GHz part of the band?

If they are easily repeatable with no antenna, it would be fairly easy to try at a different location and a different computer, but a better USB cable would be a first thing to try.

Do they move with the LO frequency?

I found an interesting Q & A on Stack Exchange. Didn't know they have an amateur radio section: https://ham.stackexchange.com/questions ... spurs/7421

Hoping it helps.

Reason: No reason

kc6ete
Posts: 11
Joined: Wed Apr 26, 2017 7:37 pm

Re: Bothersome spurs

Postby kc6ete » Fri May 05, 2017 3:29 pm

It's possible that they are computer related, but I'm very puzzled that a stop and start (from the home tab in SDRPlay) changes the character of the noise completely. It sometimes, but not usually, changes if I change bandwidth. I have not seen it change when I change IF frequencies.

I'm using an RF gain of 4, IF of -35 (auto) and LO of "automatic" at the moment.

The RF gain setting is confusing, with " 8 min" and "0 max", and when I have "8 min" selected, the noise floor with no antenna is at -67dBm, and with "0 max" selected, it's at -103 or so.. Seems opposite to the labeling. I'm trying to avoid problems due to excessive signal input but then again, i'm gettin these spurs with no antennas selected.

FWIW, when I'm having spur problems, switching to the high Z input either gets rid of them, or reduces the amplitude drastically.

Another clue: Setting the IF gain to any manual setting other than -55dB causes them to appear.
Auto settings have no issue.

Reason: No reason

octave9
Posts: 45
Joined: Sun Feb 19, 2017 7:54 pm

Re: Bothersome spurs

Postby octave9 » Sat May 06, 2017 2:18 am

With 10M Sample rate, IF AGC on, no antennas connected, antenna A selected and max (67.4dB) RF Gain - I see a couple of 'spikes' around 2.28MHz. With that gain they are only about -125dB and they drift around a little. Drop the RF Gain by one click and the lowest f one shoots up to about -110dB. Drop it by four clicks and the two spikes disappear and dozens of spikes appear instead, which I guess are the same as you're seeing.

The RF Gain is 23.7dB at that setting and the highest spikes are about -80dB. With just 8dB RF gain a spike around 3.25MHz is about -65dB.

There must be some gain adjustment and amplifier switching going on regardless of IF AGC, or the AGC with slow/medium/fast/off settings. You can see the gain figure change and then settle as you change the RF gain between some settings.

Adjusting RF gain one click at a time with IF AGC on and then off at each click shows it gets altered depending on something the RF gain control does...

Hoping it isn't going to be like a rattle in the car that you never heard before someone pointed it out - and then you hear it all the time.

4.5872MHz seems to be the peak of the effect for me. Tune to the spike there and switch IF Gain off and on again and change the levels. Not nice to watch. Looks like resonance. Almost looks like AM.

4.62M gets a peak with some RF Gain settings and not others...

I switched to 2M sample rate and still see them. Crashed SDRUno switching antenna. :(

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