Rigol DSA815-TG strange "spur"

[Solder_Junkie]

I was checking some amateur radio equipment recently and came across a spurious signal that is generated in my Rigol SA. I have checked with another 815 owner and his produces the same spurious signal.

The test is to feed the spectrum analyser from a signal generator with a carrier around 144.320 MHz and look for the spur some 11 KHz lower and at a level close to -40 dBc. This spurious signal only seems to appear around 144.3 and is not present on another spectrum analyser that I checked it with.

Any ideas what may be the cause and is it something that happens with other 815s?

Thanks
SJ

[wn1fju]

Same results with mine.  At 144.32 MHz, I get the spur about 10 kHz lower.  At 144.33 MHz, the spur is about 15 kHz lower.  And so forth, 144.34-->20 kHz, 144.35-->25 kHz. 

OK, how about lower than 144.32.  The spur is about 5 kHz lower at 144.31.  But then something mysterious happens precisely at 144.3 MHz:  the entire noise floor on the spectrum analyzer jumps up about 20-30 dB.  At 144.29 MHz, it is back to normal.  I was using an HP 8642B signal generator, which is pretty good, and I verified the same thing using a different signal generator, so it is definitely the DSA815-TG.

So whatever is causing the spur seems to be related to 144.3 MHz.  Perhaps some internal clock?

[Solder_Junkie]

My signal generator is an IFR 2025, which is also pretty clean.

I have raised the matter with the UK Rigol distributor, who is taking it up with Rigol. It’s a really strange one.

Thanks for the feedback

73 SJ

[thm_w]

There is mention of 14.3MHz in the original dsa815 thread: https://www.eevblog.com/forum/testgear/spectrum-analyzer-rigol-dsa815/?action=printpage

Though I see no mention of 144MHz.

[shabaz]

I don't own a DSA815 so I can't try it myself, and I've not done the sums either, but there's a couple of things you could try (maybe you have already tried these, otherwise definitely worth a shot):

(1) Reduce your RBW from 100 Hz down to 10 Hz and see if anything changes
(2) Reduce your input signal power level down to (say) -20 dBm instead of the current -10 dBm, without changing any other setting, and again, see if anything changes (i.e. does the spurious signal level change, and if so, by how much).

[Solder_Junkie]

I don't own a DSA815 so I can't try it myself, and I've not done the sums either, but there's a couple of things you could try (maybe you have already tried these, otherwise definitely worth a shot):

(1) Reduce your RBW from 100 Hz down to 10 Hz and see if anything changes
(2) Reduce your input signal power level down to (say) -20 dBm instead of the current -10 dBm, without changing any other setting, and again, see if anything changes (i.e. does the spurious signal level change, and if so, by how much).

Changing levels and RBW makes no difference. I am not sure of other frequencies as I have only seen it when inputting a signal over a small range of frequencies around 144.32 MHz.

SJ

[Evariste]

Hello,
I confirm, I have exactly the same problem, test carried out with the DG4162 generator, and the problem appears from 144.303 Mhz 

[Solder_Junkie]

Hello,
I confirm, I have exactly the same problem, test carried out with the DG4162 generator, and the problem appears from 144.303 Mhz 

Sorry, I didn't reply earlier. Telonic (the UK Rigol distributor) had a response from Rigol in China, the problem does not exist on the latest boards but is there on earlier ones. Rigol don't have a solution, other than scrap your old SA and buy a new one. Well, I suppose we pay our money and take our chances with Chinese junk test equipment.

SJ

[Evariste]

Thanks for the info, my next one will be in SIGLENT  👍

[enjoi4283]

Anecdotaly I've heard that updating to the latest firmware and running the autoalignment sometimes fixes the issue. Its worth a try if you're having issues.