RSP2 Band Pass not working?

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Phoenix
Posts: 11
Joined: Tue Aug 08, 2017 4:22 pm

RSP2 Band Pass not working?

Postby Phoenix » Fri Oct 06, 2017 1:50 pm

Hello,
i have the problem that i get ghost signals in my spectrum. For example there is a strong TETRA station at 390 MHz. I am also picking it up around 130 MHz. it is not supposed to be here. I confirmed that with my other 2 Scanners. Now my question is: There is a bandpass filter from 380-420 MHz in the RSP2. Why am i getting ghost signals around 130 MHz? Do i need to enable the bandpass filters or are they automatically enabled? I never had such a problem with a 30$ RTL2832 Dongle so i am a bit disappointed with the RSP2.

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Tech_Support
Posts: 202
Joined: Mon Jun 01, 2015 7:00 pm

Re: RSP2 Band Pass not working?

Postby Tech_Support » Fri Oct 06, 2017 4:59 pm

Hello Phoenix,
If you are using SDRuno, please measure the power of both the original Tetra station at 390 MHz and the Spurious response at 130 MHz. Such spurious responses occur because the Synthesiser is tuned to 130 MHz and it has a 3rd harmonic at 390 MHz. This third harmonic will mix with the Tetra signal at 390 MHz to produce an in-band response. Now, as the Synthesizer/LO is effectively a square wave, the third harmonic is only -10 dBc. If your band-pass filter was giving no rejection, then the level of the spurious signal would only be 10 dB lower than the original signal. If the spurious signal is x dB below that of the original signal, then the filter rejection will be (x-10) dB.

The RSP2 has an extremely low noise figure and is very sensitive in the VHF band. It is therefore more likely to be able to resolve weak spurious responses than a less sensitive receiver.

Sincerely

SDRplay Tech_Support

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Tech_Support
Posts: 202
Joined: Mon Jun 01, 2015 7:00 pm

Re: RSP2 Band Pass not working?

Postby Tech_Support » Fri Oct 06, 2017 5:38 pm

I should add, that if your RTL dongle uses a Rafael Micro tuner, it uses a low IF architecture whereas the RSP uses a zero IF architecture. This means that that your RTL dongle will still have a spurious response withyour Tetra signal, but it will appear in a different part of the VHF spectrum. If your RTL dongle uses an Elonics E4000 tuner, it will almost certainly see exactly the same response in the same place as that also uses a ZIF architecture.

Sincerely

SDRplay Tech_Support

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Phoenix
Posts: 11
Joined: Tue Aug 08, 2017 4:22 pm

Re: RSP2 Band Pass not working?

Postby Phoenix » Fri Oct 06, 2017 7:14 pm

Thank you for your answer
I just measured the signal strengths. The original one is at 397,740MHz and yields about -59,7 dbm RMS. The one at around 130 MHz yields about -99,9 dbm RMS. So i guess everything is fine with my unit? I should also add that the ghost signal moves in the opposite directions as all the other normal signals when i am tuning.

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Tech_Support
Posts: 202
Joined: Mon Jun 01, 2015 7:00 pm

Re: RSP2 Band Pass not working?

Postby Tech_Support » Fri Oct 06, 2017 8:52 pm

Hello Phoenix,
What matters is the frequency of the LO, not the VFO. If your Tetra signal is at 397.74 MHz, if the spurious response is cause by mixing of this signal with the third harmonic of the synthesizer, then if you set the LO to 132.58 MHz, I would expect the ghost signal to sit precisely on top of the LO. Unless you have locked the LO in SDRuno, as you move the VFO, you will also be moving the LO. The movement of the ghost signal in the opposite direction as you tune the LO simply indicates a phase reversal in the I/Q signal paths. This happens because in a zero IF receiver, for the fundamental of the LO, Q leads I by 90 degrees, but for the third harmonic, I leads Q by 90 degrees. When you get such a phase reversal, positive frequency shifts become negative frequency shits and vice-versa.
With your LO at 130 MHz, you have selected the 120-250 MHz pre-selection filter. Your measurements of power for the ghost signal relative to the original Tetra signal indicate that the filter (whose upper cut off is 250 MHz) is giving you around 30 dB of rejection at 400 MHz (which is less than 1 octave away). This is exactly in line with what we would expect from this filter design, so I do not believe there is a fault with your RSP2.
As I said previously, all wideband SDRs are prone to producing such ghost images, but depending upon the receiver architecture, these images may appear in different parts of the RF spectrum.

Sincerely

SDRplay Tech_Support

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g1hbe
Posts: 216
Joined: Sat Jan 17, 2015 3:28 pm
Location: Cheshire

Re: RSP2 Band Pass not working?

Postby g1hbe » Sat Oct 07, 2017 10:40 am

It's also worth bearing in mind that the noise floor of the 8 bit dongles is much higher than the 12-bit SDRPlay. The dynamic range of a cheap dongle will be around 48 dB, whereas the SDRPlay will manage 72 dB. The ghost signals may well be there on your RTL dongle, but the poor dynamic range is hiding them.

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Andy


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