SDR Upgrade

Talk about other SDR products like the FunCube, HackRF, BladeRF etc.
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SDR Upgrade

Post by Jekko » Mon Apr 29, 2019 12:28 pm


first of all let me say that i am really enthusiast of rtl-sdr v3. I bought it in October and with very cheap price i got into a world with a ton of signals, voices and things to do.

I am really grateful to the rtl-sdr blog to have developed a product with such elevate price/quality ratio. It is for sure the best entry level product.

But now, after months of trials i think that i pushed it to the limit.

My equipment is the rtl-sdr V3 dongle, the 88-108 filter and the lna, all from the blog, and some balcony antennas.

The main problems that i find are:

-a large amount of spurs in various bands (ie:around 145-160mhz)

-plenty of ghost signals in the HF (13-30mhz)

-If i have a strong signal on a frequency, it repeats itself in many other frequencies.

I think that these problems are limits of the cheap dongle.

What should be the "next level" sdr that i should consider?
I am oriented to the Sdrplay RSP1A. Do you think that it should resolve the problems mentioned above? Do you know other sdr that should resolve this problems?
Thank you very much

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Re: SDR Upgrade

Post by rtlsdrblog » Tue Apr 30, 2019 2:43 am

The next level is indeed an RSP1A or an Airspy with Spyverter.

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Re: SDR Upgrade

Post by hotpaw2 » Wed May 01, 2019 2:53 pm

No SDR upgrade will help with spurs if they are due to locally generated RF pollution (your digital electronics, network wiring and LED lights (etc.), or those of your neighbor's). Getting an antenna high above or well away from all this local RF noise is a better solution.

You might also want to consider a bandpass filter or switchable bandpass filter box between your antenna and your SDR (or LNA).
Last edited by hotpaw2 on Sat May 04, 2019 2:10 am, edited 2 times in total.

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Re: SDR Upgrade

Post by aurgathor » Thu May 02, 2019 4:06 pm

Yeah, I'd second the suggestion for a bandpass filter, either a switchable filter, or standalone filters for the desired band(s) based on the issues listed.

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Re: SDR Upgrade

Post by snn47 » Sat May 04, 2019 9:59 am

-If i have a strong signal on a frequency, it repeats itself in many other frequencies.
-a large amount of spurs in various bands (ie:around 145-160mhz)
-plenty of ghost signals in the HF (13-30mhz)
All of the above are most likely all from receiver (Rx) Intermodulation (IM). Even if you would spent 100 000 € or more on a commercial Spectrum Analyzer or Receiver with high dynamic range and sensitivity you still will have Rx-IM and need internal attenuators and/or (optional) filter to eliminate Rx-IM in presence of unwanted strong signals.

If you add 10 dB, 20 dB or 40 dB of attenuation and most of the signals are gone on the other frequencies than you have Rx IM. IM means your Rx is not able to process signals linearly any more, all signals should be amplified by the same factor. If strong signals are not or only partly amplified, because the receiver dynamic range is exceeded than you also get additionals signals/products generated. If you have a 1 dB resolution for selecting attenuation you can step by step determine the min. attenuation needed to eliminate Rx-IM, and maintain the max. Rx-sensitivity.

By adding attenuation you reduce all signals on all frequencies. Unless you add sufficient attenuation your Rx is not able to amplify in a linear fashion. Linear means if you add e.g. 20 dB of attenuation all signals should be 20 dB weaker, if you add 10 dB of amplification alls signals should be 10 dB stronger.
If signals that were much stronger than 20 dB previously and are gone or much weeker reduced by more than 20 dB in strength, they are Intermodulation products of your Rx.
Rx-IM means your receiver is generating additional signals by mixing existing strong signals with each signals, starting with IM 3.order (2*f1 -f2, 2*f2-f1), or higher e.g. 5th order.
The more strong signals your receiver is subjected to which exceed the dynamic range of your receiver, the more signals generated internally in your Rx by IM you will receive.

If you add Filter-, Band-pass-, Band-Reject-, High- or Low-Pass-Filter you can suppress the strong signals within the range and attenuation factor that the filter can suppress. This is another way to reduce or eliminate Rx-IM. Unlike attenuators, it depends on the quality of the Filter how much suppression you can achieve, especially if the strong signals are relative to the filter attenuation outside the passband to close to be suppressed sufficiently.

If signals are attenuate by the same factor of attenuation you add, they are most likely real signals that you receive and you have to live with them. These signals can have many sources e.g.
- spurious radiation a re signals generated outside ±250% of the defined signal bandwidth in a transmitter,
- Transmitter (Tx) Intermodulation when two or more transmitter generate IM products outside their defined channel, e.g. cell phone base stations. Such interference may not be consistanly but will occur when the transmitter use their max.power output
- leakage of equipment which by definition is not specified to radiate, but all receiver or clocked electronics also radiate more or less strong signals if you know at which frequencies to look for
e.g. electronic-clock-and databus-signals, power-line, UWB, signals of SAT-or CATV-distribution to name a few.
- Maybe a bit confusing, but intermodulation products can be generated outside electronic equipment in presence of strong signals if non linear devices like diodes, transistors, IC are part of a wired system.
But such IM has also been detected incorroded screws, which can form also non linear devises e.g. screws in a glass roof garden house. Once the corroded screws were replace the interference was gone.

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Re: SDR Upgrade

Post by JEL » Sun May 12, 2019 3:31 pm

Jekko wrote:
Mon Apr 29, 2019 12:28 pm
I am oriented to the Sdrplay RSP1A. Do you think that it should resolve the problems mentioned above? Do you know other sdr that should resolve this problems?
Thank you very much
Before choosing SDR, ask yourself which software you prefer to use.

If you like SDR#, then the SdrPlay won't work with that (as far as I am aware at least)

The Airspy (with spyverter to get HF) is by the same person behind SDR#, so fully compatible with all the extra software in that package.

So do not focus on the SDR alone, but the entire SDR+software combo :)

I have both a 'cheap' dongle and an Airspy, and the Airspy is a step up.
I still use both though :) The Airspy does not make the 'cheap' dongle bad, but rather supplements it (2 radios are better than 1 :) )

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