Receiver overload?

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delta89
Posts: 2
Joined: Wed Dec 23, 2015 8:23 pm

Receiver overload?

Postby delta89 » Wed Dec 23, 2015 9:28 pm

Hello,

I am getting lots of VHF traffic on UHF with the RSP. Before you roll your eyes let me say these signals are not very strong and consist of business users and random FM BCB carriers scattered across the screen in the mil air band (240 - 380 MHZ). My cheap R820T2 is not having this issue and the spectrum is clean. I first noticed this problem in the 30 - 49 MHZ range, FM BCB stations would show up when they do not on the realtek. I live in a remote town with no road access and certainly no strong transmitters (our ONLY FM station is 50 watts) and a few low powered handhelds around town. (2-5Watts) I tested the RSP and the realtek together far out in the woods and the realtek was the winner; No images, no USB noise or random junk flowing down the screen and especially no out of band transmissions. I also do not get ANY of these problems when back home with the realtek!

My other question is why does lowering the RF front end gain in the above bands not making any difference in removing these out of band signals?
Setting the IF bandwidth lower for more dynamic range also does not help! What is going on here??

I was using SDR# 1361 for the test with the latest RSP driver (3.5 build 1222)
The realtek driver is from rtl-sdr.ru
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Tech_Support
Posts: 202
Joined: Mon Jun 01, 2015 7:00 pm

Re: Receiver overload?

Postby Tech_Support » Thu Dec 24, 2015 8:29 am

Hello delta89,
What you are seeing is not an overload or intermodulation issue. These are spurious responses that occur as a result of the use of a block frequency converter to cover the frequency range of 250 - 420 MHz. The block frequency converter uses a fixed LO and the FM signals mix with harmonics of the LO to produce an unwanted IF image. These unwanted images can then mix with either the fundamental or harmonics of the synthesizer to put the images within the bandwidth of the IF.

This only occurs when the block frequency converter is used which is at HF (up to 60 MHz) and in the range 250 - 420 MHz. Elsewhere a conventional zero IF or low IF architecture is used which is not subject to the same issues.

There are two ways to deal with this. Firstly under the Advanced tab in the ExtIO window, try unchecking 'auto' under the LO Frequency Plan heading and instead select one of the three different IF frequencies available to you. This may move the images away from the specific region of spectrum that you are interested in.

The other way is simply to add extra external filtering to reduce the FM signal present at the input to the block converter. For broadcast FM signals, external trap filters are readily available at a low cost.

Sincerely

SDRplay Tech_Support
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wd4elg-2
Posts: 7
Joined: Thu Dec 24, 2015 12:15 am

Re: Receiver overload?

Postby wd4elg-2 » Thu Dec 24, 2015 7:13 pm

Tech Support - are you describing options in SDR# build 1361 software? Or SDR Console?
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Tech_Support
Posts: 202
Joined: Mon Jun 01, 2015 7:00 pm

Re: Receiver overload?

Postby Tech_Support » Thu Dec 24, 2015 10:39 pm

This option is only currently available in the ExtIO plugin used by HDSDR and SDR# releases 1361 or earlier. It is not yet available in SDR Console, but we hope it will be in the near future.

Sincerely

SDRplay Tech_Support
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