FM "ghost" signals on UHF

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jschall
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FM "ghost" signals on UHF

Post by jschall » Wed Dec 23, 2015 8:39 pm

Local FM stations (88-108 MHz) booming in on the UHF band around 330 MHz. Third harmonic?

This doesn't happen with my RTL-SDR.

Can anybody explain this, suggest how to avoid this?
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delta89
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Re: FM "ghost" signals on UHF

Post by delta89 » Thu Dec 24, 2015 12:42 am

I get the very same thing you have and just posted about that today. Lowering the IF GAIN and disabling the LNA keeps it to a trickle but wrecks the performance and purpose of the receiver! The 'popular bands' do not seems to suffer from this problem on the RSP; I do not have any problems with HF, VHF airband, or even UHF (Past 500 MHZ); with No images, RFI, and USB noise on these bands! Switch back to military UHF and the problems start.
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vsboost
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Re: FM "ghost" signals on UHF

Post by vsboost » Thu Dec 24, 2015 1:53 am

Try change the LO Frequency Plan to 168MHZ
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Tech_Support
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Re: FM "ghost" signals on UHF

Post by Tech_Support » Thu Dec 24, 2015 8:36 am

As already posted elsewhere on this forum:

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

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mbott
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Re: FM "ghost" signals on UHF

Post by mbott » Wed Dec 30, 2015 6:15 pm

This is the filter I'm planning on ordering after the first of the year: http://www.parelectronics.com/fm-broadcast.php. I already use their BCST-HPF to remove the am-band stations and it does exactly what it should.

--
Mike
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IW2NYX
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Re: FM "ghost" signals on UHF

Post by IW2NYX » Fri Jan 01, 2016 8:21 pm

@mbott :

you need a more performant notch filter, the one you pointed out has a min notch depth of "only" 25dB, mine has a min depth of 40dB / max depth of 60dB (tested on rf network) and does not fully solve the problem, see my "review" on this section of community forum.

In addition you have to deal with mobile phone network and TV signals, that happen to fall inside 250-420Mhz as images, at least in my case.

@Tech_support :

"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"


Maybe this "detail" should appear on the published technical information paper, because the image rejection in the range 250 -420 is subpar at best, you cannot fairly consider it as "covered"; in contrast the 0-60Mhz range seems to work like a charm.

IF the source of the images was just FM broadcast i would not be here to complain so much, but we have also mobile phone stations and TV transmitters signals (among the other) to filter out and there is no a reasonable way to do it.

I just had a look to the internal 250-420Mhz bandpass filter response curve, with its -8dB @100Mhz (FM) and -7dB @900Mhz (GSM/LTE) sadly does not come in help at all.

Just one question : there is space/hope for a future improvement working on API level or this problem has been already verified as not fixable ?
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