Shifted freqency values

[László Monda]

Hi guys,

I purchased an RTL-SDR v4 with a dipole antenna from AliExpress.

I've installed Gqrx 2.15.8 and noticed that an FM radio station that is supposed to be at 97.7 Mhz is receivable at 53.1 Mhz in Gqrx. I think the whole frequency range is proportionally shifted.

Any ideas why this is happening and how to fix it?

Thanks in advance!

- Laci

[Charlie Hotel]

Hi Laci

At this point, many things may be the cause! (HW problem, LO/upconverter settings in GQRX, saturation and harmonics generation,…)

Is the station also received at 97.7MHz or just at 53.1? Are you receiving anything in the 88-108 range? Are you receiving the whole FM band in the 50’s or just that particular station?

Cheers

Bernard 

Le 27 févr. 2024 à 03:15, László Monda <la...@monda.hu> a écrit :

Hi guys,

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[Charlie Hotel]

Also make sure direct sampling is not activated… (somewhere in the device chain….)

Bernard 

Le 27 févr. 2024 à 03:15, László Monda <la...@monda.hu> a écrit :

Hi guys,

--

[László Monda]

Thanks for the quick reply, Charlie!

I've checked the FM radios extensively; the entire FM band is in the 50s.

See the frequencies per radio according to Gqrx leftward of "->" and the actual frequency rightward of it.

53.1, 54.9 -> 97.7 (Hir FM Kecskemet)
54.2 -> 98.8 (Petofi radio)
55, 57.1, 58.6, 58.9, 59.2 -> 104.9 (Kossuth radio)
55.1 -> 103.3 (Retro radio)
55.4 -> 106.4 (Danko radio Tiszakecske)
57.2, 57.5, 58.1, 58.7 -> (105.9 Bartok radio Csengod)

I see the following in the 88-108 range. Oddly, when stepping through this range, Mhz by Mhz, the received signals don't seem to change.

As for direct sampling, to my knowledge, it's not enabled according to the following settings:

Another oddity: See this station at 54.9 Mhz:

When I increment the "4" number of "54.895.000" Mhz by rolling up my mouse wheel over it and then select 54.9 Mhz, the station is not there anymore:

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[Richard Bown]

In the "input controls" is LNB offset LO set to ZERO, and also in receiver options is the hardware frequency set to Zero

Both should be

Richard

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[László Monda]

On Wed, Feb 28, 2024 at 12:37 AM Richard Bown <ric...@g8jvm.com> wrote:

In the "input controls" is LNB offset LO set to ZERO, and also in receiver options is the hardware frequency set to Zero

Both should be

Your reply seems to be cut off. Would you please resend your email?

 

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[Richard Bown]

Ehh ????

I only sent the two lines

Richard

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[László Monda]

Sorry, I misread your reply!

> In the "input controls" is LNB offset LO set to ZERO

It's already zero:

> and also in receiver options is the hardware frequency set to Zero

I can't see how. The lower limit of my RTL-SDR v4 seems to be 24 Mhz:

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[Sandor Fellegi]

This is not software error. RTL-SDR very simple hardware, no bandpass filters in input. 8 bit SDR processing, it can be easily overdrive, the result is false RX. Suitable antenna and input filters makes it better.

--

[László Monda]

Thanks, Sandor!

Are you sure that the RTL-SDR dongle is not the culprit? I want to figure it out quickly because if it is, I'll try to send it back to the merchant.

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[Dave B]

Hi.

For comparison, this is what I have for BBC 3 Counties FM (104.5MHz) here in the UK, using a RTL dongle.
(Antenna indoors, so lots of QRM spikes, but it works OK.)

Receiver and Input settings.



The spectrum and waterfall.



The 271.010kHz value, is the IF offset from the RTL's LO setting.  You very rarely need to have the
"Receiver" working at or straddling the 0Hz IF, where the Dongle LO is tuned to.

This is using GQRX v2.16 on Linux, using a v3 dongle from RTL-SDR.com via eBay.
CPU usage about 11%, with Thunderbird email client also running (that I'm typing this into.)

(I just noticed, I had "Hardware AGC turned off.  It works better turned on!  For me anyway.)

Perhaps something here helps.

Best Regards.

Dave G8KBV.

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[Charlie Hotel]

Hi Laci

Looks weird…

Have you tried your dongle on another software like sdr++?

Make sure your settings are as per Richard reply:

LO setting to 0, offset frequency to 0 (or anything less than 600khz ).

Cheers

Bernard 

Le 28 févr. 2024 à 00:56, Richard Bown <ric...@g8jvm.com> a écrit :



Ehh ????

I only sent the two lines

Richard

On 27/02/2024 23:47, László Monda wrote:

On Wed, Feb 28, 2024 at 12:37 AM Richard Bown <ric...@g8jvm.com> wrote:

In the "input controls" is LNB offset LO set to ZERO, and also in receiver options is the hardware frequency set to Zero

Both should be

Your reply seems to be cut off. Would you please resend your email?

 

Richard

On 27/02/2024 23:27, László Monda wrote:

Thanks for the quick reply, Charlie!

I've checked the FM radios extensively; the entire FM band is in the 50s.

See the frequencies per radio according to Gqrx leftward of "->" and the actual frequency rightward of it.

53.1, 54.9 -> 97.7 (Hir FM Kecskemet)
54.2 -> 98.8 (Petofi radio)
55, 57.1, 58.6, 58.9, 59.2 -> 104.9 (Kossuth radio)
55.1 -> 103.3 (Retro radio)
55.4 -> 106.4 (Danko radio Tiszakecske)
57.2, 57.5, 58.1, 58.7 -> (105.9 Bartok radio Csengod)

I see the following in the 88-108 range. Oddly, when stepping through this range, Mhz by Mhz, the received signals don't seem to change.

As for direct sampling, to my knowledge, it's not enabled according to the following settings:

Another oddity: See this station at 54.9 Mhz:

<image.png>

When I increment the "4" number of "54.895.000" Mhz by rolling up my mouse wheel over it and then select 54.9 Mhz, the station is not there anymore:

<image.png>

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[Richard Bown]

I suspect the chances of the Merchant accepting responsibility are very low,check if the offset is constant, try some other known stations,

There are plenty of know signals , Ham Radio Repeaters, for example that you can determine the offset from, and IF the offset is constant, use the LNB offset to correct the displayed frequencies.

Probably a simpler solution than trying to get a replacement , especially if the real source is in China

GL

Richard

On 28/02/2024 09:33, László Monda wrote:

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[Charlie Hotel]

You could try the dongle on a other PC/SW…

Bernard 

Le 28 févr. 2024 à 12:20, Richard Bown <ric...@g8jvm.com> a écrit :



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[Richard Andrews]

Maybe I am wasting my breath, but it would be good to know the OP's set-up. 

In house, outside, antenna used, grounding, local surroundings, desktop, laptop?

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[László Monda]

I've figured it out, guys!

I originally installed the gqrx-sdr-2.15.8-1build1 Ubuntu package on Linux Mint, exhibiting the issues mentioned.

This time, however, I've installed Gqrx-2.17.4-x86_64.AppImage from GitHub releases, and the frequencies are completely accurate now.

The Ubuntu package is messed up. Where should I report it?

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[Richard Andrews]

Http://packages.ubuntu.com/mantic/gqrx-sdr

Look for bug reports on the right. Tell them where you got the package too.

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[László Monda]

Thanks! Just reported the issue.

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[Richard Andrews]

Maybe next time you can install from source for such things? 

It's not that hard, only difficult because you need to know the spells involved.

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[László Monda]

I go crazy if I install every software I use from the source. It's not for everybody, and I haven't encountered such a case yet.

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[Richard Andrews]

Well at least it's working and the fun can begin.

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[László Monda]

Absolutely! Thanks to everyone!

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