ARPS traffic heard well above 144.800MHz (+13.310kHz, actually)

[Juan Carlos Borrás]

Hi all,

I'm using gqrx-2.2.0.137-eca0ea7 with libgnuradio-fcd3.7.3 and a Funcube Dongle Pro (not the most recent Pro+ version).

I tried to replicate Csete's demo on youtube decoding my local APRS traffic and I have to say that it works beautifully.

But, I'm tuning at 144.813310MHz and not 144.800Mhz. That's 13.310kHz above the official APRS frequency for VHF in IARU-R1.

To me it looks like a hardware issue with the FCD but I'll appreciate a second opinion.

Thanks in advance,

jcb!

[Simon Kennedy]

Hi Juan Carlos,

my guess is that you have not adjusted the frequency using the Frequency Correction box. FCD and RTL dongles are not exactly on frequency and you have to adjust the frequency manually. 

Try putting in a correction of -30 ppm and you should notice a small shift in the received frequency. Then, adjust the setting until the APRS traffic is exactly on 144.8.

Regards

Simon.

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[Juan Carlos Borrás]

Simon,

Thanks for the answer. That would explain also why satellite beacons appear in places that can't be explained by the Doppler effect only...

Cheers,

jcb!

[Robin Gape]

Juan,

my recollection is that there are 2 sets of FCD frequency offsets. For the early batches the offset was around 100ppm (parts per million), and for later FCDs the offset was around 24-30ppm. (For an FCDPP the offset is around 2ppm.) All these offsets arise because the master oscillator is not exactly on frequency, for whatever reason, when mass-produced.

73,

Robin, G8DQX

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[Juan Carlos Borrás]

Robin,

...which means I got one of the later batches (I recall not being exactly an early bird).

Now I need to minimize the missive spike in the middle of the spectrum...

Thanks for the follow-up.

73,

jcb!