SDR# Tetra PlugIn frequency changes independently

[MikeTheListener]

Hello all,
since today the receive frequency changes by itself when receiving Tetra with the Tetra PlugIn from SDR#.
Yesterday everything was still normal, since today the PlugIn leads apparently a life of its own.
If I set the frequency 426,335.5 MHz everything is still OK. If I now activate the tetrademodulator,
the frequency changes automatically to 426.337.833 MHz (and the voice is only distorted).
I have SDR# in version 1855, with version 1732 this does not happen (just tested).
Does anyone have an idea what this could be?
Is there some kind of AFC (automatic frequency control) that is active ?

73 Mike

 

[Ubbe]

There is a AFC you can enable but it seems that you receiver isn't calibrated to be frequency accurate. The frequency you should enter and use are probably 426.3375 but you receive that frequency the best when you set the receiver to 426.3355 which is 2KHz too low. Set the receiver frequency correction (ppm) to -4 or -5

/Ubbe

 

[MikeTheListener]

Thanks for the tip Ubbe,
I will try. What surprises me only, it has yesterday, and the days before, everything worked without problems (frequency deviation).
As I said the frequency deviation makes it only with Tetra reception, as soon as I turn off Tetra decoding (or select FM-N) the QRG
remains stable on the set frequency.

 

[MikeTheListener]

It was the AFC, for some unknown reason it was active.
However, I did not (knowingly) switch it on. Anyway, now it works again.

 

[Ubbe]

now it works again.

But if you still have to enter 426,335.5 then you are still not frequency calibrated. When you enable AFC it corrects any frequency errors you have.

/Ubbe

 

[MikeTheListener]

426.335.5 MHz is the exact frequency it is broadcasting on. The signal is now clean again.

 

[Ubbe]

No tetra radio or tetra system can use that frequency as it doesn't match the channel steps allowed in tetra.
The closet it can be are 426.33625 using a 6,25KHz step size. But tetra are always using a 25KHz channel bandwidth so that seems a bit unusual for a frequency in tetra. It looks more probable to be 426.3375 or 426.325

The tetra plugin will decode the data that contains the channel number. Are that the frequency that the tetra plugin displays in the Main: and Current: values?

/Ubbe

 

[MikeTheListener]

Are that the frequency that the tetra plugin displays in the Main: and Current: values?

Yes Ubbe you are right, a typo has crept in here ..... SORRY. The correct frequency should be 426.3375 MHz.