OP25 RTL AGC Implementation Maybe??

[simpilo]

I was trying to listen to a P25 PII system about 15 to 20 miles south. My RTL-SDR dongle is able to decode the control channel. Voice channels were very shaky. I didn't find a RTL AGC option in the README file. I believe that if RTL AGC is implemented I could drop the LNA gain and the AGC should pump the signal up enough to decode it better at 850-860MHZ... Thanks.

 

[kb5udf]

You don't mention what software you are using, but my general experience is control channels decode well/easily even with mediocre antennas/signal and a cheap sdr. Voice channels, IMHO escpecially for PII are more picky. I tend to either use a better SDR or a better antenna.

 

[kj6psg]

The RTL AGC doesn't improve the noise figure, it just makes the noise louder to bring the input I/Q stream up to full scale. Using an external LNA and lowering the tuner gain can improve dynamic range.

 

[simpilo]

You don't mention what software you are using, but my general experience is control channels decode well/easily even with mediocre antennas/signal and a cheap sdr. Voice channels, IMHO escpecially for PII are more picky. I tend to either use a better SDR or a better antenna.

Its in green "OP25" so yes it is mentioned left of the thread title.

The RTL AGC doesn't improve the noise figure, it just makes the noise louder to bring the input I/Q stream up to full scale. Using an external LNA and lowering the tuner gain can improve dynamic range.

Gotta know how to set it. There is a sweet spot to find. You may disagree but i don't care. I know it can work with AGC and turning down the gain just right. Just for UHF not so for VHF. Same works for ADS-B. AGC pumps it up while tuning down the RF gain brings the noise down. AGC doesn't pump it up a whole lot but enough to work with. Worked well for listening to a jail radio system with a indoor repeater. Hard to receive it but I used RTL-AGC and turned down the RF Gain. The result was a clean signal. I tune my antenna to resonate on the 850MHZ band. It helps.

 

[jamesxqiu]

Its in green "OP25" so yes it is mentioned left of the thread title.


Gotta know how to set it. There is a sweet spot to find. You may disagree but i don't care. I know it can work with AGC and turning down the gain just right. Just for UHF not so for VHF. Same works for ADS-B. AGC pumps it up while tuning down the RF gain brings the noise down. AGC doesn't pump it up a whole lot but enough to work with. Worked well for listening to a jail radio system with a indoor repeater. Hard to receive it but I used RTL-AGC and turned down the RF Gain. The result was a clean signal. I tune my antenna to resonate on the 850MHZ band. It helps.

simpilo, did you figure out how to do AGC with OP25 and RTL-SDR? Or at least how to change the gains other than just LNA gain? The way I know is through a command like: ./rx.py --args 'rtl' -N 'LNA:49'

 

[boatbod]

simpilo, did you figure out how to do AGC with OP25 and RTL-SDR? Or at least how to change the gains other than just LNA gain? The way I know is through a command like: ./rx.py --args 'rtl' -N 'LNA:49'

The parameter is "--gain-mode=1" (default is 0)