[op25-dev] rx.py Airspy MASTER GAIN Setting option?????

wllmbecks at gmail.com wllmbecks at gmail.com
Sat Sep 5 12:26:17 UTC 2020

I didn't pay particular attention to the original post but a (-q 2) would really be unheard of with an Airspy device 
unless it is way out of calibration.  I suspect that the spike you're seeing in is not a "real' signal.

Although it's hard to reproduce on demand, I have run into conditions where my FFT plot displayed a spike at the 
center of the spectrum that turned out to be false signal.   Removing the antenna and setting the LNA and Mixer gain
controls to zero made no difference in the amplitude of the spike at the center of the spectrum.  

As a suggestion, retest rx.py using  (-q 0)  as a starting point but add (-X) to let op25 try to resolve tuning errors to
see if you're actually getting a live signal, and check to see that op25 indicates it's decoding trunk signaling blocks
and live traffic.

Raising and lowering LNA, MIXER, and IF gains should result in varying signal amplitude as observed in the FFT plot
Typically, on my Airspy devices the IF gain maxes out at about a setting of 8-10 as far as increasing signal amplitude
and whereby increasing IF gain only serves to raise the apparent noise floor and sometimes introducing unwanted
IM products.  But LNA and MIXER gains should play a very significant role in the signal level as observed in the FFT plot


-----Original Message-----
From: op25-dev <op25-dev-bounces at lists.osmocom.org> On Behalf Of op25 at zellners.com
Sent: Saturday, September 5, 2020 6:23 AM
To: op25-dev at lists.osmocom.org
Subject: Re: [op25-dev] rx.py Airspy MASTER GAIN Setting option?????

Quoting Graham Norbury <gnorbury at bondcar.com>:

> I'm not aware of a master gain parameter.   LNA, MIX, IF are the three gain
> controls.  All take value between 0-15.
>> ./rx.py --args 'airspy' -N 'IF:10,MIX:5,LNA:15' -S 2500000 -f
>> 774.55625e6 -v 7 -T P25PC.tsv -q 2 -V -2  -U -x 2 2>> airspy.log

I reran my tests on this again, using -N 'IF:,MIX:,LNA:' like above with 0-15 values.

The spike for the CC, as a test, didn't budge at all, all 15, or anything in between, no changes.

It is significantly lower to start -20 to -40db lower v. other programs and depending on antenna.

Its almost like the gains are set to 0 or something and never change??? ? ? ? ?

Even using a duck direct to the Airspy with another program results in much higher signals on its spectrum, and gains will adjust accordingly.

Changes will occur in another program with adjustments to all 3 (outside its faked single slider mode, using just the REAL GAINS), gains shows changes in the signals and noise floor.

In op25, not a bit of change at all in any thing. noise or signal.  
Like these are not being sent? Or not taking affect?? ? ? 15 for all, combos of values, even things like 50 for all three, no changes.

I am not sure what to even try at this point? Other than missing lib when it was compiled??? I didn't have the airspy then or airspy pacakges installed when compiled.. does that change anything??? It seems to work regardless of that, it tunes, it will trunk, voice etc., just the gains don't seem to do a thing at all. IF there was something missing in this, I would expect it to fail to tune, audio etc..


More information about the op25-dev mailing list