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

wllmbecks at gmail.com wllmbecks at gmail.com
Sun Sep 6 18:28:48 UTC 2020


A good tool is to take a look at your symbols and mixer plots to verify if the (-q 2) is the 
right prescription for your particular device.    Start with (-q 0) and if you get TSBK's then
let it run for five to ten minutes before checking the reported frequency error and then
make your corrections accordingly. 

However, I have run the calibration utility on all of my Airspy devices using a GPSDO
as a stable and accurate frequency reference.    In my case, the corrections added to
the rx.py command line were (-q 0.1) & (-o 30) that results in an average frequency
error of no more than 10-20 Hz as reported by op25.

Even though the R2's and Mini's are TCXO based, they are far from perfect and do drift
the first five to ten minutes when starting up from cold.    So, this has to be taken into
consideration when running the calibration utility, or in making your PPM and offset
corrections in op25.   

Bill


-----Original Message-----
From: op25-dev <op25-dev-bounces at lists.osmocom.org> On Behalf Of op25 at zellners.com
Sent: Sunday, September 6, 2020 11:33 AM
To: op25-dev at lists.osmocom.org
Subject: Re: [op25-dev] rx.py Airspy MASTER GAIN Setting option?????


Quoting wllmbecks at gmail.com:

> 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.


The -q 2 for PPM of 2, is based on another programs output of its auto ppm.

I will try with 0 as well. But with the 2 it was producing audio etc..

>
> 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.

This is not a spur, or anything of the like. Disconnect the antenna,  
its gone. I am not any sort of OP25 guru etc.... but lets just say  
this ain't my first rodeo in RF.... :) :) ;)

> 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.
>

Op25 does already indicate its a live signal, just 20-40db down. I get  
audio, TSBKS, using a -v10 I get the slew of decoded TSBK data, with  
an occasional CRC error. Which is related to the low signal.

I am also not working from a guess on the frequencies of the  
system(s). Lets just leave it at that I've got more knowledge about  
what frequencies and towers these things are using that a certain  
other site and the FCC does. :) ;) ;)


> 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

I think in another setup I am about 10-12 for LNA, and 8-7 on the IF  
and MIX as I recall. As I played to max signal and reduce the noise  
floor. I'll have to find my note on it.. its not on this box.

I think the other post to disable what ever those modes are is the  
key.. which when its reset, ie: powered off and moved to another box,  
my OP25 setups is not that box since op25 is not Python3 compatible,  
yet, and so I move it to the 18.04, with the ulitimate goal to  
possibly move it to a Pi(s), which I was hoping to dump, but there is  
a bug in another program to use them there. I am guessing the firmware  
of the AS's resets it to use these modes??? Where as the other program  
stores those settings and likely restores them when it sees the same   
tuner... as it does this with RTL's...

We'll see what the tests do with the magic disable commands....





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