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

wllmbecks at gmail.com wllmbecks at gmail.com
Tue Sep 8 16:42:24 UTC 2020


I just completed running a few tests with one of my Airspy R2's operating with CLKIN
from an frequency agile GPSDO.   The R2 works perfectly while the external reference
is set to 10 MHz but will not run at any other frequency.

The almost nonexistent Airspy literature states the R2 will operate with an external clock
reference from 10 MHz to 100 MHz.  However, from my tests is would seem that the user
must somehow configure the external reference frequency.   

Therefore, if the specifications are accurate, then one can only conclude that the default
external reference is set to 10 MHz where the R2 will initialize and operate when CLKIN
applied to the MCX input connector.

My test configuration with the external GPSDO reference tied to the R2 was tested 
with op25 configured to MPSCS RFSS 5-15 (hex) and whereby rx.py was operating at defaults
having removed (-q x), (-o x) and (-d x) arguments from the command line with op25
reporting a frequency error of + 41 Hz on the control channel.

I am fairly confident that the MPSCS base stations (repeaters) at the above referenced site are
all GTR8000's operating on external GPS referenced clock that if one were to make such 
assumptions that the reported frequency error would be zero or very close to it but I don't
know that I can accurately measure the site off air with my R2670B on external clock.

Further, I am not clear on just exactly how op25 measures frequency but want to recall that
it takes into consideration symbol rate and spacing of each of the symbols in the constellation?
Perhaps Max or Graham can shed some light on this aspect.   Still in all, 41 Hz is close enough for
government work that is stable and consistent as opposed to the R2 running on its own TXCO.


-----Original Message-----
From: op25-dev <op25-dev-bounces at lists.osmocom.org> On Behalf Of op25 at zellners.com
Sent: Tuesday, September 8, 2020 10:05 AM
To: op25-dev at lists.osmocom.org
Subject: Re: [op25-dev] rx.py Airspy MASTER GAIN Setting option?????

Quoting wllmbecks at gmail.com:

> A good tool is to take a look at your symbols and mixer plots to 
> verify if the (-q 2) is the

Thats why I use OP25 for the constellation etc. that other software  
doesn't provide. More accurate to tune.

The 2 ppm is accurate as its confirmed with other programs. Which  
start with a 0 and over time it is steady at 2ppm +- a little...

> However, I have run the calibration utility on all of my Airspy  
> devices using a GPSDO

What is the CLKIN expecting? I'd have to check to see that our GPS  
simulcast controllers put out a similar or compatible signal... and I  
might drag one to a site when I go to test with...

But the -q 2 setting is accurate as anything or more less, and nothing  
decodes, nice tight constellation.

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