As Graham pointed out, there is no “Master” gain control built into the interface specification

of Airspy devices.  However, In addition to manual gain control, there are sensitivity, and linearity modes

that apply AGC over the LNA and Mixer stages.

The “Master” gain control it seems is a feature in SDRTrunk whereby some values are applied to the

LNA, Mixer, and IF gains settings in addition to allow the end user input specific Tuner gain settings.

That said, there is no need to mimic overall RF amplitude values displayed on op25 FFT plots.

The fundamental difference between typical RTL SDR devices and the Airspy R2’s and Mini’s

is the much lower noise floor and the superior signal handling characteristics of the Airspy.

At the end of the day it’s all about Signal to Noise (S/N) performance of any given radio receiver

that defines overall usable sensitivity.  

Shown below are FFT plots captured from the same P25 system with an Airspy R2 followed by

a NooElec SMArt.  Observe the Airspy exhibits an average noise floor of approximately -80 while

the RTL exhibits an average noise floor of approximately -63 to -64 that clearly demonstrates

that the Airspy has a 16 to 17 dB advantage over the RTL device.

In addition to the lower noise performance,  the signal handling in crowed RF spectrums

of the Airspy is also a plus.  I happen to live in a rural area where there are few in-band 800 MHz

signals, but it should be noted that most RTL’s suffer from self-induced intermodulation interference

when operated in crowded RF spectrums.   

Airspy R2  ./rx.py --args 'airspy' -S 2500000 -N 'LNA:15,MIX:15,IF:10'

NooElec RTL ./rx.py --args 'rtl' --gains 'lna:49'

The third FFT plot below does demonstrate the relatively poor IM performance of an RTL SDR

captured from one of my remote instances of op25 tuned to a VHF P25 system where there

are numerous in-band signals.   In this case I am using a five element Yagi antenna aimed at the

distant RFSS in combination with minimum LNA settings to mitigate as much of the IM products

as are possible.

Signals shown in the plot below approximately -45 are IM products and are not real in-band signals

when at times there are legitimate in-band signals that do produce IM products that cause bit errors

to the desired P25 control or voice channels.

NooElec RTL ./rx.py --args 'rtl' --gains 'lna:49'

Bill

-----Original Message-----
From: op25-dev <op25-dev-bounces@lists.osmocom.org> On Behalf Of op25@zellners.com
Sent: Friday, September 4, 2020 5:35 AM
To: op25-dev@lists.osmocom.org
Subject: Re: [op25-dev] rx.py Airspy MASTER GAIN Setting option?????

Quoting Graham Norbury <gnorbury@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.

Ummm...mmm...hmmmmmm...

SDRTrunk exposes:

4 CONTROLS and 3 options in a drop down.

A guess is these "modes" are some sort of custom setup? As you can set CUSTOM MODE and you can then adjust IF,MIX,LNA separately... in SENSITIVITY or LINEARITY MODES it has a "MASTER" slider which I will take it that if there really are only 3 real gains must be a custom adjust to all 3 via that slider???? Which it doesn't reflect in the GUI that its adjusting them.... but thats not really relevant here.

I am trying to match performance from an RTL to the Airspy, and the plot on the spectrum is at least 20db down from the RTL (RTL Gain 49)..unless the Airspys are significantly less sensitive which SDRT doesn't seem to show.

I get -20db power level with the RTL and and the airspy is at -40db...and playing with those gains didn't seem to budge it... hmmm..

I'll give another whack tonight using only the 3 controls and see what I get...

Thanks...