On Fri, Sep 4, 2020 at 11:10 AM <wllmbecks(a)gmail.com> wrote:
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.
Er, no.. it means that *after all of the gains have been applied*, the
noise floor measures differently. As you pointed out above, then apparently
immediately forgot, it's SNR that matters. Eyeballing your plots, it seems
like the SNR is similar - somewhere in the 35-40dB range (can't tell more
than that, given the resolution). Note that the vertical scale is not the
same between the two plots. Also, the RF frequency is not the same, so
that's probably not a fair comparison either.
~iain
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(a)lists.osmocom.org> On Behalf Of
op25(a)zellners.com
Sent: Friday, September 4, 2020 5:35 AM
To: op25-dev(a)lists.osmocom.org
Subject: Re: [op25-dev] rx.py Airspy MASTER GAIN Setting option?????
Quoting Graham Norbury <gnorbury(a)bondcar.com>om>:
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...