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

iain macdonnell - N6ML ar at dseven.org
Fri Sep 4 18:24:12 UTC 2020


On Fri, Sep 4, 2020 at 11:10 AM <wllmbecks at 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 at lists.osmocom.org> On Behalf Of
> op25 at zellners.com
> Sent: Friday, September 4, 2020 5:35 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.
>
>
>
> 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...
>
>
>
>
>
>
>
>
>
>
>
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