Quoting Graham Norbury <gnorbury(a)bondcar.com>om>: 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...

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 &lt;op25-dev-bounces(a)lists.osmocom.org&gt; 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 < <mailto:gnorbury@bondcar.com> gnorbury(a)bondcar.com&gt;gt;: gain 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...

Quoting Graham Norbury &lt;gnorbury(a)bondcar.com&gt;om>: I reran my tests on this again, using -N 'IF:,MIX:,LNA:' like above with 0-15 values. The spike for the CC, as a test, didn't budge at all, all 15, or anything in between, no changes. It is significantly lower to start -20 to -40db lower v. other programs and depending on antenna. Its almost like the gains are set to 0 or something and never change??? ? ? ? ? Even using a duck direct to the Airspy with another program results in much higher signals on its spectrum, and gains will adjust accordingly. Changes will occur in another program with adjustments to all 3 (outside its faked single slider mode, using just the REAL GAINS), gains shows changes in the signals and noise floor. In op25, not a bit of change at all in any thing. noise or signal. Like these are not being sent? Or not taking affect?? ? ? 15 for all, combos of values, even things like 50 for all three, no changes. I am not sure what to even try at this point? Other than missing lib when it was compiled??? I didn't have the airspy then or airspy pacakges installed when compiled.. does that change anything??? It seems to work regardless of that, it tunes, it will trunk, voice etc., just the gains don't seem to do a thing at all. IF there was something missing in this, I would expect it to fail to tune, audio etc.. Ideas?????

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. 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. 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. 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 screen. Bill -----Original Message----- From: op25-dev &lt;op25-dev-bounces(a)lists.osmocom.org&gt; On Behalf Of op25(a)zellners.com Sent: Saturday, September 5, 2020 6:23 AM To: op25-dev(a)lists.osmocom.org Subject: Re: [op25-dev] rx.py Airspy MASTER GAIN Setting option????? Quoting Graham Norbury &lt;gnorbury(a)bondcar.com&gt;om>: I reran my tests on this again, using -N 'IF:,MIX:,LNA:' like above with 0-15 values. The spike for the CC, as a test, didn't budge at all, all 15, or anything in between, no changes. It is significantly lower to start -20 to -40db lower v. other programs and depending on antenna. Its almost like the gains are set to 0 or something and never change??? ? ? ? ? Even using a duck direct to the Airspy with another program results in much higher signals on its spectrum, and gains will adjust accordingly. Changes will occur in another program with adjustments to all 3 (outside its faked single slider mode, using just the REAL GAINS), gains shows changes in the signals and noise floor. In op25, not a bit of change at all in any thing. noise or signal. Like these are not being sent? Or not taking affect?? ? ? 15 for all, combos of values, even things like 50 for all three, no changes. I am not sure what to even try at this point? Other than missing lib when it was compiled??? I didn't have the airspy then or airspy pacakges installed when compiled.. does that change anything??? It seems to work regardless of that, it tunes, it will trunk, voice etc., just the gains don't seem to do a thing at all. IF there was something missing in this, I would expect it to fail to tune, audio etc.. Ideas?????

Gents, Graham Norbury dug into the Airspy library sometime back and came up with a fix to enable/disable the various gain modes. I want to recall that he patched some of the existing code to allow additional args to control these gain modes. So it’s likely the following will only apply to the boatbod version of op25. The yellow highlighted text below in my rx.py command line is an example of how my R2 device was set to manual gain mode whereby the device responds to user specified LNA, MIX, and IF gain settings. ./rx.py --args "airspy=0,sensitivity=0,linearity=0,bias=0" -N 'LNA:15,MIX:15,IF:10' -S 2500000 -T trunk.tsv ……… Bill From: op25-dev &lt;op25-dev-bounces(a)lists.osmocom.org&gt; On Behalf Of Graham Norbury Sent: Saturday, September 5, 2020 7:25 AM To: op25-dev(a)lists.osmocom.org Subject: Re: [op25-dev] rx.py Airspy MASTER GAIN Setting option????? This happens when the agc is turned on. Bill Becks and I experimented with it a while back and found a way to enable/disable, but at this moment I don't recall the magic incantation. Graham On Sat, Sep 5, 2020, 7:23 AM &lt;op25(a)zellners.com <mailto:op25@zellners.com> > wrote: Quoting Graham Norbury &lt;gnorbury(a)bondcar.com <mailto:gnorbury@bondcar.com> >: I reran my tests on this again, using -N 'IF:,MIX:,LNA:' like above with 0-15 values. The spike for the CC, as a test, didn't budge at all, all 15, or anything in between, no changes. It is significantly lower to start -20 to -40db lower v. other programs and depending on antenna. Its almost like the gains are set to 0 or something and never change??? ? ? ? ? Even using a duck direct to the Airspy with another program results in much higher signals on its spectrum, and gains will adjust accordingly. Changes will occur in another program with adjustments to all 3 (outside its faked single slider mode, using just the REAL GAINS), gains shows changes in the signals and noise floor. In op25, not a bit of change at all in any thing. noise or signal. Like these are not being sent? Or not taking affect?? ? ? 15 for all, combos of values, even things like 50 for all three, no changes. I am not sure what to even try at this point? Other than missing lib when it was compiled??? I didn't have the airspy then or airspy pacakges installed when compiled.. does that change anything??? It seems to work regardless of that, it tunes, it will trunk, voice etc., just the gains don't seem to do a thing at all. IF there was something missing in this, I would expect it to fail to tune, audio etc.. Ideas?????

Setting the manual gain mode in op25 will take away AGC and more importantly will remove the false spike in the center of the spectrum. This is quite so that you can visualize the "Real" control channel from a figment of the Airspy's imagination. Note that a full reboot or removal and re-plugging of the SDR into the USB port will be required to clear the Airspy from previous linearity or sensitivity modes. Changing of the rx.py command line arguments and restarting rx.py will not clear the internal registers of the device, something that it took me a while to figure out. I would still recommend starting with (-q 0) while checking for the presence of TSBK's coming from the control channel, and/or verification of a legitimate p25 signal by observing the constellation or symbols plots. But I think you'll be ok in using FFT once you have the AGC gain modes disabled. Please let us know how you make out. Good luck! Bill -----Original Message----- From: op25-dev &lt;op25-dev-bounces(a)lists.osmocom.org&gt; On Behalf Of op25(a)zellners.com Sent: Sunday, September 6, 2020 9:29 AM To: op25-dev(a)lists.osmocom.org Subject: Re: [op25-dev] rx.py Airspy MASTER GAIN Setting option????? Quoting wllmbecks(a)gmail.com: Thanks... I think that may be the magic,airspy=0,sensitivity=0,linearity=0,bias=0, as those names are present in the other program, but hmm... I would expect since I nuked them in that program it would persist? I guess not.. I will try this during some tests today... have to work on a more mission critical option at first. Thanks....

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 &lt;op25-dev-bounces(a)lists.osmocom.org&gt; On Behalf Of op25(a)zellners.com Sent: Sunday, September 6, 2020 11:33 AM To: op25-dev(a)lists.osmocom.org Subject: Re: [op25-dev] rx.py Airspy MASTER GAIN Setting option????? Quoting wllmbecks(a)gmail.com: 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.. 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.... :) :) ;) 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. :) ;) ;) 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....

You should be good to go if you have 10 MHz reference clock available. The Airspy's accept anything from 10 MHz to 100 MHz external clock reference on the MCX connector. Don't ask me how the internals figure this out or if any configuration is necessary to specify the source reference. I haven't tried this but will go back to my shop and connect a 10 MHz reference to an R2 to see what if any affect it has on op25 receiving a live p25 system. Bill -----Original Message----- From: op25-dev &lt;op25-dev-bounces(a)lists.osmocom.org&gt; On Behalf Of op25(a)zellners.com Sent: Tuesday, September 8, 2020 10:05 AM To: op25-dev(a)lists.osmocom.org Subject: Re: [op25-dev] rx.py Airspy MASTER GAIN Setting option????? Quoting wllmbecks(a)gmail.com: 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... 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.