Frequency Scanner experience

General SDR and HW discussion for anything not covered elsewhere
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nbfm
Posts: 12
Joined: Fri Nov 03, 2023 6:21 pm

Frequency Scanner experience

Post by nbfm »

Hi all, and especially srcjon!

I thought I'd write up a few notes on Frequency Scanner, specifically towards NBFM scanning. I've got years of experience with SDRs with various software, and even more years as a systems engineer for large scale RF networks.

Overall, I've found SDR "scanners" to be fairly problematic in a plethora of ways, and usually after an hour or so tend to give up as there are too many issues or idiosyncratic features.

With the Frequency Scanner in SDRA, I was pretty happy to see that it's fairly configurable. I was downright ecstatic when I found I could use high amounts of decimation coupled with high sample rates to get at least some amount of dynamic range - I live in a challenging RF environment with nearby (~200m) transmitters keying up from time to time, and this is within the frequency range of interest. Definitely living the near-far situation here.

At this time, I'm using Airspy at 10 MSPS, with decimation of 64. I would use 128 or 256 if they were available.

(I do have a SDRplay RSP1A that I got just a few days ago, but in using it in other software I'm finding odd behavior I haven't been able to characterize yet - I do want higher bit width SDRs again for dynamic range. 14 bits should be standard these days, and we should be seeing 16 bits as well. Perhaps the Jupiter SDR from Analog will help here...)

I use both LNA and Mixer AGC and set VGA gain on the Airspy to whatever works in the situation at hand - it's easier to use that slider than the THreshold knob.

I narrow down the bandwidth in both the Frequency Scanner window as well as in the NFM Demodulator down to US standards, and to lessen adjacent channel interference.

I set squelch to -100 in NFM Demodulator, as it seems that the Frequency Scanner is another control for the audio gate. If the gate opens in Frequency Scanner and we "stop" on a frequency, I want to hear it. Otherwise, it can stop and no audio will be forthcoming.

In Frequency Scanner, TH is set as appropriate for the set of channels and as noted previously, I leave it be for the most part and use VGA gain to compensate for conditions.

T delta s (time to settle) is set to 0 ms. I'm pleased that this works at essentially no delay whatsoever.

Power measurement time is set to the minimum of 0.1 s.

Trtx is a personal preference, I have it set to 0.2 s.

These settings work very well, with an apparent "scan rate" over 20 frequencies per second. Now, with power measurement being 0.1 I'm not quite sure how that winds up that way, but I've verified it. Even with the high decimation I use it could be that several frequencies can be sampled for power levels, but to be honest the frequencies go by so fast it's hard to tell. But I'm not unhappy about this!

Do I get a few unsquelch bursts? Yep, but that's power-based squelch for you. Does it cure my overload when the nearby transmitters key up? Not entirely, but it's manageable and much better than setting up a bunch of parallel NFM decoders. I'd prefer parallel NFM decoders all with CTCSS squelch, but I've found the near-far dynamic range issues too much to overcome for today's SDRs.

Ideas for enhancements:

• Feeding the NFM decoder with CTCSS/DCS values would be extremely helpful. There are share frequencies here and being able to understand who is transmitting would be great. Plus CTCSS/DCS should help with the squelch bursts

• Tracking a rolling average for a noise floor on a frequency basis could help in being able to programmatically set a power squelch level per frequency vs the overall THreshold setting (e.g., 8-12 db above the rolling average for that frequency).

Note, I haven't tried annotations yet but having a frequency description track all the way through the interface would be nice; at some point I'd like leverage the API to go headless; once things are set up correctly the GUI shouldn't be needed...

Good stuff, thanks!
Axel
Posts: 27
Joined: Mon Oct 30, 2023 4:46 pm

Re: Frequency Scanner experience

Post by Axel »

Thank you for sharing.

What is your experience with the scanner on Airband? I'm finding it very challenging to get the right mix of parameters so that I can scrape in the slightest microvolt of signal from weak stations while not being overloaded by transmissions from planes right overhead. I find that in such situations analogue scanners such as Uniden or Icom generally fare much better overall.
tbryant2K2023
Posts: 17
Joined: Sun Oct 22, 2023 11:24 pm

Re: Frequency Scanner experience

Post by tbryant2K2023 »

Getting it to work with airband has been a challenge. Even with airband filtered SDR and airband antenna, both AirNav RadarBox. My Grecom scanner still seems to pickup better. The challenge I have is that I don't live near a highly active airport, I have a small city airport and enroute traffic which at times are sparse in communications. And this means I have a range of signal strength and trying to find ideal settings is still being figured out.
Axel
Posts: 27
Joined: Mon Oct 30, 2023 4:46 pm

Re: Frequency Scanner experience

Post by Axel »

Ver 7.17.2 should fix most of these problems as it enables granular control on variables affecting reception on airband. One has to bear in mind that contrary to "hardware" scanners such as Icom, Whistler or Uniden which read discrete stored memories/frequencies/modes/squelch settings etc, SDRAngel scans a whole chunk of spectrum at a time. This makes it extremely fast but at the same time harder to fine tune in terms of settings that would apply to individual stations. Let's test Ver 7.17.2 and see how it goes.
srcejon
Posts: 297
Joined: Thu Jan 19, 2023 2:30 pm
Location: London
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Re: Frequency Scanner experience

Post by srcejon »

There's still no control over gain, I'm afriad.
Axel
Posts: 27
Joined: Mon Oct 30, 2023 4:46 pm

Re: Frequency Scanner experience

Post by Axel »

Hello again.

Following is first feedback on Ver. 7.17.2 — I'm using a MacBookPro running Catalina 10.15.7

1. Columns TH and Sq do not retain values typed into them. Looks like a bug.
3. Very handy to be able to specify CH bandwidth for each stored frequency. This works flawlessly.
3. The lack of fine control over gain remains a major drawback. One has to adjust the gain to one's best estimate between risk of overload and of missing weaker transmissions.
4. Overall the updated plug-in has improved the control over scanning parameters, with crisp audio.
5. Once the bug in point 1. above is fixed I am sure I will notice further improvements.
nbfm
Posts: 12
Joined: Fri Nov 03, 2023 6:21 pm

Re: Frequency Scanner experience

Post by nbfm »

Axel wrote: Tue Dec 05, 2023 5:15 pm 3. The lack of fine control over gain remains a major drawback. One has to adjust the gain to one's best estimate between risk of overload and of missing weaker transmissions.
This entirely the nature of any wide-band SDR.

The main way to overcome this will be with higher sampling bit widths - 14 and even 16. Dynamic range with 12 bits is challenging unless decimating a huge amount.

I'm hoping we start seeing the higher bit SDRs in volume in the next year or so. The chips have been out for some time now, but they haven't filtered down to the hobby level yet.
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