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EDIT: I have succeeded in getting the AM receiver to work, by replacing the AM Demod block with a Complex To Mag, and a Band Pass filter (this is noted as an alternative in the AM Demod docs).

Consequently, while I've proved that most of what I did was valid, I'm still interested to know what I did wrong in the use of AM Demod below, if anyone can spot my error.

I'm completely new to SDR, and I fear I'm making very basic mistakes. I want to receive AM transmissions in the air band, but so far my only success has been with FM public broadcast.

Here's the gnuradio companion flowgraph that is the best I've been able to come up with so far.

enter image description here

This is based on the example of the AM Demod block, described here: enter link description here

This, runs and outputs static in a moderately convincing way, but no meaningful audio. I live 1 mile away from my local airport, and their ATIS signal is on 126.1, yet, when I select that frequency, I simply get the same static.

I have also assembled a trivial waterfall diagram flowgraph which shows a trace at 126.1 too, which seems to suggest the signal is strong enough for this receiver, even with it's feeble 1-foot-long whip antenna.

I do get a warning message when the graph runs:

Using device #0 Realtek RTL2838UHIDIR SN: 00000001
Found Rafael Micro R820T tuner
[R82XX] PLL not locked!

However, I'm inclined to think this is not significant, since I get the same message when I run the FM receiver that works quite well.

Can anyone tell me if there's something wrong with the flowgraph, or the values therein? Or perhaps offer any other suggestions?

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    $\begingroup$ Hello Toby, and welcome to ham.stackexchange.com! $\endgroup$
    – rclocher3
    Commented Oct 11, 2021 at 23:42
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    $\begingroup$ I know I'm late to the party, but can confirm that "PLL not locked" seems to be a harmless message all of these similar devices issue when brought up, even with non GNU Radio based software. $\endgroup$
    – user21417
    Commented Feb 1, 2022 at 17:08
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    $\begingroup$ I can't explain why this worked when changing the blocks as you did, all else being equal, but I did not there was no frequency correction on the RTL-SDR Source block. Mine are off by more than 5 kHz at the aircraft band frequencies, which would put signal outside your low-pass filter range. $\endgroup$
    – Deepstop
    Commented Oct 23, 2022 at 12:48
  • $\begingroup$ Fair point @Deepstop, I was tuning by hand, but did, in later experiments, add a GUI control for frequency correction to make that aspect simpler. $\endgroup$
    – Toby Eggitt
    Commented Oct 24, 2022 at 15:06

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