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I need a FM receiver that is capable of receiving ultrasonic audio frequencies above the standard wide band FM range of 15 KHz. I'd like something that can receive signals up to 60 kHz if possible: I'm using JQAM software to generate QAM with a soundcard.

Are there any options for this that are relatively inexpensive? Could I build one? I am not a master at electronics, but if there were not too complicated schematics I could follow, that would be good.

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  • $\begingroup$ Do you need 0 to 60 kHz or just some smaller band of ultrasonic frequencies? $\endgroup$
    – Phil Frost - W8II
    Jul 23, 2018 at 21:04

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Well, it'd be trivial to modify an existing software FM receiver to have a larger audio bandwidth (typically that means a larger control loop bandwidth). The hardware cost would actually be below 10€ with a china-imported RTL-SDR dongle.

For example, GNU Radio comes with a simple, parameterized FM receiver and you can set that to basically any bandwidth you'd like. In fact, it comes with three from which you can choose:

FM demodulator chains

I've quickly built this for you, in hopes it'll be helpful at understanding the ideas. You shouldn't be expecting the same output from the three, but I hope the whets your interest in SDR; yes, that's three different FM receivers; and yes, that's Not Much WorkTM to have something to play with.

Note that the question really is where you'd get the FM signal, and what that signal actually should contain – from the top of my head, very wide FM doesn't make much sense in many potential radio channels, signal model-wise, so I'm a bit curious what your application is. Please don't build yet another PSK-in-Audio-over-FM transceiver. These things were spectrum-wise a bad idea from the start, and given the existence of SDRs there's no excuse to build one in 2018 (as you don't need to pack your data into audio for FM transmission to use affordable hardware anymore).

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  • $\begingroup$ Thanks for the detailed response! You got it right (almost). I am using JQAM software, so it uses QAM instead of PSK. I am only using a simple VCO with 10dBm and I don't plan on amplifying it, so I'm not really concerned about spectrum. You talk about packing the data into audio. So would using a QAM scheme in SDR somehow be more effecient than just using it with an audio FM transmitter? I got the impression it's doing more or less the same thing. $\endgroup$
    – Synaps3
    Jul 23, 2018 at 17:41
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    $\begingroup$ Encapsulating anything in an FM is absolutely not the same as sending it directly! Yes, putting QAM into the audio input to an FM transmitter is very wasteful in terms of bandwidth for a given data rate, and in terms of power needed for a given error rate. That's the reason noone but Hams do such things anymore. It's mathematically bad and technically obsolete! $\endgroup$
    – Marcus Müller
    Jul 24, 2018 at 0:19

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