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I wanted to explore digital modes by connecting a Raspberry Pi via audio cables to a transceiver. However, I realized that thanks to the clock generator, the raspberry pi can produce RF signals directly. Adding a square to sine wave converter, a band pass filter and an amplifier, this could be used as the transmitter. There is good software in GNU Radio and PiTx. FM signals can be produced. As far as I can tell it is possible to generate SSB directly on the pi but I haven't understood yet how.

Q1: Is it possible to directly generate SSB on the pi?

On the receiver side, things look a bit more tricky. It requires a down converter and a very fast analog to digital converter. The good news is that they appear to be very cheap, e.g. R820T2

Q2: Would combining a cheap dongle such as the R820T2 enable me to operate in half-duplex mode with the Raspberry Pi?

I'm mainly interested in playing with the radio, so any band is fine. If I were forced to choose, I would prefer 50MHz.

Apologies for this open-ended question, any suggestions welcome.

M0HXS

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Yes, with some important qualifications, you can "directly" generate SSB on the Raspberry Pi. The rpitx software you mention does that, and in combination with an RTL-SDR type dongle you would indeed have a sort of half-duplex transceiver. So in short: yes (Q1) and yes (Q2).

Based on that exact combination, qtcsdr is an app that aims to make your Raspberry Pi into a amateur radio transceiver.

Of course, all the filtering that goes into both a good receiver and — more exigently — a legal transmitter are "sold separately"! This is the most important qualification.

I would assume that rpitx (and qtcsdr, which uses it) generates a much, much messier signal than even the most infamous CB amplifier driven hard. (At least your signal will be much quieter :-) Check out the QRPi shield — it's not just a low-pass filter like is often needed in a typical RF chain, but rather needs to be a full band-pass filter to reign in the Pi's spurious emissions.

As far as how the SSB generation actually works, that is a fascinating subject and I wish I knew too: How does rpitx generate arbitrary SSB data with a clock peripheral?

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  • $\begingroup$ Am I mistaken that with a square to sine wave converter and a band pass filter the Pi should be able to produce a decent signal i.e. all the harmonics filtered out? $\endgroup$ – Edgar H Oct 26 '16 at 8:53
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    $\begingroup$ @NegativeProbability Take a look at the QRPi shield design notes (pdf) linked at the bottom of the TAPR page — the designer found that "a LPF solves only the harmonic content attenuation and doesn't help against the broadband noise of the RF signal synthesized with the BCM2835’s 'General Purpose GPIO Clocks'". $\endgroup$ – natevw - AF7TB Oct 26 '16 at 20:55

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