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I'm interested in doing some experimentation with custom modulation in the 915 ISM band.

Off-the-shelf radio modules use specific known modulations (FSK, QAM, etc.) so those won't work for my purposes.

I've never built my own transmitter before and wanted to know what parts are recommended to do this.

At the moment I am thinking of using an AD9850 for the IF of a superhet design, but I don't know what I should use for the up converter.

Any suggestions or warnings are appreciated.

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What you're describing sounds a lot like a software-defined radio :)

Now, first of all, you can buy these in various degrees of impressiveness.

Then, you could also consider getting an Quadrature modulator, i.e. a mixer that takes IQ baseband signals on one side and spits out RF signal on the other. Connect that to the cheap DAC you have lying around - your soundcard, and, if it doesn't come with an integrated LO, to an oscillator. Component-wise, analog devices is probably the go-to supplier of such mixers.

Regarding warnings: well, whatever you do, a mixer doesn't only produce the intermodulations you want, so be sure to check the spectrum and add band pass filters as appropriate. For quick checking, having a RX-only SDR, even if its only a dozen-of-bucks RTL-SDR, is a helpful tool :)

Regarding suggestions on how to generate the signals: no matter whether you use a commercial off-the-shelf SDR or a selfbuilt one, you now can generate your signal in software! That's way more flexible than always having to fiddle with hardware to generate a slightly different waveform. If you're in for a bit of fun on signal generation and analysis: maybe start with the GNU Radio Guided Tutorials; it's how I generate my custom modulations, it comes with an extensive toolbox of existing math and modulation building blocks, and there's plenty of applications out there that use GNU Radio to implement different transmission modes – be it something mundane as simple PSK31 transceivers, or something cool as a full DAB+ implementation. In fact, GNU Radio itself comes with an implementation of many DVB standards, and that's maybe something you'd want to have a look at.

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