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dawsonjon / PicoRX

Build a SDR SW/MW/LW Receiver with a Raspberry Pi Pico
MIT License
214 stars 30 forks source link

QSD Question. #1

Closed boxerbomb closed 10 months ago

boxerbomb commented 10 months ago

Why is it that in this design you only have a single RF source going into the FST3253, while in other designs including your own, you also supply an inverted input signal to pin 7?

dawsonjon commented 10 months ago

Interesting question, there are a few versions of the qsd out there. You might have been refering to this design that uses a transformer as a phase splitter generating a normal and inverted copy of the signal. https://github.com/dawsonjon/OpenXcvr/wiki/Hardware-Design#quadrature-sampling-mixer

In this design I am using a differencing amplifier to achieve the same effect, switching between the inverting and non-inverting inputs of the amplifier instead. I think that the two approaches give equivalent results. The advantage is that it removes a (hand wound and potentially harder to obtain) transformer. I like the simplicity, especially in a stripped down project like this one. There are some designs that use both approaches, some even use 'proper' differential amplifiers too. I don't think that these changes will make very much difference to the performance though.

I got the idea from this project https://circuitsalad.com/2020/01/06/compact-si5351-based-sdr/ which is another very minimal receiver. Although this is one of the configurations in Dan Tayloe's paper https://www.norcalqrp.org/files/Tayloe_mixer_x3a.pdf.

boxerbomb commented 10 months ago

Interesting. Thanks for the information, I might try out a few variations.