SDR Musings

Here's a thought... for a very integrated SDR TX/RX. The idea sprung from the bi-directional mixer in the "Minima" TX/RX design plus the IQ and audio parts of the Softrock Ensemble TX/RX design. This is the idea:



Sorry for the bad photo...

The 20MHz fixed oscillator is divided by 4 to give the 0deg and 90deg phase signal for the encoder/decoder. The RX & TX mixers are the well known SDR DSB generators with audio in and out. The "Minima" balanced mixer is bi -directional (!) and interfaces on both TX & RX with the antenna through a LPF.

My guess is that this would take

1 transistor for the oscillator,

a dual JK flip flop for the diviered,

two CMOS switches for encode/decode and

a couple of FETs for the balanced mixer.

Plus a quad op amp for the audio in/out.

Pretty simple eh? and NO tuning coils or caps.

The balanced mixer is fed with I & Q signals from the local VFO/divider chain . But this could be an AD9854 synthesiser module controlled by an Arduino. The AD9854 outputs both I & Q (0 & 90deg) signals at the fundamental frequency - avoiding the need for the /4 Johnson counter. Unfortunately no one seems to make an Arduino module for the chip.