The theory behind how the I & Q signals are produced are described in this excellent YouTube video by Craig, devttyS0. In short the LO outputs are produced by dividing the signal from the Si5351 by 4, there are two outputs from the 74AC74 at the LO frequency and one is 90 degrees out of phase from the other. When each one of these is mixed with the RF in the ADE-1 we end up with two outputs at baseband audio frequency, I & Q, one being 90 degrees out of phase from the other as shown below:
The audio LPF that follows each RF output I copied from EA4NZ
I note from devttyS0's video that the FST3253 device removes the need for the two ADE-1's by implementing a Tayloe Detector - something to be tried.
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