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Wednesday 27 December 2017

Homebrew SDR HF Transceiver - Adding the ADE-1 Mixers

The ADE-1 is a passive device with an RF and LO input and an IF output. Two ADE-1's are used with the RF port on each being fed with output from port 1 and 2 of the RF splitter respectively (Note: I changed the design of the RF splitter from that which I first published, here is a link to the updated page). The LO ports will be fed from one of the two outputs of the 74AC74 flip-flop respectively. The output of the IF ports goes to an audio frequency LPF (one LPF per port).

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

Source: EA4NZ
Below you can see a picture of the RF splitter, ADE-1's and audio LPF's. The 7805 and assorted capacitors in the top right is the 5V supply for the 74AC74.

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.

Next post: The Si5351

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