Having convinced myself that the Si5351 can directly drive a pair of ADE-1 mixers, I decided to prototype the mixer stage of the receiver. The main goal was to confirm that the mixing action is working with this set-up, and to make some measurements of the outputs. Here's the circuit diagram:

And here's what the build looks like:

In order to make some initial measurements I drove the RF input with an LC oscillator I happen to have built previously. This generates a ~120 MHz tone with an amplitude of 400 mV peak-to-peak into 50 ohms. The LO inputs were driven directly by the Si5351 set to generate quadrature outputs at ~110 MHz.

With this set-up the main mixing products are at ~10 MHz and ~230 MHz. In the final receiver I'll only be interested in the first of these, so I decided to take a look at just that component of the IF outputs by enabling the scope's 20 MHz bandwidth limit. Here's what that looks like with the inputs AC coupled:

This seems to be working pretty well! I expect a 6 dB loss from the splitter and a further 5 dB loss from the mixer, so for a 400 mV p-p input I'd expect 113 mV p-p outputs, which is very close to what I'm getting. Further, the scope is measuring a phase difference of close to 90 degrees between the outputs as desired.

One wrinkle is the small difference in amplitude between the outputs of around 3 mV. I expect asymmetry here to eventually show up as degraded receiver performance. I don't know what form that will take or how significant it will be, but for now I'll assume it won't prevent me getting something out of an initial build.

Finally, I note that I also measured the DC offset of each output to be around -4 mV. Such offsets present a challenge for zero-IF designs since they are within the band of interest of the IF signals. Knowing just their approximate magnitude is hopefully a first step towards me understanding how I might handle this.