Spectrum Analyser Mixer Dissection - Page 4

By now, I am feeling depressed and bloody-minded enough to haul it all out and clean up, no matter what gets broken on the way. The (dc) short circuits in the shapes mean we only now discover that one of the diodes in the ring mixer is kaput  ie. about 1 ohm in either direction, only after we get it off the board. In the process, the corroded and bent back tab on the diode component breaks off, as does one of the others for good measure.

It's a diode ring as in "all the diodes chase each other around a kite-square". OK, maybe our mystery mixer modifier got it right, but it all depends on how you draw the darn thing. Do we need a diode ring where they all chase each other in a crossover connection, or not. I think we need the "crossover" type. It seems so obvious, but the one fitted was a non-crossover layout. Click here to see why it should not be so.

The board is about 1mm thick slippy feel teflon type stuff. From the 3mm width of the strips, I guess Er= approx 2.8, but (OK - you check!). Now that we have wiped off some solder, we can see that there is something not quite right on the edge of the groundplane.. WHAT..is going on here??

Its not so easy to lay on heat from the soldering iron into all that groundplane, but we suck off solder in small stages, until we can prise the corner up. Well OK. I'm doing this unlikely mix of laughing and starting to sob. My family think I am having some kind of breakdown (well I was ...sort of!).

Keep with the heat. Carefully now! Keep lifting the "extra" metal foil. Well..hello.., what have we here? Its got another stub hiding underneath. Its a microstrip directly over groundplane on the other side.

I am stunned. I just cannot believe what I am finding!

As the foil peels up, we see a pair of strip stubs shorted at the end, exactly as on the IF side, just ready to take a piece of hardline. So that is what I do, even if I don't yet understand precisely why this is a "good idea".

I am thinking there may be hope for the old mixer yet. Of course, I am also thinking that nobody tries to build mixers for themselves anymore. Its probably quite hard to find the little diode packs! Maybe I should let these printed strips just help a little ready-made mixer (eg - MiniCircuits® or somesuch).




Page 1 Special Mixer Mess (description of the problem)

Page 2 - Spectrum Analyser Mixer Dissection - Taking things apart

Page 3 - Spectrum Analyser Mixer Dissection - A closer look

Page 4 - Spectrum Analyser Mixer Dissection - Oh Yes - it is still full of surprises!

Page 5 - Spectrum Analyser Mixer Dissection - How it ended up

How We Finally Got it Right (and all the secrets too!)

The Result - Performance Traces

Some (not so good) Traces


Author : Graham Seale