Hi all,
I'm not an RF engineer - as will very quickly become apparent - I freely admit that I really don't know what I'm talking about here. But.. this issue of the quadrature sample detecting at higher frequencies has been buzzing around my head for the last few days and I need to get it out. The analogy I see of the problem at hand is that in having active QSD switching you have a "reciprocating engine" which is hitting a revs upper bound - ideally one would want to move to something with "free rotation to be able to spin faster"; from piston to jet.
As I understand it (warning klaxon) the current multiplexer implementation is essentially arbitrary - i.e. capable of going to any switch position from any other as commanded - whereas the requirement here is for a fixed rotary sequence with the critical factor being phase lock. Early PAL colour decoder delay lines, RC timing, transistor chains, and voltage control all come to mind, but things get very messy very quickly.
I had wondered if simple logic hardware might provide a solution - something like a 4-bit shift register in a cascade or even flywheel loop configuration turning on fast transistors; but a random sample indicates those clocks max out even before the multiplexers. High speed logic gates in flip-flop or cycling through a simple truth table? PIO bit-banging? At 50MHz x4 even microcontroller clocks look slow... Hmmm.
I can't help but think that there "should" be a demod complement to a diode ring mixer or even bridge rectifier; it would be really elegant if the issue could be solved by 4 varicaps acting as both the switch *and* the integration capacitor. I've got Han's "poor man's PIN diode" alternative-use philosophy stuck in my head, in digging around I found some notes he published a few years back on "poor man's varicaps" (http://www.hanssummers.com/varicap/) - it would be very satisfying if a high-speed simple QSD could be implemented by a quadrant of FETs , or even LEDs - "no moving parts" - but that's very wishful thinking indeed.
Anyway, very happy to put right on all the misunderstandings above - I get it, if it was that easy it would be have been done - but just sharing random thoughts in the faint hope that might trigger a line of enquiry, but otherwise I'll get my coat...
73 Henry M7FYI
