Re: ADC "stacking"

Hello John,


Just summing wouldn't give the bandwidth I need, either. It's sort of
the dual of the ADC problem: staggered adc's need fast s/h's, and
staggered dacs need a fast gated multiplexer.

I was thinking diode current steering, maybe some 0.04 pF chip
schottkies on a hybrid substrate. I know a guy who makes hybrids, and
he owes me some favors.


Sure, you'd have to decouple from the DACs and do current summing or something like that. That should be possible with hotshot RF FETs in common base configuration, preferably the ones without protection diodes. With stripline and matched impedances it could possibly be done on a good FR4. Last time I looked at 5GHz telco stuff they had FR4 in there. Might have to be milled out under the diode.

Another option could be Pi-filter coupling. The DAC or transistor is the input capacitor and they'd all have a common output capacitor. Of course this would have to be nicely matched or it'll ring like crazy. Then there are hybrid couplers or even simple resistive combiners. In the microwave world they use these to combine dozens of channels.

I miss hybrids. Had done a few of them (thickfilm) but then the technology fell from grace and became expensive.


I sort of know how to do that already (start an lc oscillator
asynchronously, but phase-lock it to a good crystal clock) but I've
only done that at 50 MHz or so. I'm thinking a triggereble coaxial
ceramic resonator oscillator at 4 GHz as the starting point.


When you do that the LC would have to catch up to the phase of the crystal oscillator. The resonator sounds like a good idea. Something that acts like a pendulum, you let it go and it wants to stay at it's characteristic frequency and keeps going as long as energy is regularly replenished.


That'll be two beers :-)

Any time. Alcohol lubricates brainstorming, up to a point at least.


Gordon Biersch under the Bay Bridge used to be a good place. Except that it's kind of noisy in there.

Regards, Joerg

http://www.analogconsultants.com
.