Re: Prob. a very basic Q re: capacitors

Joel Koltner wrote:
Hi Joerg,

Thanks in advance for your advice here; as usual it's quite good.

"Joerg" <notthisjoergsch@xxxxxxxxxxxxxxxxxxxxx> wrote in message news:n5Ojk.9186$L_.6160@xxxxxxxxxxxxxxxxxxxxxxx
Question: What kind of switch? Relay?

MMICs... stuff like HMC349s. There are many of these paths and of course it's supposed to be on a tiny little box, so relays are just too big.

What dynamic range do you need?

120-140dB, give or take -- upper end is typically at 10-20dBm, and you'd like to still be able to hear something down at -120dBm.


Ok, sound like normal comms receive gear then, or a good scanner.


What noise figure?

3-6dB, although the upper levels would probably require a lot of justification that it can't get any better without huge price/power/size/etc. increases.


3dB almost requires a common preamp before the switch to overcome switch losses but that isn't too expensive.


How much mismatch?

1.5:1 ballpark -- 2:1 would probably still be OK.


But hopefully not during switching times?


Any sacrifices ok in those domains?

Mainly price :-) -- hence the viewpoint of doing this more for the educational value than any return on investment.

Additionally, specifications are "typical" -- it's unlikely that anyone would complain if, e.g., we miss the noise figure or isolation or VSR, etc. over various narrow frequency ranges. (E.g., diplexer crossovers, perhaps...)

Production quantity per year?

High hundreds, in all likelihood.


That sure puts a discrete solution into the educational domain.


Then I'd build a T network each and ramp the current.

Yeah, but you need a different current for the leg of the T than the arms if you don't want large mismatches, right? (And the arms need to be at least reasonably matched...) That too would tend to suggest to me a microcontroller/DAC approach where you can calibrate the DAC output on a unit-by-unit basis to get the best SWR.


Why would a mismatch during switching matter? In the off state the receiver input see a pretty high-Z anyhow. Else you'd sacrifice quite some noise figure whether fully integrated or discrete.


I really need to start building some test boards for this sort of thing (RF switches, attenuators, amplifiers, etc.) and posting the schematics/results on ABSE for commentary rather than just using someone's pre-packaged device all the time...


Yes, but don't use a.b.s.e. The wonderful NY attorney general has caused most ISPs including mine to kill it. I wrote to him but, as expected, he did not care to answer. That guy must really feel like a hero now, having taken a sledgehammer and smashed one of our windows. I guess a.b.s.e. must be pretty low in US traffic these days.

--
Regards, Joerg

http://www.analogconsultants.com/

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