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Subject: Re: High-Q Microwave Parts?
X-Newsreader: Microsoft Outlook Express 6.00.2600.0000
Date: Thu, 21 Nov 2002 12:51:09 -0600
NNTP-Posting-Date: Thu, 21 Nov 2002 13:20:23 CST
Organization: University of Illinois at Urbana-Champaign
Thanks for those nice folks who have replied!
Okay here is what we are actually doing.
We are building an NMR (nuclear magnetic resonance) probe to detect weak
RF magnetic field from the sample being observed. The circuit is
conceptually simple, which consists of a solenoidal coil wrapping around the
sample tube, and a few tuning capacitors to tune and match the probe to 50
Ohm pre-amplifier at 600MHz. Because the voltage across the coil is
proportional to the number of turns providing the RF magnetic field from the
sample is constant, we always want to put as many turns into the coil as
possible. So here comes the problem. Too many turns give you too much
inductance as well as parasitic capacitance that makes the self-resonant
frequency below 600MHz. Until now we have to limit the number of turns of
the coil to make the self-resonant frequency above 600MHz.
However if we can put a VERY GOOD inductor directly in parallel with the
sample coil, the total inductance will be smaller than that of the sample
coil, which makes it possible to put even more turns and to still tune it at
600MHz! If the new inductor has a Q a lot higher than the sample coil
(around 300 @600MHz), it won't consume much RF signal. Most of the extra
signal gained from the extra turns will eventually go to the pre-amplifier
because there is nowhere else to go....
We are about to try coaxial transmission lines as well as line
We are in search of the top line microwave parts in the market. Any
tunable-length transmission line or whatever that do the job will be worth
The inductance range we are looking at is less than 100nH. I doubt
piezo-electric parts, e.g. quartz, dielectric resonator, will work because
of their inductance is way too big.
Looking forward to hearing from you guys!
"Tom Bruhns" wrote in message
> If you "need ... Q AS HIGH AS POSSIBLE" then you better just go
> straight to the superconductors, because anything else won't give you
> a Q that is as high as you can get with superconductors. But if you
> can be more explicit about the Q you actually need, and what you need
> for range of adjustability, we may be able to help a bit more.
> Have you made a calculation of the maximum possible Q you can get with
> silver or copper condutors? Is it adequate? If it isn't, don't waste
> our time asking for something that can't possibly meet your needs.
> (Just _why_ do you think you need such a high Q in an "RF detector
> "xzhang3" wrote in message
> > Hi everyone,
> > I'm currently involved in a design of a 600MHz RF detector system.
> > really need an inductor about tens of nano-Henry, with a Q AS HIGH AS
> > POSSIBLE.
> > Silver coating wires are not good enough. We are going to try to
> > piece of transmission line, shorted at one end, as a inductor at 600MHz.
> > Could anyone let me know what companies sell the best-quality
> > components, i.e. Line Stretchers; Variable Phase Shifters; or whatever
> > tunable-length transmission line?? (Co-axial preferred because other
> > of the system are lumped.)
> > We really want to exhaust everything commercially available before
> > to helium-cooled superconductive wires/cavities. (Some people are
> > trying that, of course, they have got a lot of difficulties:(....)
> > Thanks a lot,
> > Mark
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