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From: firstname.lastname@example.org (Tom Bruhns)
Subject: Re: High-Q Microwave Parts?
Date: 21 Nov 2002 16:46:34 -0800
NNTP-Posting-Date: 22 Nov 2002 00:46:35 GMT
"xzhang3" wrote in message news:...
> 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....
Hey, wait a minute here! You may put more turns on the coil and get a
higher voltage, but you have NOT convinced me that you will then have
more signal _power_ to work with. If I'm right about that, then it
becomes a matter of building an amplifier with as low a noise figure
as possible, and matching the coil's output to that amplifier's
optimum source impedance (which in general will not be its input
impedance nor will it be the conjugate of its input impedance).
As far as making a "very good inductor" with a coaxial TEM
transmission line, I believe that you can, by using 77 ohm
air-insulated line made with smooth copper conductors, get a Qu of
about 107*D*sqrt(f), where D is the line's outer conductor inside
diameter, and f is the frequency in MHz. So if you used a 2-inch line
at 600MHz, you should be able to get a Q about 5000. Probably you
won't want to use a line any bigger than that. It will not be easy to
realize that high a Q by the time you connect it to anything.
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