Re: Noiseless Damping?
- From: Tim Wescott <tim@xxxxxxxxxxxxxxxx>
- Date: Fri, 14 Mar 2008 23:31:18 -0500
On Thu, 13 Mar 2008 09:02:31 -0500, Vladimir Vassilevsky wrote:
Tim Wescott wrote:No, Mr. arrogant-but-human, I'm mistaking you for someone who is asking
Vladimir Vassilevsky wrote:
There is a loop receive antenna. It is the LC circuit with the Q atI suggest you do some basic analysis, because I'm not sure that you're
the order of 100. It has a high resonant peak. The frequency and the
amplitude of the peak are sensitive to the component values,
temperature, particular parts, etc. I would like to damp the Q down to
something like 10, so the parameters would be much more stable and
predictable. This can be done by adding a series or a parallel
resistor to LC.
However the SNR of the antenna drops approx. proportionally to sqrt(Q)
due to the thermal noise of the damping resistor. I wonder if it could
be possible to implement the noiseless damping. Cooling down the
resistor is not an option.
One idea is to switch the damping resistor on-off with a high
frequency and a low duty ratio. Or to switch or vary the value of the
capacitor so the resonance peak will be dithered. It has many side
effects, though.
Any ideas, suggestions?
going to do yourself any good at all.
You are mistaking me for some other idiot, pehaps. Of course, I have
accounted for the tradeoffs.
for assistance. Just because you're Russian doesn't mean you have to act
like the stereotype.
But it's good that you're thinking.
Even if you had a dewar full of liquid nitrogen handy to keep your
loading resistor in, all the resistor is going to do is burn up signal
that would have otherwise reached the detector.
So what? The noise of the electronics is well under the other sources of
noise. I trade off a tiny part of signal to get the robust operation.
At no point in the
spectrum will the amount of energy reaching your detector be greater
with loading than without
And this is actually bad. The sharp resonance peak can slip off the
frequency of interest and pick up some interference. Hence the dynamic
range has to be increased by Q times, which is problematic.
BTW, I have tried the automatic tuning with the GIC, too. The simple
straightforward solutions are too noisy, the good solutions take too
many parts. Varactors are inapplicable.
-- on the contrary, the resistive loading will
just lower the response of the antenna where it had previously been
more sensitive.
The absolute sensitivity is not a problem. The S/N and the dynamic range
is what matters.
The only place that I could see such resistive loading being an
advantage is if you are attempting to receive a signal so broad that it
is filtered by the antenna -- then you may gain something.
In the addition to the above mentioned reasons, the gain and the phase
shift of the loaded antenna are very stable and predictable.
The only two choices that I can suggest are to use an RF amplifier that
itself loads the antenna down,
That was considered, too. The total noise balance is going to be worse.
I'm not sure how this is consistent with your other statements about the
total noise. Is this one of the things that you've analyzed so
thoroughly that you can diss me for suggesting that you analyze it, or is
it one of the things that you know you don't have to do your homework on
because Everything is Bigger in Russia?
or take the suggestion of one of your other respondents and change the
antenna to more efficiently capture energy over a broader band.
This misses the point. There is no problem with capturing enough of
energy.
Well, why didn't you say so in your original post? You're perfect, so it
can't be that you forgot to mention it.
Knowing your frequency of interest would help, too, and whether you're
interested in sky waves, ground waves, or whatever happens to impinge
upon your antenna.
At 300kHz, a one-meter capacitive probe connected to the gate of a JFET
works nicely -- it receives enough atmospheric noise that any energy loss
to inefficiencies is negligible, and it's quite small compared to a
wavelength.
--
Tim Wescott
Control systems and communications consulting
http://www.wescottdesign.com
Need to learn how to apply control theory in your embedded system?
"Applied Control Theory for Embedded Systems" by Tim Wescott
Elsevier/Newnes, http://www.wescottdesign.com/actfes/actfes.html
.
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