Noiseless Damping?

De-tune the antenna?

-- Paul Hovnanian snipped-for-privacy@hovnanian.com

----------------------------------------------------------------------- Procrastinators: The leaders for tomorrow.

What's the frequency?

1. If this is below, say 30 MHz, ambient noise could well be high enough that the resistor doesn't do much additional harm, especially if you stop at Q=10.

1. You can synthesize a "cold" resistor using a super-low-noise semiconductor, if you get the impedances right. Numbers like 40K might be possible. But a perfect resistor still eats signal energy.

2. Or you could turn the loop into a bandpass filter; add a small additional LC resonator to make a double-hump resonance. Aren't you a filter guy?

4... Oops, latte is gone, back to work.

John

If possible you can lower the L and increase the C both by a factor of

5 or so.

Cheers,

Joop

I have it on good authority and experience that "Antennas make poor Filters..."

-mpm

I thought that, too. Unfortunately, the antenna efficiency is too low so the resistor noise is dominating at Q = 10.

Do you mean something like a small value resistor bootstrapped by the active circuit? Thank you for the good idea! That's what I am going to try.

The values appear to be inconvenient for the passive inductors. GIC cound be the option, however got to watch for the noise.

Thank you for your advice, John.

Vladimir Vassilevsky DSP and Mixed Signal Design Consultant

.

ext -

Couple the loop more tightly to your receiver input?

Mark

Hello Vladimir,

Your information is very limited but is it an option to generate the resistance by active parallel Feedback? As the feedback resistor is large enough, the impact on noise performance is minimal, but the effect is larger because of the gain of the input amplifier.

Maybe you can do something with the size of the loop. When you can extend the loop in a third dimension, inductance reduces; hence current noise of amplifier will generate less noise. Try to use as much as volume you have available.

Best regards,

Wim PA3DJS

(Dutch).

snip duhhh

see US2787704 .......

Actually I have to go the opposite direction: generate a large resistance from the small resistance using the positive feedback (bootstraping or something like that).

For this application, the noise of the input amplifier is not an issue; the dominant component is the thermal noise of R. The absolute values of the signal and the noise are not critical. The goal is to optimize the S/N of antenna while keeping the Q at 10.

Understood. The energy caught in the loop is approximately proportional to the volume of the loop. However we have what we have.

Thank you, Wim

VLV

If you had a high-impedance noiseless amplifier with a high inverting gain G, and use a negative feedback resistor R, the composite input impedance looks like R/(G+1), but has the current noise of the high-value R, so the apparent resistor noise temperature is below room temp. I've seen nuclear-sensor amps that use feedback transformers, to make a preamp that looks like a 50 ohm load to the detector but has much less Johnson noise; I may have a paper around here somewhere.

You may as well combine the fake "cold" resistor with the preamp, namely design a preamp that kills your Q to the desired extent and looks like a cold resistor and has a basically low noise figure.

Some of the phemts have noise figures of 0.4 dB, equivalent to about

28 Kelvins, when properly matched. They tend to get noisier at low frequencies, for certain values of "low."

Hey, how about this?

John

Would a parallel loop that intercepted the flux of the main loop lower the Q if this parallel loop were terminated?

Sure, but the termination will be a noise source.

John

I suggest you do some basic analysis, because I'm not sure that you're going to do yourself any good at all.

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. At no point in the spectrum will the amount of energy reaching your detector be greater with loading than without -- on the contrary, the resistive loading will just lower the response of the antenna where it had previously been more sensitive.

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.

The only two choices that I can suggest are to use an RF amplifier that itself loads the antenna down, or take the suggestion of one of your other respondents and change the antenna to more efficiently capture energy over a broader band.

--

Tim Wescott
Wescott Design Services
http://www.wescottdesign.com

Do you need to implement control loops in software?
"Applied Control Theory for Embedded Systems" gives you just what it says.
See details at http://www.wescottdesign.com/actfes/actfes.html

Based on your information, that is not obliged. Generating a certain low noise input impedance/resistance (to damp or load a circuit) can be done with or a low value resistor with series feedback or a large resistor with parallel feedback.

I did the same in a 8 MHz circuit to generate a prediscribed load for a 2 resonator LC filter (to avoid resonance and keep the curve).

Best regards,

Wim PA3DJS

(Dutch)

You are mistaking me for some other idiot, pehaps. Of course, I have accounted for the tradeoffs.

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.

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

The absolute sensitivity is not a problem. The S/N and the dynamic range is what matters.

In the addition to the above mentioned reasons, the gain and the phase shift of the loaded antenna are very stable and predictable.

That was considered, too. The total noise balance is going to be worse.

This misses the point. There is no problem with capturing enough of energy.

Vladimir Vassilevsky DSP and Mixed Signal Design Consultant

Drat! Foiled again.

Thermodynamics proves that the universe is cruel.

John

No, Mr. arrogant-but-human, I'm mistaking you for someone who is asking 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.

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?

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

What a shame.

I used to be of better oppinion about you, Mr. Wescott. Never mind.

VLV

Tim Wescott wrote: