Tuning of varicap diode in a magnetic loop antenna based on modulating signal

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rodv92

Newbie level 4
I was wondering if it was possible to remedy to the extremely high Q of magnetic loop antennas in the case of an analog modulation that requires more bandwidth than the Q can offer, and hence produces distortion)
That is for transmission of course at very low power (so as not to breakdown the varicap)

To explain it clearly :
Imagine an FM modulated carrier. The idea would be to adjust the center frequency to match the "instantaneous frequency" of the modulated signal by suppling a varying voltage to the tuning varicap (that would theoretically make the resonant frequency "ride" the modulated carrier)

Since varicaps are quite linear, this seems only a choice of a good resistor choice for the analog modulating signal and a simple biasing circuit to inject the modulating signal on top of the DC center frequency tuning voltage.

In the case of AM, that would probably involve some more advanced circuitry to perform the transfer function,
So I am investigating FM first.

Is this idea worth exploring, or maybe it simply exists already?
Or would it induce even more distortion (I think of slow response issues from the varicap diode)?

kripacharya

Banned
This is an interesting concept, but in your FM context how would you predict what your next received frequency will be ? This is where the concept breaks down imho.

I would think it's easier for the AM case, but maybe I'm missing something?

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kripacharya

Banned
..... The idea would be to adjust the center frequency to match the "instantaneous frequency" of the modulated signal by suppling a varying voltage to the tuning varicap (that would theoretically make the resonant frequency "ride" the modulated carrier)

Since varicaps are quite linear, this seems only a choice of a good resistor choice for the analog modulating signal and a simple biasing circuit to inject the modulating signal on top of the DC center frequency tuning voltage.

...
I believe the real challenge here would be to ensure that your varicap/ magnetic loop IS tuned to the incoming frequency. There would be no 'feedback path' to correct for any errors.

You could generate a voltage proportional to the FM carrier deviation to drive the varicap. But this voltage would have to be very carefully sized beforehand. I.e. you would have to calibrate your tuning for various frequency steps and somehow use this calibration to feed the varicap at runtime.

andre_teprom

Super Moderator
Staff member
I was wondering if it was possible to remedy to the extremely high Q of magnetic loop antennas in the case of an analog modulation that requires more bandwidth than the Q can offer, and hence produces distortion)
Should we assume that your doubt concerns an unconventional case of a demodulation without use of intermediate frequency in the receiver side? This feature is designed to allow the transmission of more channels within the same carrier, i.e. being transmission optimized for the same antenna geometry.

betwixt

Super Moderator
Staff member
AFC was (until PLL came along) used extensively to keep receiver local oscillators tracking an incoming signal. For receiving, there would be little point in tuning the antenna so accurately, even with magloops high 'Q' it still has a few KHz of bandwidth, certainly enough for an AM signal. Tracking modulation of AM or FM signals would be very problematic because the bandwidth changes rather than the center frequency and the lag between demodulation and tuning would in any case produce an unpredictable pass band.

I think Rodv92 may be talking about transmitter antennas and deliberately re-tuning the antenna in sympathy with the modulated carrier frequency.
In theory it would work but the trouble with magnetic loop antennas is they can produce exceptionally high voltages across the tuning point. One tested here with a 50W carrier produced over 7.5KV across the tuning capacitor and I somehow doubt a varicap would survive that!

If it could be done, I think a signal matching the modulation but at high voltage would be needed so the carrier was relatively small compared to the tuning voltage. It would be difficult.

Brian.

biff44

Advanced Member level 5
i understand your concept, and it might work.
consider though that FM and PM are related. So if you have a source that is FM modulated, and you now vary the resonant frequency of the actual antenna with a tunable reactance, you are adding PM modulation on top of your FM signal (by a variable capacitance in the antenna moving its resonant frequency). Distortions of the FM modulation will ensue, unless you do it very carefully

it IS a sound method though, and has been used before. Like an impatt amplifier...has a limited bandwidth...so add a varactor diode tuning element to the cavity and drive it with a DAC, and voila, you have a virtual huge bandwidth now. The same is used in modern on chip VCO circuits, where they switch in/out fixed lumped capacitances to coarse tune the frequency, and use the varactor to fine tune the VCO.

and on the magnetic loop tuning...i wonder if a ferrite device with a varying DC magnetic bias might work better than a varactor diode in series with the gap capacitance? Might handle a much higher transmit power. the varactor is at a voltage maximum point, and will see huge RF voltages with high power.

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kripacharya

Banned
Excellent discussion and analysis.
In summary, it seems like a bad idea to try 'widening' the mag-antenna bandwidth by your methods.

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