In message , Richard Clark
writes
On Wed, 28 Jan 2009 13:03:45 -0000, "christofire"
wrote:

My next question is about using varicap diodes to tune the loop. Most
designs use a conventional capacitor, but a varicap would enable the loop
to be located (say) in the loft, and the tuning varied from the shack
downstairs. Is this technology worth perusing (ie Googling)? Or has it
been found to be a waste of time?

Varicaps are fine and you will find devices containing pairs like the BB212,
specifically intended for tuning domestic receivers. They're used in all
the modern radios that are digitally controlled. The trick is to use them
in balanced pairs so strong signals don't push a single diode into
conduction.

Thanks for the advice, however......

Hi All,

The strong signal issue goes well beyond the possibility of
conduction. When you think about a voltage controlled capacitor, you
have to account for the effects of the voltage of the signal you are
tuning (and those you are not tuning). The BB212 application is found
characterized for signal levels less than 100mV (typically far less).

Why? A half volt signal level

(that's surely one hell of an RF signal?)

at the terminals of the device (taking
the BB212 for example) biased at 1V will swing the capacitance 100pF
above and below where it would be thought to be tuned. Of course,
correspondingly smaller signals have correspondingly smaller swings -
it all depends on how much you can tolerate.

Varicap diodes will always suffer from having their capacitance
modulated by an RF signal impressed upon them. However, they do seem to
work well enough - even when the RF level is pretty high (which must be
the case especially with varicap-tuned oscillators - ie most VCOs and
PLL systems).

Presumably, the effect of this modulation will be to generate
intermodulation products. In VCOs, this will simply appear as harmonics
of the oscillator signal (which you would get anyway - even with a
conventional tuning capacitor). Where multiple-frequency signals are
present (like you have with the receiving loop), the most apparent
effect of the modulation of the diode capacity will appear as
crossmodulation and other nasties on the other signals in the passband.
However, as varicaps ARE used for the tuning of the input of receiver RF
stages, how do they 'get away with it'?

This swing is also a problem for PIN diodes used as voltage variable
attenuators.

My understanding of things is that the effectiveness of PIN diodes
relies on them having a very poor performance at RF (especially at VHF
and UHF). In attenuator circuits, they are forward biassed, and the DC
current passing through them varies their RF resistance. However, the
charges flowing through the junction are so 'sluggish' that they don't
react to the rapidly-changing RF voltages. There is therefore negligible
modulation of the RF resistance by the RF signals passing though them.
--
Ian