[sdiy] VC short wave radio?

Trevor Page trevor at resonance.fsnet.co.uk
Mon Aug 12 00:20:53 CEST 2002


Hi.

The fact that your shortware receiver has a digital tuner would make it easy
to perform the modification that you describe. If you have a bit of
knowledge of PLL circuits, that is.

Somewhere within the PLL tuning circuit will be a varactor, or
variable-capacitance diode. This diode actually forms a VCO, from which the
radio's local oscillator is derived. The PLL circuit, or synthesiser chip,
presents a control voltage across this diode to tune the radio. The varactor
is like a voltage-controlled capacitor.

With a datasheet for the PLL IC that you radio uses, it should be fairly
easy to locate the variable-capacitance diode within the VCO circuit. In
fact you can do it with just a bit of probing about. Try and find a DC
voltage somewhere that slowly creeps up or down as you vary the tuning. Then
hack your own control voltage into that point, with the PLL chip disabled.
You could make some measurements first to determine the voltage range the
diode expects to see (i.e. from one end of the band to the other).

There might well be a number of VCO circuits, i.e. for each band.

Similarly the synthesiser IC will change bands by wiggling a logic level on
a pin somewhere, so you can maybe hack a band select voltage into that
point.

Trev




----- Original Message -----
From: <Ordeish at aol.com>
To: <synth-diy at dropmix.xs4all.nl>
Sent: Sunday, August 11, 2002 7:15 PM
Subject: [sdiy] VC short wave radio?


> Hi all. Ive been fooling with a shortwave that i have, and was thinking
how wonderful it would be to go through the bands with an LFO, or envelope.
>  Does anyone know how I can achieve this? I have done some kits, but have
no idea exactly what to do for this idea. The shortwave I have has a digital
tuner. Does that make it more difficult to use control voltages for it?
>  Any suggestions would be greatly appreciated.
>
> thankyou
> edward schultheis
>




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