View single post by sparks
 Posted: Jun 3rd, 2014 04:51 PM
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Joined: Jan 15th, 2008
Location: New Zealand
Posts: 77
Yes, in most instances, the control of the varicap voltage can be done by an external circuit, where an introduced voltage waveform makes the radio scan up and down the band.
A resistor / capacitor network will not be ideal, as it is an exponentially shaped wave, meaning the scan is non linear in nature. This means the speed of the scan is fast at one end of the band and slow at the oither.
The best sort of waveform is either a triangle or sawtooth, Both are linear, so constant speed. The triangle wave means the scan goes up and down the band, and the sawtooth makes it scan to the end and then reset and start again.
The stepped waveform diagram you mentioned will replicate the stepping of hacked radios, but the stepping itself isnt really necessary.
There are function generator kits that can be adapted to interface with such a radio. One that has been used by others is made by
Kit 23 Opamp Function Generator
The kit can be interfaced with a radio, but it is not a straight forward procedure, as the waveform will need to match the unique scan voltage limits of the radio, and thus will require customisation of components in the kit in order to work properly.
I hope this helps.