Solid State Shortwave Receivers For Beginners - The Listeners Guide

Electrical bandspread is an electrical equivalent of a mechanical
reduction drive, and simply consists of adding a low value variable
capacitor in parallel with the main tuning capacitor. This is shown in
the circuit of Fig.13. The value of the additional capacitor is not critical,
and can be anything from about 15 to 50 pF.
In use the main tuning control, or ‘Bandset’ control as it is normally
termed when a bandspread capacitor is fitted, is set to the band
that is to be searched for transmissions. Then the actual tuning is
accomplished using the bandspread control.
As the bandspread capacitor has a much lower value than the
bandset one, it only covers a very small part of each tuning range.
Tuning is therefore far less cramped using the bandspread control.
This type of electrical bandspread used to be the standard method
used in simple receivers, but variable capacitors are relatively expensive
these days, and mechanica1 or other electrical methods are now often
employed instead.
A special type of diode known as a variable capacitance diode
(often abbreviated to varicap.) can be used as a cheaper alternative. This
component has the property of changing its capacity in proportion to the
level of reverse bias applied to it.
What is happening is that the two pieces of silicon that form the
diode are acting as the plates of the capacitor. The insulating layer that
exists between the two pieces of silicon, and which is called the depletion
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