How to Build Advanced Short Wave Receivers - The Listeners Guide

mica washer and plastic bush set. Use a continuity tester to
make absolutely sure that this insulation is effective as otherwise
the output of the P.S.U. will be short circuited (the metal
pad on the underside of the transistor connects to its collector
terminal).
For reasons of safety, the receiver must be enclosed in a proper
casing which leaves no mains wiring exposed, or within easy
access. Also, whether an earth connection is used or not, the
chassis of the receiver must be connected to the mains earth.
Crystal Calibrator
Home constructed receivers are easier and more convenient to
use if they are equipped with a calibrated tuning dial. If a good
quality wide range R.F. signal generator is available, this can
be used to supply the calibration signal. An alternative, and
probably more popular method of obtaining a calibration signal
is to use a device known as a crystal calibration oscillator.
This is an oscillator which operates on a single frequency, or
perhaps on two or three frequencies, and has a strong harmonic
output throughout the S.W. frequency spectrum. This
type of circuit usually operates at a fundamental frequency of
1MHZ, with a lower fundamental frequency available if
required (usually 100kHZ). Thus such a unit can provide
calibration signals in the form of 1MHZ and 100kHZ harmonics.
The harmonics are merely signals produced at multiples of
the fundamental frequency, and so a 1MHZ calibrator will
produce outputs at 2MHZ (second harmonic), 3MHZ (third
harmonic), 4MHZ, etc. to beyond 30MHZ. It therefore
enables the tuning dial of the receiver to be calibrated at
1MHZ intervals throughout its tuning range. A 100kHZ
calibration signal provides additional calibration points at
100kHZ intervals throughout the tuning range of the receiver.
Crystal calibrators sometimes have even lower fundamental
frequencies, occasionally as low as 10kHZ, but for general
coverage receivers such as those featured in this book,
100kHZ is about the practical minimum unless an exceptionally
good tuning drive and dial is fitted.
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