Electronics-World-1959-05

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M
An assortment of crystals
of random values can be
combined to yield an
amazing range of accurate
signal frequencies.
QUARTZ CRYSTAL oscillators are
quite familiar to technicians in all
branches of electronics. Two limitations,
however, have operated against
their more general use as signal generators,
either for the service shop or
in the laboratory. These limitations
are the relatively high cost of individual
crystals ground to desired frequencies
and, as a result, the inconvenience
of being limited to just a few
fundamental frequencies and their
harmonics. Thus, despite such desirable
features as constancy of frequency and
stability of amplitude, crystal oscillator
applications have been restricted.
Since World War II, the surplus
market has offered "grab-bag" lots of
surplus crystals at unusually low
prices. Unfortunately, few of these are
ground to frequencies directly usable
in either radio and TV service or in
laboratory apparatus. The method described
here permits many of these
inexpensive crystals to be used for any
service function whatsoever: as markers
for calibration of variable -frequency
generators or even directly for
circuit alignment.
The idea of this apparatus was conceived
while using a conventional crystal
oscillator with a number of surplus
crystals to check the dial calibration
of a new signal generator. This procedure
worked well, as far as it could
be applied. None of the crystals in
the assortment had fundamentals be-
Generating
Test Signals
with Crystals
THOMAS DEANE HERRIMAN
low 3000 kc. and the ranges of the
generator extended from 75 kc. to 150
mc. Of course, the higher ranges could
be checked with harmonics, but what
about the broadcast and i.f. bands?
While searching the crystal bank, a
pair was found having marked frequencies
of 4490 kc. and 4495 kc.
Curiosity prompted the construction of
a second crystal oscillator in order to
determine whether the audio beat
would be heard. A crystal -diode probe
and a pair of headphones revealed the
presence of the 5000 -cps tone with considerable
amplitude. This incident
suggested the solution to the problem
of calibrating the low- frequency
ranges. Heterodynes were obtainable
from pairs of crystals at fairly close
intervals from below 75 kc., through
the i.f. and broadcast bands, and well
above! Most of the broadcast band
could be checked at intervals of 20 kc.
or even closer.
Twin -Crystal Oscillator
Fig. 1 shows the very simple circuit
ultimately adopted for the twin oscillator.
A separate mixer proved to be
unnecessary; the common 7500 -ohm
cathode resistor provides adequate
mixing. The optional output amplifier,
V,, is helpful only in case audio or very
low radio- frequency heterodynes are
to be used. It will contribute little
gain at high frequencies and the wide
range to be covered precludes the use
66 ELECTRONICS WORLD