Electronics-World-1959-05

Generating Crystal Signals
(Continued from page 67)
The possibility of extreme error just
discussed will not usually arise and
can be easily avoided. A more typical
case. for example, would be the pairing
of units rated at 8010 kc. and 4500 kc.
to produce a difference of 3510 kc.
Since these are not too close together,
the least favorable combination of errors
(assuming the rated tolerance of
.015 for each) would result in an approximate
error of .036'; at 3510 kc.
This is probably less than could be detected
by reading the dial of a conventional
signal generator.
Sum heterodynes, of course, allow
far less possibility of error. Here the
deviation in the output will be greatest
when the individual crystals err in the
same direction. Even in this case,
however, accuracy will be no worse
than that of the individual crystals.
Additional Uses
The twin -crystal heterodyne oscillator
circuit may be used to compare
the frequencies of two crystals having
the same nominal frequency, as a
check on accuracy. To perform this
task, simply connect sensitive headphones
with good low- frequency response
(or an amplifier- speaker combination
good in the lows) to the output
of the circuit in Fig. 1 or 2. The difference
frequency. if greater than about
40 cps, will be audible. This output
can be compared with that of a calibrated
audio generator.
Another valuable application is the
determination of actual frequency for
crystals that are unmarked (or incorrectly
marked). Place the unknown
unit in one socket of the oscillator and
another crystal of known frequency in
the other socket. The difference output
they produce (the available output
signal that is lowest in frequency ) can,
in turn, be zero -beat against the already
calibrated variable signal generator,
using the arrangement shown
in Fig. 3.
This method still leaves two possible
values for the unknown, but doubt can
be resolved easily. For example, suppose
an unknown crystal has been
paired with another whose frequency
is known to be 3105 kc. The difference
output is found at 970 kc. To produce
this difference, the unknown may be
beating either above or below the frequency
of the known unit. Frequency
of the unknown may therefore be
either 4075 kc. or 2135 kc.
To determine which value is correct,
simply repeat the test using another
crystal of known frequency. For example,
the unknown is next paired
with one rated at 3550 kc. and the beat
is found at 525 kc. The results of this
test alone would indicate the frequency
of the unknown to be either 3025 or
4075 kc. Comparing both test results
obviously establishes the value of the
unknown at 4075 kc.
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111