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This simple low cost instrument was built to confirm the
calibration
accuracy of a somewhat ancient Racal 9521 counter/timer, as well as
that of the
Philips Oscilloscope used for its development. The standard uses the
carrier of the 198kHz Radio 4 AM transmitter at Droitwich
UK. The circuit below uses a version of the once ubiquitous
Ferranti ZN414
trf radio receiver chip, later cloned as the MK484.
This 3-pin device appears to be still available in the UK from Bowood Electronics
and possibly Rapid
Electronics
Although outwardly simple to use, the chip rewards attention
to
detail,
particularly in the tank circuit where careful design
of the
coil for high Q is beneficial. The prototype used a surplus MW ferrite
rod aerial from a pocket sized radio (wound with Litz wire), which was
subtantially
padded with
a 1.5nF capacitor to bring it down to LW and within range of the "set
in test" fixed trimming capacitors used. A fine tune was achieved by
sliding the coil on the ferrite rod. Choice of Q1 and Q2 is not
critical. Based on an original Ferranti design employing a 100mH choke
on the ZN414 output to remove the superfluous modulation, this circuit
performed well enough for the present purpose without this filtering,
achieving about 1 part in 200,000 stability. Perhaps this hardly
qualifies it as a "standard" compared to the Droitwich carrier's
accuracy better than 1 in 1010 but a
useful piece of lab kit nevertheless. For those seeking the ultimate in
this type of instrument, check out those at Quartzlock.
