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f
3
=
1.08 MHz for the V.C.O. frequency converter
f
4
=
120 kHz
for the frequency counter and 2020 output
f
5 =
12 kHz
for the frequency counter
5.3.2. Voltage Controlled Oscillator (V.C.O.)
The controlling DC voltage for the V.C.O. can be supplied in two ways,
a) from a potentiometer mechanically connected to the shaft in the
centre of the frequency scale, or
b) from an external voltage source.
The DC voltage signal is either fed directly to the oscillator circuit giving
a linear sweep or via a linear to logarithmic converter resulting in a log-
arithmic sweep. The output from the V.C.O. covers the range 1.2 MHz to
1.0 MHz (linear). This produces a frequency range of 0 to 200kHz. How-
ever in order to provide the necessary frequency resolution and low frequen-
cy stability, the V.C.O. output is fed to two converters which each decrease
the frequency range of the instrument by one decade. The first converter
transforms the V.C.O. range to 1.2MHz to 1.18MHz (frequency tuning
range 0 to 20kHz) and the second transforms to 1.2 MHz to 1.198 MHz
(frequency tuning range 0 to 2kHz). The same relative stability, therefore,
is obtained in all the ranges.
Note: When a varying DC control voltage is applied, there will be a delay
on the frequency change of the output signal corresponding to 1.50 msec.
If for instance the regulation voltage changes at a rate corresponding to
100Hz/sec. the signal frequency will be 1.50 x 10-
3
x 100
=
0.150 Hz
"behind". The settling down time of the VCO is 30 msec.
5.3.3. Frequency Counting and Marking
The tuned-in frequency of the analyzer is periodically sampled and
shown on a 6 digit Nixie display. The sampling time is 0.1 seconds. which
may increased to 1 second by depressing the COUNTING TIME INCREASE
push-b.
u tton. The longer sampling time gives an extra digit resolution on the
display.
If the frequency marking facility is used, the change in value of a pre-
48
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