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GENERAL
4.1
The instrument employs conventional digital counting techniques. In frequency
measurement, clock pulses, generated at a known p.r.f. by a standard oscillator,
are used to open a gate for a specified period. During this period pulses derived from the
unknown signal are accumulated in the six counting decades and displayed in the six window
readout as a measure of frequency. For period measurement the clock pulses are fed
directly into the counting decades via the gate which is held open for a single period or a
specified number of periods of the unknown input signal. In this way the number of clock
pulses accumulated when the gate is open, is displayed as a measure of the period of the
input signal. The following paragraphs 4.3 to 4.5 describe the basic operating principles
for both frequency and period measurement with reference to the Simplified Block Diagram
(Fig. 2. 1).
4.2
The control logic of the instrument incorporates a number of gates which form an
electronic changeover circuit. This changeover circuit carries out the appropriate
routeing of the signal in accordance with the setting of the Function switch on the front
panel.
FREQUENCY MEASUREMENT
4.3
Signals in the frequency range 10Hz- 80MHz are applied to the.input socket of
model 9057 (or input socket 'A • on model 9059). Signals in the frequency range
40MHz- 560MHz are applied to input socket 'B' in model 9059. Signals in the range
1OHz - 80MHz are applied directly to
.
the Amplifier and High Speed Decade Assembly
whilst signals in the range 40MHz- 560MHz are applied via the 560MHz Amplifier
and Pre-scaler. Either input may be attenuated by 20dB by appropriate setting of the
Channel Sensitivity switch;
4.4
The unknown signal is amplified and shaped to provide a pulse for every complete
cycle of that signal. The resulting train of pulses (p.r.f. equal to the frequency
of the input signal) is fed via the electronic changeover switch to the signal input of the
gating circuits. Clock pulses, in the form of 1OOHz square wave output from the frequency
standard, are fed via the changeover switch to the Time Base Divider, a selected output of
which is ther.t routed by the Control Circuit to the 'control' iAput of the gating circuit.
4.5
The Time Base Divider consists of divide-by-ten stages which provide an output pulse
for every 1, 10, 100 or 1000 periods of the 1OOkHz frequency standard. This enables
gate times of 1 Oms, 1 OOms, ls or 1 Os to be selected by the bank of four push-buttons in
conjunction with the control logic.
4.6
When the gating control circuitry in the Amplifier and High Speed Decade Assembly
is opened by a gate-time signal from the timebase in the Control Assembly, the
4-1
9057/9059
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