Output C of the prescaler goes to pulse driver A1 when the Model 5257A is
switched to the 50 to 200 MHz range.
This gives a tunable sampling rate
from 16.7 to 33.3 MHz. The other prescaler output, if passed by the inhibit
amplifier, goes to the counter input gate on line A for frequency counting
on all ranges. Thus on the lowest range, counter prescale is N and on the
three highest ranges the counter prescale is 4N.
The thumbwheel switches, A8 preset decades, and A9 gate-time extender
control the counter's start-stop and are separate from the transfer
oscillator portion.
The thumbwheels are set to the transfer oscillator
(VFO) harmonic number N which zero beats with the input signal being
measured. The thumbwheels activate switches which set the conditions of the
three binary decades of A8 in units, tens, and hundreds. The decades start
counting at start and provide a coincidence gating pulse to the gate time
extender when the count period is completed for stop.
In this way, gate
time extender A9 controls the counter's start-stop time so the readout is
the actual measured frequency. Range switch S1 turns off divide-by-four in
A9 for the 50 to 200 MHz range and turns it on in the other ranges to give a
correct count when A8 prescaler is switched.
Therefore, when the
thumbwheels are set at 001 the counter will display the sampling frequency
to pulse driver A1 in all ranges.
Figure 3.
Model 5257A block diagram.
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