Time Base Selection - Racal Instruments 9057 Technical Manual

or
lOs
for frequency measurement;,or the input signal by 1, 10, 100 or 1000 for period
measurement. In both cases the selected output from the divider chain is used to control
the Gate Time Control Bistable on the Amplifier and High Speed Decade Assembo/.
Time Base Selection
5.25 Selection of a particular time base is carried out by operating the bank of four
push buttons on the front panel in conjunction with the time base selection gates
which are:-
(a) Four two-input nand gates IC8a, IC8b, IC8c and IC8d.
(b) One four-input nand gate IC12a.
5.26 If, for example, a gate time of 1 Oms is selected for a frequency measurement {or
a single period for period measurement) logic
1
1
1
is applied to pin 10 of IC8c to
open that gate allowing the 100Hz signal to pass to the gate time control circuits via IC12ci.
Gates IC8a, IC8b and IC8dwill be closed due to logic
1
0
1
on pins 2, 4and 12 respectively
thus inhibiting the other inputs to the gate time control circuits.
5.27 A "Reset" pulse generated either manually by the Reset push button on the front
panel or automatically at the end of a measurement cycle, sets the divider stages
IC9, IC10 and IC11 to the
1
999
1
condition i.e. the output of each stage is set to logic
1
1
1
•
The leading edge of the first pulse {derived from the frequency standard or the input signal)
sets the output of IC7c to logic
1
1
1
•
This means, at this instant all four outputs from the
divider chain are at logic
1
1
1
,
thus ensuring that the output of IC12a is at logic
1
1
1
before
the trailing edge of the first pulse is generated.
5.28 Immediately the output of IC7c goes to logic
1
0
1
the divider chain registers 0, 0, 0
and all four outputs of the divider chain are at logic
1
0
1
which ensures that the
output of IC12a returns to logic
1
0
1
•
This action provides the first negative edge whi~h opens
the High Speed Decade via the gate time control circuitry. A second negative edge from the
output of IC12a is required to close the High Speed Decade, this is generated after a period
determined by the particular time base output selected by the bank of push buttons on the
front panel.
INPUT ATTENUATOR
~SSEMBLY
19-0662 Fig.4.6
5.29 Signals in the Frequency range 10Hz to 80 MHz are applied to the input socket.
The slide switch offers direct coupling to the Amplifier and High Speed Decade
Assembly or 20dB attenuation via the attenuator network. The attenuator also provides bias
for the input amplifier of the Amplifier and High Speed Decade Assembly by biasing the F. E. T.
Q5 to zero volts..
5-4
9057/9059