Low Level Inhibit; Signal Selection System - Racal Instruments 9916 Workshop Manual

AGC and Attenuator
4.15 The signal at 04 emitter is passed via 03 to the peak to peak detector C13, 05,
D4 and C11. As the input signal amplitude increases the potential across C11
increases. This level is applied via the emitter follower tolC31a, which controls the
current in D2 and D3. An increase in input signal increases the current in the diodes,
reducing their dynamic impedance. The diodes, in co':!iunction with R10, form a potential
divider across the output of 02, and a reduction in diode dynamic impedance reduces the
signal passed to IC30.
Low Level Inhibit
4.16 The output of the AGC detector is fed to the Schmitt trigger IC31b. If the input
signal ampl itude is too small for stable counting \C3 ' I b/7 is at a lower potential
than IC31b/6. The resulting output at IC31b/l0, applied to IC21q113, gives a voltage
level at IC21q115 high enough to inhibit the output of IC21b.
Signal Selection System
4.17 The selection of the lA' Channel, IB' Channel or LF multiplier signal to be
passed to the counter input IC4/22 is made by the Channel Select switch 1Sl and
gates IC18a and b and IC9a and b.
4.18 With the 'A' Channel seiected IC9b/5 is set to '1' via IC10a. Provided IC9b/4
is also at '1' (see paragraph 4.10)
'A'
Channel signals will pass to IC4/22. IC8b
is inhibited by a '0' at pin 12, and IC9a is inhibited by a '0' at pins 1 and 2 from IC10b.
At the same time the -5 volt supply is connected to the 'A' Channel and disconnected from
the 'B' Channel by the Channel Select switch. The +5 volt supply is connected to the 'A'
Channel by 013, turned on by the Channel Select switch via 1C10a and IC13c, and
disconnected from the 'B' Channel by Q6, turned off via IC13d.
4.19 With the lB' Channel selected le8b is enabled by 'l's at pin 12 (from R79),
pin 9 (from R81) and pin 10. 'B' Channel signals From Q5 will pass to 1C4/22.
IC9b
IS
inhibi'ted by a '0
1
at pin 5 from IC10a, and IC9a is inhibited by a '0' at pins 1 and
2 from IC10b. The -5 volt supply is now connected to the 'B' Channel instead of the 'A'
Channel, and Q6 and 013 connect the +5 volt supply to the 'B' Channel and not to the
I
A' Channel. The '1' from R79, inverted by ICl Oaf puts IC14a into the cleared state,
cutting off the +24V supply to IC26 (see paragraph 4.59).
4.20 With LF selected IC9a is enabled by a '11! on pins 1 and 2 from IC10b. IC8b is
inhibited by a '0' at pin 10 and IC9b is inhibited by a
'Q'
at pin 5 fromlC10a.
The power suppl ies remain connected to the ',B 1
Channel.
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9916 Vol. 2