HP 3562A Service Manual page 319

MOD E L 3 562A
Square Wave Source
The DAC l oad s ignal (DACLD) on p i n 36 is used for a n u m ber of thi ngs in t h i s part of
the analog sou rce. This 256 k H z squ are wave is divided by U450 (a d u a l 4-b it cou nter)
w h i c h yields squ are waves whose frequenc ies are 64 k H z and a 4 k H z . Both signals are
connected to the signal selection c i rcu it.
Pseudo Random Source
DACLD c l ocks U455 (serial sh ift reg i ster with 8-b it para l l e l output) w h i c h drives a group
of gates, the end res u l t of which is a pseudo random bit stream which repe ats itself every
256 clock cyc l es. T h i s signal and its i nverse is connected to the signal selecti on c i rc u it.
Signal Selection
S i gnal selection is done by U452 (a d u al 1 -of-4 selector/mux) and rec locking is done by
U451 . I n puts to this block consist of two squ are waves (64 k H z and 4 k H z ) from U450
and the pseudo random noise (PRN) s ignal and its i nverse. The control l i nes S E LCAL and
I NVCA L from U 502 sel ect one of the four signals to pass to the cal i brator c i rc u i t and
to send to the trigger board as the s ignal CALT R I G . Pseudo random noise is not u sed for
a CAL T R I G signal, but a si gnal from the PRN sou rce is conn ected to both C2 and C3 of
the CAL T R I G selector, U452A. See tab le A30-2 i n secti on V I I I for s pecific control
i n formatio n .
Calibrator
The 6.2 volt refe rence is d ivided by R5 and R6 s u c h that 0.2 volts appears on the i nverting
i n put of U1 . A feed back loop through Q1 , Q2/Q3, and R7 forces the vol tage across R8
to be the same as across R5. T h i s m akes Q1 a stable cu rrent sou rce for Q2 and Q 3 . The
bases of th i s trans istor pair are d r iven by the TT L s ignal from the s i g n a l sel ecti on b lock
s u c h that one of the pair is on and the other is off at any given time. W hen Q3 i s off
there is no voltage d rop across R1 6 and R1 0; when Q3 is on there is a voltage d ro p . S i n ce
the cu rrent through Q2 and Q3 is ve ry we l l reg u l ated, the voltage appeari ng on its c o l l ec­
tor is very stable.
C I RC U IT D E SC R I PT I O N S
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