Fluke PM6681 - Page 38

Figure
4-18
Oscillator
circuits
functional
description.
her with the signal PD1 from
OQ0504, the
delayed
signal
is fed back to the
ARDI
input on OQ0502.
Instead of a
time delay, the
delay can be
programmed
to an
event
de-
lay. The
EDC1 input of OQ0504
are therefore
connected to
the B input enabling,
event
delay for
events
on the B chan-
nel.
Hold-Off
A
second
programmable
delay with a
resolution
of 10 ns is
used
as hold off. It is
triggered from OQ0502
by the signal
HOS. The counter U53
is loaded with a
value and clocked
with 100
MHz.
When the
counter
has come
to
zero
this
in-
formation is
sent to OQ0504,
EDC2 and the
circuits
are re-
seted.
The delayed
signal is fed back to
the HOF input on
OQ0502.
Gate Open
The
signal GATO
from OQ0502
gives a
real
time indication
of the
state of the
measuring logic. Main
gate open is indi-
cated by a
high
level
and
main gate
closed
is indicated by
a low level. VI
22 makes it
possible to make
the
high level
1.4
V
in
50a.
Divider
The signal from
input A is
divided by two during
frequency A
measurements
by the divider U85.
The reason
for this is that
the
OQ0502
can
not
handle
frequencies
above 225 MHz.
To be
able to measure
frequency bursts also
on
input C the si-
gnal from the
prescaler
is divided by
two by the other half of
U85 before
it enter the OQ0502..
Inputs
The signals A
(A-channel), B
(B-channel),
C (prescaler si-
gnal), and
E (rear panel
external
arming input) go to
an in-
put
multiplexer in OQ0502, In
OQ0502 the A and
B inputs
also
have slope selections
(positive edge
and negative
edge). R257 and C117
terminates the
C signal.
External
Arming
The rear panel
input
EXTERNAL
ARMING
is a DC-coupled
TTL
level
input. R258 to R261
with D32 and D33
protects
the input.
V8 and V42 are a
Schmitt-trigger with
approxima-
tely 1.4 V
threshold level. The
external
arming signal is
connected to
E
input on
OQ0502.
Burst
The signal
HOS
from OQ0502 are
also used when measu-
ring
at bursts. The
External Arming
input
is switched off by
the signal HS0.4
from the iti-processor
via V66 and
V65.
The HOS signal is
then
fed via V68
and V67 back to the
OQ0502
input
E.
All
ECL-inputs in OQ0502
get their reference
(VBB) from
an
external ECL-circuit U132.
The
GET-signal from
an optional
GPIB-interface can con-
trol the start of
the measurement.
4-14
Hardware
Functional
Description