HP 8340B Manual page 491

PULSE MODULATION ASSEMBLY
A21
Pulse Modulator Driver
Pulse modulation
is
produced
by the
A21
pulse modulator driver assembly driving
one of the two
pulse modulators.
The front panel pulse modulation input
is
a
TTL
compatible
input.
A
low
signal
on
this
input turns the RF
off,
and
a
high turns the RF on. With no connection, the pulse
modulation input is internally
pulled to
a
TTL high level.
When
a
low signal
is
present
on the pulse modulation input, the
A21 pulse
modulator driver
delivers
20 mA
to one of the two pulse modulators. An output multiplexer on the
A21
assembly
directs the
20 mA modulator current
to
the A9 low band pulse modulator when the instrument
is in
low band, or to the pulse modulator in
the A16 mod/splitter when the synthesizer
is in
high
band.
The
leveled instrument pulse modulation capability requires timing signals be sent to the ALC
assemblies. The
timing signals
coordinate the
leveling
operation with the pulse operation. When
a
pulse
is initiated, three timing signals are generated
on the A21 pulse
modulator driver assembly:
Sample/Hold Timing
This signal controls the sample/hold gate on the A25 detector assembly. It is
adjusted to close the
gate (sample) when
the
RF
pulse is
on and open the gate (hold) when the RF
is off. The sample/
hold
output voltage represents the
peak RF amplitude.
Analog to Digital Converter (ADC) Timing
This signal
enables the
analog to
digital converter on the
A27
level control
assembly to monitor the
output of the
sample/hold. The ADC
is enabled when the
RF
is on, and for
1
ms after the
RF
is
turned
off.
After
1
ms, voltage droop renders the sample/hold value inaccurate.
Integrator Timing
This signal controls the integrate and hold gate on the A26 linear modulator assembly. This gate
is
closed when the RF
is
on and stable.
To speed response time for narrow pulses, the gate is held
closed for
a
minimum
of 10ns. The sample/hold
maintains
a
valid integrator input after the pulse
is
turned off
HP 8340B/41B
RF
Section Theory of Operation
H-ll/H-12