Theory Of Operation; Theory Of Operation; Leveling - HP 8340B Manual

RF
Section
Theory of Operation
The RF
section contains the microcircuits and board assemblies that produce, amplify, and
control
the RF output. Refer to Figure H-2 for
a
simplified RF
functional group block diagram.
RF power generated by
the
YIG
oscillator
is
delivered to the mod/splitter, which couples off
a
portion
of the signal for the
rear
panel auxiliary output.
The
mod/splitter also
splits
the power to provide RF to
the high band power amplifier, and the low band mixer.
LOW BAND
Low band is
produced by mixing
a
high-power swept LO signal with
a
fixed RF signal.
The
fixed
signal
is
generated by
the 3.7 GHz
oscillator, and
passes through
a
linear modulator, pulse modulator, low
pass
filter,
and isolator, before
it
reaches the
mixer.
The mixer output frequency
(IF)
is the difference between the
LO and the RF frequencies. Because
the
LO
sweeps from
3.71
to 6.0 GHz, and the RF
is
fixed
at 3.7 GHz, the
IF
sweeps
from 0.01 to 2.3 GHz.
The
low band power amplifier amplifies the
IF,
and sends
it
to the low band splitter/detector, where
a
small portion of the signal
is split off and
detected for leveling.
The
leveled
IF
(RF) enters
the
SYTM
(switched YIG-tuned multiplier), which provides
a
straight
through path for RF from the low band splitter/detector when low band
is activated. After passing
through the
SYTM,
the signal
passes through the directional coupler, which performs no function in
low band, and the step attenuator, before reaching the instrument
output
connector.
HIGH BAND
The
high bands are produced by feeding the mod/splitter RF output through
the high band
power
amplifier.
The SYTM generates harmonics from
the high level output of the
power amplifier,
to
pro¬
duce bands
2
through
4.
The SYTM contains
a
bandpass
filter
tuned to the desired RF output
fre¬
quency, which allows
it
to
pass
the desired
RF output frequency,
and reject the unwanted harmonics.
The directional coupler couples off part of the
SYTM RF output to the high band detector
for high
band
leveling. The step attenuator provides low power output
levels
in all
bands.
LEVELING
The ALC (automatic
level control circuit) uses
feedback to
hold the
RF output constant throughout the
full
frequency range.
When internally leveled in
high band, voltage from the high band detector
is fed through
a
log amplifier
and summed with
a
correction level that closely
approximates
the variations of the
RF attenuator
and
the
RF
cables.
The corrected detector
level
is
then fed through
the sample/hold, and routed to the
linear modulator driver assembly where the
corrected detector voltage
is
summed
with the reference
voltage
at the
integrator
input.
H-2
RF
Section Theory of Operation
HP 8340B/41B