A9 Low Band Pulse Modulator; A10 Directional Coupler - HP 8340B Manual

A9 LOW BAND PULSE MODULATOR
The A9
pulse modulator
is
a
shunt PIN diode modulator
that provides
an
on/off
ratio
greater than
80
dB
in
low band. Modulator control
is the
LOPMOD DR (low band pulse mod drive) signal from the A21
pulse modulator driver assembly.
A10 DIRECTIONAL COUPLER
The A10
directional coupler has
a
—16 dB coupling coefficient.
The reverse-coupled port is termi¬
nated. The coupled output
is sent to
the
AH high band
detector for leveling in the higher bands.
Although the low band output
passes through the A10 directional coupler,
it
does not affect low band
leveling.
The
insertion loss
is
less than 1.3 dB at
26.5 GHz.
All HIGH BAND DETECTOR
The
AH high band detector detects the RF amplitude for leveling
in
high band, producing
a
positive
output voltage proportional to
the RF power.
The
detector output
is
sent to the A25 ALC detector
assembly for processing.
A12 LOW BAND SPLITTER/DETECTOR
The
A12 low band splitter/detector samples and detects the RF amplitude for low band leveling. It
consists of
a resistive power
splitter, and
a
schottky diode detector
that
produces
a
positive output
voltage proportional
to
the RF power.
The LDET BW
(low detector bandwidth control) input (from the A26
linear modulator assembly)
switches
in
additional filtering for leveling below 400 MHz.
A
thermistor (mounted inside
the splitter/
detector package) compensates for detector
temperature variations.
A13
SWITCHED YIG-TUNED MULTIPLIER (SYTM)
The
A13 SYTM
uses a
PIN diode switch to turn the band 0
signal off
when
you select band
1-4. In
band
0,
the
SYTM
provides
a
straight path for the 0.01 to
2.3
GHz
RF signal from the A12 low band splitter/
detector. Low band insertion loss is
typically less than
0.5 dB.
For high band, the RF input from the AT1
isolator is
fed to the
SYTM.
This signal
is applied through
an
impedance matching circuit to an
SRD
(step recovery diode) to produce an output rich in
harmonics.
The SRD BIAS
applied to the diode
is
different for each band to optimize the harmonic required
for
that band;
Band
1
=
fundamental
Band
2
=
second harmonic
Band
3
=
third harmonic
Band
4
=
fourth harmonic (HP 8340B)
H-4
RF
Section Theory of Operation
HP 8340B/41B