Pulse Modulation; Sytm Driver Compensation; A8 3.7 Ghz Oscillator - HP 8340B Manual

If
the corrected detector voltage
is
not equal to
the reference voltage, the integrator output voltage
changes,
and drives an exponential driver that feeds current to the linear
modulator
in
the mod/
splitter. This
changes
the RF
output level,
changing the detected voltage from the high band detector.
The modulator driver is also switched to the
A9 high band modulator
in the
3.7 GHz
oscillator.
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PULSE MODULATION
Pulse modulation requires special drive circuitry and modulators to
achieve narrow pulse widths.
The
high band pulse modulator
is in the mod/splitter, and the low band
pulse
modulator is located
just
after
the 3.7
GHz
oscillator.
The
pulse modulator
drive circuitry coordinates ALC operation
with the pulse
signal.
When the pulse
input is
driven low to turn
off the RF,
the ALC
is triggered to sample
and hold
the current
RF level. During the time the pulse is held low, the
output
is
attenuated by more than 80 dB.
When the pulse
input
is
returned to
its
normal
high level, the ALC
releases the hold circuits, and the
instrument returns to the RF
level
it had
before the RF was turned
off.
STEP RECOVERY DIODE (SRD)
The
SYTM
contains
a
step recovery diode that generates the harmonics for the high bands. Power
level, frequency, and
DC
bias effect the
SRD's ability
to generate harmonics. For maximum
conver¬
sion efficiency, SRD
bias is varied with
power
level and
frequency.
In
band
1
(2.3 to
7
GHz), the
SRD
is biased on to allow the fundamental to pass through. The
SYTM
also contains
a
YIG-tuned bandpass
filter that can be
tuned over
the
high bands by changing
the
magnetic field.
The filter passband must closely
track the YO frequency, or some harmonic of
it,
to
cover the high bands.
SYTM DRIVER COMPENSATIONS
The
SYTM
driver compensates for nonlinearities In the tuning magnet, hysteresis, and magnetic
delay.
The SYTM pin diode switch allows the low band signal
to
be switched off when high band is
selected. The bias for this switch
is
generated
on
the attenuator driver/SRO bias assembly.
A8 3.7 GHZ OSCILLATOR
The
A8 3.7 GHz oscillator provides
a
fixed
3.70 GHz
RF output.
The A8A1 oscillator microcircuit and
the
A8A2 oscillator
bias assembly are located
inside the
A8 3.7 GHz oscillator housing. This source
is
phase-locked to
a
100
MHz
internal standard.
A
linear modulator, at the output of the oscillator,
provides amplitude control from nominally
+1
to —70 dBm.
The A8A2
bias
assembly provides the necessary circuitry to amplify the 100
MHz
input signal and
phase-lock the 3.70 GHz
signal
to
it. A
lock signal
is
generated when the two signals are phase-
locked. A8A2 also provides the
+20V
and —10V microcircuit bias voltages.
HP 8340B/41B
RF
Section Theory of Operation H.3