HP 8340B Manual page 336

The
YIG sphere magnetic
field is proportional to the current through the
SYTM
coil
(with
a
sensitivity
of approximately
15
mA/GHz), which
is generated from the
PRETUNE signal
from the A54 YO pretune
assembly, and the latched band information
from the A27 level control assembly.
To track the
correct
YO frequency harmonic,
the
SYTM driver has circuitry that adds corrections to the SYTM tuning
current that accounts for the effects of the nonlinear tuning curve, hysteresis, and magnetic delay.
Nonlinear Tuning Curve Compensation. The A28 SYTM
driver has three breakpoint adjustments to
correct for the nonlinearities in the
SYTM tuning
curve (one fixed; two adjustable)
Magnetic Delay Compensation. The SYTM
magnet does not respond fast enough to track the
cor¬
rect frequency when the current drive is
changed
rapidly. This
causes
an increasingly severe delay
problem as the instrument sweep rate
is
increased. To compensate for the slow SYTM response,
the
delay compensation current, which
is
a
function of sweep rate and change in
frequency from the
start
of sweep, adds to the
SYTM
tuning current.
A24 Attenuator Driver/SRD
Bias Assembly
The A24 assembly
controls three
SYTM
functions:
•
Step recovery
diode bias
•
PIN switch driver
•
YIG sphere temperature control
Step Recovery Diode Bias. In band
1,
the SRD (step recovery diode)
is
forward biased with
a
nega¬
tive voltage.
The SYTM
conversion
efficiency
in the higher bands is related to the
SRD DC bias
voltage. Proper
SRD biasing
is also necessary to avoid squegging.
NOTE;
Squegging
is an undesired oscillation in
the A13
switched YIG-tuned multiplier. Symptoms
of
squegging are power reversal
(RF power
decreases as selected power
is increased), power loss at
specific frequencies, and spurious signals
on the
RF output.
To maintain optimum
SRD
bias
for the higher bands, the bias voltage
is
generated as
a
function of
both frequency and
power
level.
The
SRD
bias voltage variation with frequency is derived from 1.4V/
GHz
(from the A28 assembly), which
is proportional to the
YO frequency. The SRD
bias is adjusted for
power
level correction with the
SRD
bias control
from the A26
linear modulator
assembly (a
voltage
corresponding to the modulator voltage).
Pin
Switch. In low
band, the low band
RF
input
passes unattenuated through the SYTM, and the
YIG
sphere is
tuned by the A28 assembly to
a
fixed non-interfering frequency.
In
the high bands, the SYTM
driver provides
a
tuning current, as described previously, while the
SYTM low band RF input port
is
grounded to keep
it
from interfering with the desired high band RF output. The process
of selectively
grounding the low band RF signal
is facilitated
by the PIN diode switch
In the
SYTM. The PIN diode
switch driver is located on the
A24 attenuator driver/SRD bias assembly.
YIG Sphere Temperature Control. Because
the
SYTM passband
drifts
if
the
temperature
varies, the
YIG sphere temperature
is
held
constant with heater resistors and
a
thermistor (located on the
SYTM
substrate), and the heater
drive circuitry (located on the
A24 attenuator
driver/SRD bias assembly).
SYTM Peaking
SYTM
peaking tunes the
SYTM such that the RF signal (YO frequency or multiple of
the
YO
frequency)
is in the center of
the
SYTM 1
dB passband, ensuring that maximum RF power
is
available.
To manually access the peaking
routine (stored in
ROM), press [PEAK].
To remotely access the
peaking routine, use
"RP1" (to turn peaking on) and "RPO" (to turn peaking
off).
When
you access
the
peaking routine this way,
it
executes only
if
the instrument is in
CW
or manual mode, and the RF
is
on.
HP 8340B/41B
RF
Section Theory
of Operation H-9