HP 8340b User Manual

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

HP 8340b Specifications

General

Brand	HP
Model	8340b
Category	Inverter
Language	English