HP 8340b User Manual

Because
the
response
of
the
90
dB
step attenuator
(10
dB
steps)
between
the
coupler
and
the
RF
output connector
is
not
flat,
as
the
leveling
loop
holds
the
detector
output
voltage
constant,
the
RF
output
power
varies
with
frequency
due to
the
flatness of
the
detector,
coupler, attenuator,
and
RF
hardware.
This
variation
can
be
plotted
on
a
graph,
and
approximated
with
several
straight
line
segments
to
within
±1
dB.
While the
detector
output voltage
is
a
function
of
RF
power
to
the
detector,
because
the
output voltage
also
varies
with
both
temperature
and
RF
frequency,
forcing
the
detector
output
voltage
to
a
particular
level
does
not
guarantee
the
RP
power
will
remain constant
as
the
temperature or frequency
changes.
By
making
the
reference
voltage
change
as
a
function
of
frequency,
this
circuit
can compensate
for
straight
line
variations.
This
is
essentially
what
is
done
with the
level
correction
voltage from
the
A27
level
control
assembly.
The
detector
temperature
characteristics
are
corrected
by
temperature
com¬
pensation
circuits
on
the
A25
ALC
detector
assembly.
The
normal
range
of
power
at the
coupler
output is from
0
to
—10 dBm; the
attenuator
provides
lower
powers.
For
example,
—56
dBm
is
produced
with
50 dB
of
attenuation and
the
ALC
loop
set
to
—6
dBm.
Because
of
noise
and
drift,
the
ALC
loop
is
not normally
set
to
less
than
—10
dBm
(the
detector output
is
approximately
1
mV for —10 dBm
RF
output).
The
maximum
attenuation
is
90
dB.
To
go
from
—100
to
-110 dBm,
the
ALC
runs
from
-10
to
-20
dBm.
For
power
levels
greater
than
0
dBm,
the
ALC
loop
runs
at
the
desired
power
without
attenuation.
At
some frequencies,
the
synthesizer
can
produce
+20
dBm,
the
maximum
ALC
power.
To
get +20
dBm,
the
power
amplifier
is
driven
into
saturation
by
approximately
10
dB.
To
reduce
its
output
30
dB
(to
-10
dBm),
the amplifier
input
is
reduced
40
dB by
the
modulators.
Modulator
Attenuation
The
modulator
attenuation
is a
non-linear
function
of
drive
current.
Plotted
on
log-log
paper,
the
plot
is
straight
over
the
high
current
end,
and
if
fed
from
a
current
source
that
is
offset
approximately
1
mA,
the
plot of
attenuation
in
dB
versus
log
is
straight
over
the
entire
range.
Because
the
ALC
loop
bandwidth
and
stability
are
a
function
of
the
gain
and
frequency response
of
each
element
in
the
loop,
ideally, these
parameters
do
not change
with
operating
conditions
(RF
frequency
and
power
level).
Although
not
linear,
the
modulator
characteristic
described
above
lends
itself
to
a
constant gain
ALC
loop.
SYTM
RELATED
ASSEMBLIES
The
two
assemblies
that
directly provide
control
circuitry
for
the
SYTM
are
the
A28 SYTM
driver,
and
the
A24
attenuator
driver/SRD
bias
assembly.
A28
SYTM
Driver Assembly
The
SYTM
passband
must
track
the
appropriate
harmonic
of
YO
frequency
to
an
accuracy
of
approx¬
imately
0.1%,
without
feedback.
The
primary
function
of the
A28
assembly
is
to
tune
the
center
of the
SYTM
passband to
the
correct frequency
for
high
bands
by
varying
the
magnetic
field
about
its
YIG
sphere.
H-8
RF
Section
Theory
of
Operation
HP 8340B/41B

HP 8340b Specifications

General

Brand	HP
Model	8340b
Category	Inverter
Language	English