HP 8340B Manual page 337

If
you
access the peaking routine by pressing
[SHIFT] [AMPT MKR],
or by remotely
programming
"SHAK",
it
executes immediately,
regardless of instrument state, but only
a
fine search around the
most recent slope
DAC setting
is done. Because the
SYTM passband is not tuned to track the output
frequency in
low band, the peaking routine does not execute if the instrument is in
low band.
MOD LVL
is
fed to the A27 level control assembly, where
it becomes one
input
to
the ADC input
multiplexer.
The
peak
routine programs the
ADC
multiplexer to route the
MOD
LVL voltage to the test
ADC. By monitoring the
test
ADC
output while tuning the
SYTM,
the program finds the peak
RF signal
level.
When you select the peak function from the front panel, the instrument does
a
full
peak (coarse
and
fine search)
if
in
CW or
manual.
If
you
leave the peaking function on, the instrument
repeaks every
seven minutes,
doing
a
fine
search around the most recent slope DAC
setting.
If,
while peaking is
on,
you change
the
CW or
manual frequency
with the front panel knob, the instrument performs
a
fine
search around the most recent slope
DAC setting; if
you change the frequency
using
the step or
numerical entry keys, the instrument performs
a
full
peak.
The SYTM passband has
a second, narrower
peak 200 to 300
MHz
lower
in
frequency than the
desired peak. The coarse search routine starts at the top of the passband and searches for
a
peak in
15 MHz
steps. Once
a
peak is
found, the
coarse search
continues for
another
150
MHz
to verify
that
it
found the correct peak, stopping before it reaches the second peak. The step
size
is
15 MHz
to ensure
that at least
one
step falls within
1 dB of the
peak
(the minimum 1
dB bandwidth
is
24 MHz).
Once the coarse search peak is located, the microprocessor reads the modulator level as
a
reference
for the
—1
dB points. The
level control
DAC
is
stepped down
1
dB, causing the modulator level to drop
1 dB (because the ALC loop generates the
1
dB change).
Using the
—1dB reference, the
micro¬
processor
steps the slope
DAC
until
the modulator level is
equal to or less than the
—1
dB reference
level.
The
slope DAC
Is
stepped down from the
peak
until the same reference threshold is
reached.
If
the slope
DAC
goes out of range before
it
reaches the —1dB reference
level,
the
routine
searches the
other
side of the passband to the same modulator threshold present when the slope
DAC
reached the
end of its
range.
The
slope DAC
is
set to the midpoint of the two values, and the original power
is
restored.
Because the
SYTM
passband can vary with power
level, peaking is
done at the current ALC power
level.
In the fundamental band,
the SYTM YIG sphere
may squeg
if
too
much power
is
applied.
To
prevent
this
from interfering with peaking, the maximum ALC power setting (in the
fundamental band
only)
is 0
dBm during the coarse search, and
+10
dBm for fine
search. Once peaking is completed,
the original
power
is
restored.
When the instrument goes unleveled, the current
driving the ALC modulator
is
turned
off
to give
maximum available power. The MODLVL
line
still
gives an indication of the detected power out of the
SYTM, with
a
sensitivity of
30 mV/dB. The absolute voltage
level shifts by
about
1V
when the
instru¬
ment goes unleveled,
but the incremental level
remains
valid.
Because the peaking routine uses
incremental changes, the absolute level shift
poses no problem
to the peaking routine.
Initiate
SYTM
tracking by either pressing [SHIFT] [PEAK]
on
the front panel, or remotely program¬
ming
"SH RP" over the HP-IB.
Each band
is
tracked independently. To track
a
band,
a
single band sweep
is
set and the sweep is
stopped at
several points across the band. The number stored
in the calibration constant represents
the least squares analysis of the straight line fit for the slope DAC numbers returned from the peaking
routine.
If
the least squares number is
out of the slope DAC range, the fault light turns on with the
TRK
indicator flashing.
In band
1,
the SYTM
is
peaked at 2.3 GHz, and at
1
GHz
steps from
3
to
7
GHz.
In
band
2,
the
SYTM
is peaked
at 6.9
GHz,
and at
1
GHz
steps from 7.5 to 13.5
GHz.
In band
3,
the SYTM
is peaked at
13.4 GHz, and
at
1
GHz
steps from 14 to 20
GHz.
In
band 4 (HP 8340B), the SYTM
is peaked at 19.8 GHz, and at
1
GHz steps
from 20.5
to 26.5 GHz.
H-IO
RF
Section Theory of Operation
HP 8340B/41B