HP 8340B Manual page 335

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