HP 8340B Manual page 177

YO Tune. The YO TUNE
voltage
is
generated
by the
A49 YO loop phase detector from
the
20-30 MHz
input (from the
20-30 loop) and the
Sampler
IF,
which
is
the down-converted product of the
YO RF
output and
the Nth harmonic of the M/N
output.
The A45
directional coupler couples
a
portion of the
YO
output back through the AT2 15 dB attenuator and A46 7
GHz
low pass
filter to the
A48 YO
loop
sampler. There
it is mixed with the Nth harmonic of the
M/N
output to produce the Sampler
IF,
that is
in the
frequency range of 20-30 MHz.
This signal and the
one
from the 20-30
loop are compared
in the
A49 YO loop phase detector.
The
resulting error signal
is
amplified and integrated to produce
YO
TUNE,
with
a
sensitivity
of -3 MHz/V.
When HLEY (high lock enable YIG oscillator)
is
high, the YO
loop is
closed. When HLEY
is low, the sample and
hold circuit holds the
phase-locked
value of
YO
TUNE
constant, breaking the loop to allow
a
voltage ramp (VSWP) to be
summed
into
PRETUNE
to
sweep
the YO.
YO FIVI
Coil Drive. The
A49
FM
coil driver has
a
100 Hz high
pass filter.
Only high frequency (>100 Hz)
portion
of YO TUNE
is
applied to the YO FM
coil.
The
resulting output
is
the YO FM COIL DRIVE
current. The low frequency portion
(<100 Hz) of YO TUNE
is summed with
PRETUNE
and applied to
the
YO
main
coil.
YO Main
Coil Drive.
The YO MAIN COIL DRIVE current
is
derived at the ASS YO
driver from the
PRETUNE
and
YO TUNE
voltages.
The
ASS YO driver sums YO TUNE
with the
OFFSET
voltage, and
scales
PRETUNE
via the
GAIN adjustment.
The YO MAIN COIL DRIVE
current
is
constructed as
a
linear function of the desired YO frequency.
The OFFSET adjustment
varies the offset of the curve; the
GAIN adjustment varies the slope. These two adjustments are used to ensure the tuning accuracy of
the
YO versus PRETUNE voltage over
its
full range of operating frequency. OFFSET has greater
effect on the low end of the
range (2.30 GHz), and GAIN has greater effect on the high end (6.99
GHz).
The
ASS YO
driver
voltage-to-current
converter and the A47 sense resistor assembly
are part of the
same
circuit. This circuit converts the (scaled) sum
of the PRETUNE and YO TUNE
voltages to the
YO
MAIN COIL DRIVE current that
drives the
YO
main
coil.
This completes the YO loop, which tunes and
phase-locks the YO.
Sweep Generator Detailed Theory Of Operation
The
following
assemblies cause the frequency of the instrument to sweep:
•
ASS
sweep generator
•
AS4 YO pretune/delay compensation
assembly
•
ASS YO
driver.
When the YO loop phase-locks at the start frequency
of an instrument sweep or at the start of
a
new
band, the AS4 YO pretune/delay compensation assembly provides PRETUNE voltage to the ASS YO
driver.
PRETUNE
is
a
DC voltage proportional
to the frequency at which the
YO
is
locking.
The
sensitivity
of this voltage is
—2.S
volts/GHz (YO
frequency).
The
ASS YO driver converts
this
voltage
to
a
current that is also proportional to the lock frequency. This current is fed to the YO main
coil
where
it
tunes the
frequency of the YO to
be within the
capture range
of the
YO phase
lock loop.
For sweep
widths >S
MHz,
the A49 YO loop phase detector opens the
YO phase-locked
loop by
activating the
sample and
hold, thus holding YO TUNE
constant
at its phase-locked value. The
ASS
sweep generator provides
a
voltage ramp which is summed with the
PRETUNE
voltage on the AS4 YO
pretune/delay compensation assembly. The
resulting voltage ramp
produces
a
current ramp from the
ASS YO
driver assembly that in
turn
sweeps
the frequency
of
the YO.
VSWP
is also
used on the AS4 YO pretune/delay compensation assembly
to produce another nega¬
tive going
ramp called VCOMP, This is added
to
PRETUNE
on the ASS YO driver assembly
to compen¬
sate for the eddy-current induced swept frequency error inherent
in
the
YO magnetic structure.
HP 8340B/41B
Sweep Generator
-
YO
Loop Theory of Operation D-7