HP 8340B Manual page 176

The mixing and multiplying make bandcrosslngs
necessary. At each crossing, VSWP
resets to zero
and the YO
is phase-locked to
the appropriate frequency for
the start of the next band. The result is
that
in
a
multi-band sweep the
MKR RMP
is
a
ramp that pauses for bandcrossings, and VSWP
is
a
series of ramps, each
starting at
zero
volts.
The slope of any particular ramp depends on the
YO
harmonic being used
in
that band.
The MKR RMP
is
a
0
to -HOV
ramp used
as the reference for
all
sweep events. In
a
single band
sweep, the voltage goes
linearly
from
0
to
+10
volts
in the selected sweep time. In
a
multi-band
sweep, the voltage is
a
segmented ramp where the
linear
ramp portions correspond to the particular
bands swept, and
the flat portions correspond to the time necessary to re-phase-lock the instrument
for the next band. In the multi-band case the
sum of the times for the various
linear
ramp portions
equal the selected instrument
sweep
time. That
is
to say, the instrument sweep time is the time during
which the instrument
is actually sweeping and does not
include the
re-phase-lock time
at each band-
crossing.
Narrow Band Sweep. YO sweep
widths
:s5 MHz are achieved by sweeping the 20-30
synthesizer.
The
ASS sweep generator assembly produces the 20-30 SWP
(from inputs from the
processor),
that
causes the 20-30 output to sweep in
frequency. The A49 YO loop phase detector keeps the YO phase-
locked to
this signal
for the duration of the
sweep.
For
narrow
band
YO sweep widths between 500
kHz and
5 MHz, PRETUNE
is
also
swept (VSWP summed
in)
a
proportional amount so that the
pretuned YO frequency remains
within the
capture range
of the
YO phase lock loop. This
is
in addition
to the phase-locked error correction by YO
TUNE. For
YO sweep
widths
<
500 kHz, VSWP is
dis¬
abled, and YO TUNE alone
supplies the correction to keep the
YO phase-locked for the
entire
sweep
ramp voltage.
YO Loop Detailed Theory Of Operation
The
assemblies (documented in this functional group) that comprise the
YO
loop are
show below:
A44 YIG
oscillator
A45
directional coupler
AT2 15 dB attenuator
A46 7
GHz
low pass
filter
A48 YO loop sampler assembly
A49 YO
loop phase detector assembly
A50 YO
loop interconnect assembly
A54 YO pretune/delay compensation assembly
A55 YO driver assembly
In
CW
and manual modes, the
YO loop phase-locks the YO at the appropriate frequency.
The
informa¬
tion to tune the YO comes from two
sources:
PRETUNE
and
YO TUNE,
that combine to form the YO
FM COIL DRIVE and the YO MAIN COIL DRIVE.
Pretune. The
PRETUNE
voltage is
generated
on the A54
YO
pretune/delay compensation assembly.
It is
the scaled sum of VDAC,
a
tune voltage generated from
inputs from
the instrument processor,
and VREF,
a
constant DC
offset voltage.
PRETUNE
has
a
sensitivity of
—2.5V/GHz
of YO sweep.
VCOMP
makes
no contribution to
PRETUNE
in the
CW or Manual modes. LVSX (low voltage
sweep
disable) is low to shunt out any unwanted noise contribution from the
VSWP
line.
D-6
Sweep Generator
-
YO
Loop Theory of Operation
HP 8340B/41B