HP 8340B Manual page 465

20-30
LOOPS
The 20-30
loops contain three phase-locked loops used together to provide the YO loop with
1
Hz CW
resolution, and with analog
sweep
widths from 100
Hz to
5
MHz.
For
sweep widths of
5 MHz
or less,
the
YO
Loop remains phase-locked during the
sweep,
and the 20-30
Loop is swept
the desired
amount. For
sweep
widths
greater than
5
MHz,
all
phase-lock loops
lock at the beginning of
sweep,
the
20-30 Loop remains
fixed,
and the YO
is
swept. This
is called lock and
roll,
and
will be
discussed
later.
The 20-30
loop generates
an output between
20
and
30 MHz
in
CW
mode, and between
15
and
30
MHz when
swept. This
loop
has an output resolution of
1
Hz. Comparing and
locking the
down-
converted YO frequency
to the 20-30
output produces
a
1
Hz
resolution
in
the YO frequency.
YO LOOP
The YO Loop contains the YIG oscillator (YO), which
is the tunable local oscillator source for
all
frequency bands. When the instrument is
set to
a
specific
CW
frequency, the instrument processor
sets the frequency of the
M/N
and
20-30
loops
accordingly. The instrument processor also sets the
YO
pretune voltage, which tunes the
YO to
the
approximate frequency.
The
output of the YO
is fed through the A45 directional coupler to the
RF
section.
The directional
coupler
splits off part
of the YO signal, which goes
to the sampler. The
M/N
output frequency and its
harmonics are mixed with the YO frequency
in the sampler to produce
an IF signal between 20
and
30
MHz. The 20 to 30
MHz
IF
from the sampler is
compared
to the 20-30
loop output
in
the phase/
frequency detector.
The error-induced voltage from the
phase/frequency detector is
fed through
a
sample/hold circuit and
is summed with the pretune
voltage that drives the YO tuning
coils.
The YO frequency changes
until
the output voltage from the phase/frequency detector goes to zero and phase-lock is
achieved. For
sweep
widths greater than
5
MHz,
the
YO
is
phase-locked at the start of
the
sweep.
The sample/hold
circuit
is
then set to hold, breaking the loop, and allowing the
YO to sweep. The
sweep generator
initiates
a
voltage sweep (VSWP) ramp that
is
summed
with the
YO pretune voltage.
This voltage causes the YO to sweep to the STOP frequency.
This action is
referred to as Lock and
Roll.
During multi-band sweeps, the
YO
phase-locks at
the start of each band before continuing on
with the
sweep, ensuring frequency accuracy at the beginning
of each band.
CONTROLLER SECTION
The
controller section performs all
data transfer, and
coordinates the control signals that operate the
instrument.
It
contains
a
16-bit
microprocessor, a
total of
34K X 16 ROM and 8K X 16
RAM.
This
section also contains Interface circuitry for communicating with the rest of the instrument.
Digital information
is
exchanged between the microprocessor and other sections of the
instrument on
a
bi-directional bus. At power on instrument preset, the controller runs through an Instrument self-
test.
The microprocessor
also sets the front panel controls to preset conditions if
instrument preset
was pressed.
HP 8340B/41B
Assembly-Level Service Overall Instrument Theory A-5