Loops; Yo Loop; Controller Section - HP 8340B Manual

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
I
instrument.
It
contains
a
16-bit microprocessor,
a
total of
64K 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
Change
7
Assembly-Level Service Overall Instrument Theory A-5