HP 8340b - 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