Reference Loop Theory Of Operation - HP 8340B Manual

Reference Loop
Theory of Operation
The
reference loop produces
all of the reference signals that
are
used
in
instrument phase-locked
loops. Each
of these
signals
are referenced to either the internal
A5110 MHz
standard or
an
external
5
or 10
MHz
source connected to the EXT REF BNC connector on the rear panel. The frequency stability
of the instrument is
directly
related to the
stability of the reference.
The
reference loop consists of four assemblies, shown
in
functional order:
•
A30100 MHz VCXO
assembly
•
A29 reference phase detector assembly
•
A34 reference
loop
—
M/N
loop motherboard
•
A5110 MHz
standard
A30 100 MHz VOLTAGE CONTROLLED CRYSTAL OSCILLATOR
ASSEMBLY
The A30 VCXO assembly
is
a
crystal-stabilized voltage controlled oscillator with
a
±1 kHz
tuning
range.
It
provides
a
100
MHz reference signal
for four instrument
circuits:
•
20-30 phase lock loops
(PLL3)
•
3.7 GHz
oscillator (in the RF section)
•
M/N
phase lock
loop (after being multiplied by four)
•
A29 reference phase detector for phase-locking to the 10
MHz
standard
The A30 VCXO
is
phase-locked
to the
A51 frequency standard by the A29 reference phase detector
assembly.
A29 REFERENCE PHASE DETECTOR ASSEMBLY
The reference phase detector
phase-locks the A30 VCXO
to the
A5110 MHz
reference standard.
This
assembly
also divides
the output of the A30 VCXO
and distributes it
to the following circuits:
•
M/N
phase
lock loop
•
20-30 phase
lock loops (PLL1, PLL2)
•
10
MHz OUT
on the rear panel
The
100 MHz VCXO
output
is divided by ten, providing
a
10
MHz input to the reference phase detector.
The 10
MHz (internal
or external) frequency standard
is the second input
to
the reference phase
detector.
The VCXO
tuning voltage output
is proportional to the error in
the
100
MHz VCXO.
This
assembly also
divides the 100 MHz VCXO
output frequency by
five and
ten, providing
20
MHz
and 10
MHz respectively. The 20 MHz
signal
is output to the
M/N
phase
lock loops. 10
MHz
is output
to the 20-30
phase lock loops and to the
rear panel
10
MHz REFERENCE OUTPUT
connector.
B-2
Reference Loop Theory
of Operation
HP
8340B/41B