Operation; Loop Characteristics; Pll2 Theory Of Operation - HP 8340B Manual

20-30
LOOP CHARACTERISTICS
20-30 Loop assemblies
form
a
frequency synthesizer
with the following performance characteristics:
•
Fixed frequency output range from 20 to 30
MHz
(required by the YO loop)
• Analog
sweep
widths between 100 Hz and 5
MHz
•
CW frequency
resolution as low as
1
Hz
The 20-30 Loops contain three phase locked loops that are used in three different configurations
to
achieve the required
CW
resolution and sweep range. The loops are referred
to as PLL1, PLL2 and
PLL3. PLL2
is
used
in all modes of operation.
PLL1 and PLL3 are
only used during
narrow sweeps or
in
CW
mode, and are designed to be
phase-locked to the output of PLL2. The purpose of PLL3 and
PLL1
are explained below.
PLL2 THEORY OF OPERATION
PLL2
Assemblies
•
A40 PLL2
VCO (voltage controlled oscillator)
•
A41 PLL2 phase detector
•
A42 PLL2 divider
•
A43 PLL2
discriminator
PLL2 VCO
The PLL2 phase
lock loop is the primary phase lock loop in the 20-30 synthesizer, and is
used
in
all
modes of operation.
Refer to
Figure C-1. PLL2 phase-locks to
a
10
MHz
signal
from the reference
loop.
The 20-30
loops must
sweep
the
YO
directly during
YO sweeps of 100 Hz to 5
Mhz,
but the 20-30
loops
output has to be divided down in steps to allow this. The
three
divided outputs
each cover
a
portion of the 100 Hz to 5
MHz
range:
YO
Sweep
width
is between
100 Hz and
5 kHz. The 75 to 150 MHz VCO
signal
is divided by
500
to provide the necessary resolution, However, the output signal can
not be used directly
by the
YO
loop.
The
PLL3 and PLL1
loops convert this high-resolution, low-frequency signal to
a
frequency
range that is
usable by the YO loop (20 to 30 MHz).
YO Sweep width
is
Between
>5 kHz and
100 kHz. The 75 to 150 MHz VCO signal
is divided by
25
(divided by five twice).
The
output
is
3
to
6
MHz,
and can not be used directly by the YO loop.
The PLL3 and PLL1
loops convert this high-resolution, low-frequency signal to
a frequency range
that is usable by the
YO loop (20 to
30 MHz).
YO Sweep width
is
Between
>100
kHz and
5 MHz. The 75 to 150 MHz VCO signal
is
divided by
five to
produce a
15
to
30 MHz
output. This signal
is
sent directly to the YO
loop.
HP 8340B/41B
20-30 Loops Overall Theory of Operation
C-3