Pll3 Theory Of Operation - HP 8340B Manual

PLL3 THEORY OF OPERATION
PLL3
Assembly
•
A39 PLL3 upconverter.
The
PLL3 and
PLL1
phase-lock loops
are
only used when
in
CW
mode, or
in
swept frequency
modes requiring YIG
oscillator
(YO) sweep
widths less than 100
kHz.
Refer to Figure
C-1. During
YO sweep
widths of
100
kHz to
5
MHz,
the output of the PLL2 VCO
(75
to 150 MHz)
is divided by
5
to provide
a
15 to 30 MHz output. This output is sent directly to the YO
loop.
For sweep widths less than 100 kHz,
this
configuration provides insufficient resolution (1 kHz)
to set the start frequency to closer than
0.5%
of the sweep width.
To provide finer resolution, the output
of
the PLL2 VCO
is
divided further (either by 25 for YO
sweep
widths between
5
kHz
and 100 kHz, or by
500 for YO sweep widths
less than
5
kHz). The
frequency, however,
is also reduced
such that it is
no longer
in
the 20
to
30 MHz
range.
PLL1 and
PLL3 Loops translate the high resolution, low frequency PLL2 output up to
a
200 to 300 MHz
range. Since frequency translation
is
a
fixed offset in
frequency,
it
does not change the
resolution
or sweep width. After the frequency translation, the output
of PLL1
is
divided by ten to reduce the
200-300 MHz
signal
to 20-30 MHz. This also increases the output resolution by
a
factor of ten.
The A39 PLL3 assembly mixes
160
MHz with
the output of the A40 PLL2 VCO, and outputs
the
sum of the two frequencies. This
Is done
using
a phase-lock
loop with
a
closed-loop bandwidth of
approximately
10 kHz. PLL3 uses
a
reference
signal from the 100 MHz VCXO
reference to
generate
the 160
MHz
offsetting frequency. As the PLL2 output changes frequency, the PLL3
output also changes frequency with the same resolution and sweep width, but at
a
higher
operating frequency.
Frequency Multiplier
x
1.6.
A 160 MHz reference signal
is
required to offset the PLL2 output by
160
MHz. The
frequency multiplier
X
1.6 generates
160
MHz
by dividing the 100
MHz
input
reference
signal by five and then selecting the eighth harmonic.
Mixer.
The mixer output is
amplified, filtered, and sent to the
phase/frequency detector. The
desired mixer output is the difference frequency and is
between
150 kHz and
6
Mhz.
Phase/Frequency Detector.
The phase/frequency detector generates
a
differential output signal
that
is
used by the loop amplifier and the
phase lock indicator.
C-6 20-30
Loops
Overall Theory of
Operation
HP 8340B/41B