HP 8340B Manual page 173

RF
Source
A44 YIG Oscillator/Bias Assembly. The A44 YIG oscillator
(YO)
provides the
fundamental RF output
frequency
for the instrument. The
frequency range of the YO
is
2.3 to 7.0 GHz.
The YO
uses two tuning coils:
•
Main
Coil
The
main
coil is
the primary tuning component of the YO. The
main
coil
tunes
the
YO
to
within the
capture range of the phase-locked YO feedback loop (within ±20
MHz of selected frequency), and
provides low-frequency
(<100 Hz) phase-error correction.
•
FMCoil
The FM
coil frequency modulates
the
YIG oscillator when
a signal is
input to the instrument's
FM
input
BNC. The FM
coil
also allows high frequency (>100 Hz) phase-error correction, that is provided by the
A49 YO loop phase detector (described
below).
Feedback Loop
Assemblies
A45 Directional Coupler. The directional coupler splits the output of the YIG oscillator. One output
goes to the RF Section (and eventually to
the
instrument's RF output) and the other goes
to the
YO
feedback
loop.
The feedback loop portion goes to the
AT2 15
dB attenuator.
AT2 15 dB Attenuator. AT2 attenuates the coupled-off
portion
of the YIG oscillator output. Signal
attenuation
is
required so
that the feedback
signal can be mixed with
a
synthesized reference and be
phase/frequency compared. These processes are
described below. AT2 also provides isolation
between
the feedback loop assemblies and the YIG oscillator.
A46
7 GHz Low Pass
Filter.
The
low pass filter reduces harmonics from the coupled-off YIG
oscillator output to prevent unwanted mixing products
in the
A48 YO loop sampler.
A48 YO
Loop
Sampler. The sampler
mixes
the
coupled-off YIG oscillator output (from the A46
assembly) with the synthesized output of the
M/N
loop.
The
output
is
a
20 to 30 MHz
difference signal,
that is
sent to the A49 YO loop phase detector.
A49 YO Loop Phase Detector.
The 20-30 MHz
IF signal from the A48 assembly
is
phase/frequency
compared to the output of the
20-30
synthesizer.
The resultant
error
signal
is
integrated and sent to
the sample and hold circuit.
SAMPLE MODE.
This no-memory mode sends
a
low frequency error correction signal (YO
TUNE)
directly to the ASS
YO driver. This mode
is used
in CW/Manual
operation, or
in swept mode when
the
YIG oscillator (YO)
is swept
5 MHz
or
less. This
completes the YO feedback loop
and phase-locks the
YO.
HOLD MODE.
This mode must be used when the YO
is
swept
>5
MHz, Sample
mode first allows the
YO to phase-lock
at the beginning of the
sweep. Afterwards,
the hold mode holds the
YO TUNE
voltage constant while
VSWP
is
summed
into the
PRETUNE
signal, which
in turn sweeps
the
YO.
This
sequence
is
repeated at the beginning of each
sweep.
HP 8340B/41B
Sweep Generator
-
YO
Loop Theory of Operation D-3