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Model 8901B
Service
To prevent mistuning on the doubler range (which can result from spurious LO signals) the input to the
Doubler is filtered by a tunable, Doubler Input Filter. The filter primarily suppresses the third harmonic
of the HF VCO which becomes the
3/2
harmonic of the doubled signal. The Doubler High-Pass Filter
following the Doubler suppresses feedthrough of the fundamental frequency (the
harmonic).
Low
Frequency VCXO and Filter Assemblies (A22 and A21)
The Low-Frequency, Voltage-Controlled, Crystal Oscillator (VCXO) is a highly stable, tunable, reference
oscillator to which the HF VCO is locked in the low-noise tune modes. It consists of two tunable
crystal oscillators (nominally 9.26 and 11.26 MHz) mixed together to produce a 2 MHz output. The
two oscillators can each be tuned approximately 6.25 kHz in opposition for a total tuning range of 2
MHz
16.25 kHz. This tuning scheme allows a broad tuning range while retaining the high stability
of the individual oscillators. The 2 MHz Low-Pass Filter and 2 MHz Bandpass Filter (A21) reject
unwanted mixing products which appear as spurious AM and FM residual tones. Careful selection of
the crystal frequencies minimizes the output of spurious mixing products.
Sampler Assembly (A23)
The Sampler is the phase detector of the phase lock loop. The tunable 2 MHz signal from the LF
VCXO (A21) drives the Sampling Bridge through the 2
MHz
Limiter and Impulse Generator. The
output of the impulse generator is a train of extremely short-duration pulses with the repetition rate
of the 2 MHz signal. The two pulses momentarily turn on the diodes (that is, close the switch) of
the Sampling Bridge and pass the signal from the HF VCO (A24). The output from the Sampling
Bridge is thus the HF VCO sampled a t a 2 MHz rate. If the two signals are harmonically coherent,
the output is a dc voltage with a level determined by the phase and amplitude of the
HF
VCO. The
action of the phase lock loop tunes the
HF
VCO to drive the voltage to zero. If the relationship is not
strictly harmonic (that is, phase lock is broken), the output is a beat note with a frequency equal to
the difference between the HF VCO and the nearest harmonic of the LF VCXO. The output of the
Sampling Bridge, which is the phase error voltage,
is
smoothed and buffered by the Sampler Amplifier.
The tune voltage for the HF VCO is supplied by the HF VCO Tune Integrator and Amplifier. The tune
integrator has several sources of input: the Sampler Amplifier, the Track Loop Amplifier, the Sweep Up
Current Source, the Sweep Down Current Source, and the DAC Control Amplifier. Only one input is
active at a time. If one of the sweep current sources is active, the tune integrator sweeps the HF VCO.
If the input is one of the amplifier outputs, the HF VCO Tune Integrator and Amplifier is configured
as part of a feedback loop.
The grounding switch at the input of the H F VCO Tune Integrator and Amplifier is open only when the
Sampler Amplifier
is
connected to its input. When the amplifier is not connected, the switch is closed to
keep signals at the Sampler Amplifier output from coupling into the tune integrator. The Out-of-Lock
Detector at the Sampler Amplifier output senses the presence of ripple and lights the OUT OF LOCK
annunciator to indicate lock has broken. A BW Control line also lights the annunciator when the Tune
Voltage Filter (in the H F VCO Assembly) has not been turned on. This line also controls the bandwidth
of the HF VCO Tune Integrator and Amplifier. The bandwidth is narrowed in the low-noise phase lock
and the track modes (that is, whenever the instrument is properly tuned).
The No-HF-VCO Detector lights the NO HF VCO annunciator if the amplitude of the signal from
the HF VCO is too low. The 700 MHz Low-Pass Filter in the Sampling Bridge input line filters out
harmonics of the HF VCO to assure proper sampler gain.
LO Control Assembly (A20)
The LO Control Assembly contains the digital decoders and latches for the entire RF Section (that
is,
the section housing assemblies A15 through A24) and the low-frequency analog circuits that control
and tune the LO.
Service Sheet BD2
8D-19
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