HP manual Model 8663A

Model 8663A Service

SERVICE SHEET 14

A5A3 FRACTIONAL-N LOOP PHASE DETECTOR

REFERENCEBLOCK DIAGRAM 4

Table 4-l. Recommended I’erfoymance Tests

After Adjustments or Repairs

Table 5-2. Post-Repair Adjustment Procedures

PRINCIPLES OF OPERATION

General

The purpose of the Fractional-N Loop Phase Detector (A5A3) is to

dwelwpthe FN Loop Error Voltage which is used to tune the VCO.

This FN Loop Error Voltage (tuning voltage) is developed by

integrating currents from the Phase Detector circuit, the

Fractional-N Correction Pulse ?Jidth to Current Converters, and the

Bias Sink circuit.These currents are integrated together by the

Current Summing Amplifier to develop a voltage. The Sample and Hold

circuit samples the voltage output from the Current Summing Amplifier once each reference period and at the same time during each reference

period. The sampled voltage becomes the FN Loop Error Voltage.

When the Fractional-N Loop (FN Loop) is phase-locked, the tune

voltage must be a constant dc value, This means the voltage output

from the Current Summing Amplifier must be the same at every sample

period. In order to meet this condition the total of the currents

being integrated must be the same each reference period. To look at it another way, the currents entering the summing node must equal the

currents leaving the summing node in order for the tune voltage to

remain constant. This concept, that when the FN Loop is

phase-locked, the currents entering the summing node equal. the

currents leaving the node is true for aI1 conditiwns, that is, for

the condition when the loop runs without a fractional part and for

the condition when it has a fractional part. The difference is that

when the loop operates with no fractional part the output from the

Phase Detector ci,rcuit remains constant. However, when the loop

operates with a fractional part, the output from the Phase Detector

circuit no longer remains constant but varies from reference period

to reference period. To compensate for the changing phase detector

output the outputs from the Fractional-N Correction Pulse Width to Current Converters must also change. For example, if the Phase

Detector circuit supplies less current to the summing node, the

Fractional-N Correction Pulse Width to Current Converters must supply more current so that the current entering the summing node is always a constant value.

Phase Detector

The Phase Detector consists of a pair of flip-flops, U5A and B, and gates U4D and U~C. The purpose of the Phase Detector is to generate

a pulse width proportional to the phase difference betw@en its two

input signals, the FN Loop IF (VCO/N) and the FN Loop PM Det

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