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compared to the 1.000 MHz reference, and used to generate a

tuning

voltage as

usual. However, the tuning voltage controls the VCO to lock on to the external

source, keeping the

1st

IF at exactly 1.000 MHz.

The analyzer normally goes through a pretune-acquire-track sequence to achieve

phase lock. In external source mode, the fractional-N

VCO

prettmes

as a

closed-loop synthesizer referenced to the 100

kHz

signal from the

Al2

reference

assembly. Then, to acquire or track, a switch causes the VCO to be tuned by the

All phase lock assembly instead. (Refer to the Overall Block Diagram at the

end of Chapter 4, “Start Troubleshooting Here.“)

A13/14

FRACTIONAL-N

30 to 60 MHz

IST

sg6235d

Figure 12-7. External Source Mode

12-24 Theory of Operation

HP 8753E Service Manual

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