Agilent Technologies 8753ES Option 011 Service Manual page 378

Theory of Operation
Source Operation in Other Modes/Features
External Source Mode
In external source mode, the analyzer phase locks its receiver to an external signal source.
This source must be CW (not swept), but it does not need to be synthesized. The user must
enter the source frequency into the analyzer. (The analyzer's internal source output is not
used.)
To accomplish this, the phase lock loop is reconnected so that the tuning voltage from the
A11 phase lock assembly controls the VCO of the A14 fractional-N assembly and not the
A3 source. See Figure
down-converts the RF signal supplied by the external source. The resulting 1st IF is fed
back to the A11 phase lock assembly, 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 pretunes as a closed-loop synthesizer
referenced to the 100 kHz signal from the A12 reference assembly. Then, to acquire or
track, a switch causes the VCO to be tuned by the A11 phase lock assembly instead. (Refer
to the overall block diagram at the end of
Figure 12-7 External Source Mode
12-24
12-7. The VCO's output still drives the 1st LO of the samplers and
Chapter 4 , "Start Troubleshooting Here.")
Chapter 12