12-20 Chapter 12
Theory of Operation
Source Theory Overview
Since the chosen VCO harmonic and the source differ by 0.500 or 0.333 MHz, then another
VCO harmonic, 2 or 3 times higher in frequency, will be exactly 1.000 MHz away from the
2nd or 3rd harmonic of the source frequency. The samplers, then, will also down-convert
these harmonics to yield the desired components in the 1st IF at 1.000 MHz. Narrow
bandpass filters in the receiver eliminate all but the 1.000 MHz signals; these pass
through to be processed and displayed.
Figure 12-6 Harmonic Analysis
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 12-7. The VCO’s output still drives the 1st LO of the samplers and
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.
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