Keysight Technologies N5247B Service Manual page 182

Theory of Operation
Synthesized Source Group Operation
The A4, A15, and A17 13.5 GHz synthesizer boards each contain their own
phase lock circuitry. The A15 board produces an independently phase locked
LO signal while the A4 and A17 boards produce independently phase locked
test signals. This makes it possible for the LO signal to be tuned to a different
frequency than the test signal. With frequency offset mode disabled, the LO
signal is a frequency value higher than the test signal. For analyzers with DSP
version 4.0, the frequency value is 7.606 MHz. For analyzers with DSP version
5.0, the frequency value is 7.438 MHz. Since the A4, A15, and A17 13.5 GHz
synthesizer boards each receive their 50 MHz input reference signal from the
exact same source, frequency drift error is eliminated.
5-12 Keysight N5247B Service Guide