HP 8560E, Service Manual

Page: 562 / 665

Chapter 10 561
Synthesizer Section
Phase Noise Problems
Phase Noise Problems
System phase noise can be a result of noise generated in many different
areas of the spectrum analyzer. When the spectrum analyzer is
functioning correctly, the noise can be observed as a function of the
distance away (the offset) from the carrier frequency. The major
contributor to system noise can be characterized as coming from specific
circuit areas depending upon the offset frequency.
Some very general recommendations can be made for identifying which
circuitry is the cause of the noise at certain offsets. The
recommendations below apply with a center frequency of 1 GHz.
Phase Noise in Locked versus Lock-and-Roll Spans
Input a signal to the spectrum analyzer. Set the center frequency to the
input signal frequency, set the span to 2 MHz, and plot the display. This
plots the system noise for a locked sweep. Plot the display again with a
span of 2.01 MHz (lock and roll sweep). The crossover point of the noise
floor of the two plots is typically at an offset of about 50 kHz, for a
functioning instrument.
If the crossover point is shifted out to a higher offset frequency, suspect
the YTO loop circuitry.
If the crossover point is shifted in to a lower offset frequency, suspect
the offset or fractional N loop circuitry.
Table 10-18 Settings of Sweep Switches
Carrier Frequency Offset Major Contributor (when
working correctly)
100 Hz Reference (OCXO or TCXO)
1 kHz 100 MHz (or 600 MHz)
reference PLL
3 kHz Fractional N PLL
10 kHz to 150 kHz Offset lock loop or YTO loop
>150 kHz YTO