HP 8560A, Service Manual

Page: 359 / 450

Frequency Span Accuracy Problems
The HP 8560A employs lock-and-roll tuning to sweep the 1st LO. The 1st LO is locked to the
start frequency immediately after the previous sweep has been completed. The 1st LO is then
unlocked, and, when a trigger signal is detected, the 1st LO sweeps (rolls).
When there is a considerable delay between the end of one sweep and the beginning of the
next, the actual 1st LO start frequency may differ from the locked start frequency. This start
frequency drift will be most noticeable in a 1.01 MHz LO span (the narrowest FM coil span).
This drift is not noticeable in either Free Run or Line Trigger modes.
The sweep signal is applied to different oscillators in the synthesizer section depending on the
desired first-LO’s span (due to harmonic mixing, this is not necessarily the same as the span
setting of the analyzer). Refer to Table
for a listing of sweep-signal destinations versus
First LO spans.
Sweeping the Roller Oscillator’s Offset Oscillator PLL results in sweeping the YTO’s FM
coil. There is a one-to-one relationship between the roller-oscillator’s frequency span and the
first-LO’s span.
Table 10-6. Sweep Signal Destination versus Span
First LO Span Sweep Signal Destination
MHz All YTO’s Main Coil
1.01 MHz to 20 MHz All YTO’s FM Coil
1.01 to 1 MHz Main Roller Oscillator
Offset Roller Oscillator
Determining the First LO Span
The first-LO’s span depends on the spectrum analyzer’s harmonic-mixing number. Use the
following steps to determine the first-LO’s span:
1. Read the span setting displayed on the HP 8560A.
2. Determine the harmonic-mixing number from the information in Table 10-7.
Table 10-7. Harmonic Mixing Number versus Center Frequency
Center Frequency Harmonic
Mixing
Number
1 to 2.9
1
18 to 325
6 through 52
depending upon
lock harmonic
selected
Synthesizer Section
15