HP 8560E, Service Manual

Page: 556 / 665

Chapter 10 555
Synthesizer Section
Frequency Span Accuracy Problems
Frequency Span Accuracy Problems
The HP 8560E/EC employs lock-and-roll tuning to sweep the first LO
for spans greater than 2.0 MHz. The first LO is locked to the start
frequency immediately after the previous sweep has been completed.
The first LO is then unlocked, and, when a trigger signal is detected,
the first LO sweeps (rolls).
When there is a considerable delay between the end of one sweep and
the beginning of the next, the actual first LO start frequency may differ
from the locked start frequency. This start frequency drift will be most
noticeable in a 2.01 MHz LO span (the narrowest FM coil span). This
drift is not noticeable in either free run or line trigger modes. The
sweep is generated by different oscillators in the synthesizer section
depending on the desired first-LO span (due to harmonic mixing, this is
not necessarily the same as the span setting of the spectrum analyzer).
Refer to
Table 10-15 on page 555 for a listing of sweep-signal
destinations versus First LO spans. Sweeping the fractional N
oscillator results in sweeping the YTO FM coil. There is a one-to-one
relationship between the fractional N oscillator frequency span and the
first-LO span. The fractional N oscillator sweep is generated digitally.
The oscillator is always synthesized, rather than employing lock and
roll tuning.
Determining the First LO Span
The first-LO span depends on the spectrum analyzer harmonic-mixing
number. Use the following steps to determine the span of the first LO:
1. Read the span setting displayed on the HP 8560E/EC.
2. Determine the harmonic-mixing number from the information in
Table 10-16 on page 556 .
Table 10-15 Sweep Signal Destination versus Span
First LO Span Sweep Signal Destination
>20 MHz A11 YTO main coil
2.01 MHz to 20 MHz A11 YTO FM coil
≤
2 MHz None Fractional N oscillator
sweeps without a sweep ramp
signal.