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Synthesizer Section
Frequency Span Accuracy Problems
Frequency Span Accuracy Problems
The spectrum analyzer employs lock-and-roll tuning to sweep the 1st
LO for spans greater than 2.0 MHz. 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 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 1st LO span (due to harmonic mixing,
this is not necessarily the same as the span setting of the analyzer).
Refer to
destinations versus 1st LO spans.
Sweeping the fractional N oscillator results in sweeping the YTO FM
coil. There is a one-to-one relationship between the frequency span of
the fractional N assembly and the 1st LO span. The fractional N
oscillator sweep is generated digitally. The oscillator is always
synthesized, rather than employing lock and roll tuning.
Table 11-12
Sweep Signal Destination versus Span
1st LO Span
>20 MHz
2.01 MHz to 20
MHz
≤2 MHz
Determining the 1st LO Span
The 1st LO span depends on the spectrum analyzer harmonic-mixing
number. Use the following steps to determine the 1st LO span:
1. Read the span setting displayed on the spectrum analyzer.
568
Table 11-12 on page 568
Sweep Signal Destination
A11 YTO main coil
A11 YTO FM coil
None Fractional N oscillator sweeps without a
sweep ramp signal.
for a listing of sweep-signal
Chapter 11
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