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Frequency Span Accuracy Problems
The spectrum analyzer 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 span of the first LO (this is not necessarily the same as the span setting of
the analyzer, due to harmonic mixing). Refer to Table 11-13 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 span of
the first LO. The fractional N oscillator sweep is generated digitally. The oscillator is always
synthesized, rather than employing lock and roll tuning.
Table 11-13. Sweep Signal Destination versus Span
I
First LO Span
1
Sweep Signal Destination
>20
MHz All YTO main coil
2.01 MHz to 20 MHz All YTO FM coil
12
MHz
None
Fractional N oscillator sweeps
without a sweep ramp signal.
Determining
the
First
LO
Span
The span of the first LO depends on the spectrum analyzer harmonic-mixing number. Use the
following steps to determine the first LO span:
1. Read the span setting displayed on the spectrum analyzer.
2. Determine the harmonic-mixing number from the information in Table 11-14.
Table 11-14. Harmonic Mixing Number versus Center Frequency
9 kHz to 2.9 GHz
2.75 GHz to 6.46 GHz
Harmonic Mixing Number
1
11-36
Synthesizer Section