Agilent Technologies 856290216 Frequency Span Accuracy Problems, Determining the First LO Span

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

Harmonic Mixing Number

1

2.75 GHz to 6.46 GHz

11-36 Synthesizer Section