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 10-6 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

>20 MHz

All YTO’s Main Coil

1.01MHz to 20 MHz All YTO’s FM Coil

1.01kHz to 1 MHz Main Roller Oscillator

5100 kHz

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

kHz to 2.9 GHz

1

18

GHz to 325 GHz

6 through 52

depending upon

lock harmonic

selected

Synthesizer Section lo-15