Agilent Technologies 856290216 Unlocked Fractional N PLL, Confirming an Unlocked Condition

Unlocked Fractional N PLL

Operation

The fractional N oscillator is used in the HP 85633 as a reference for the first LO phase locked loop. It provides the 1 Hz start-frequency resolution for the first LO, and is the means by which the first LO is swept in LO spans of 2 MHz or less (fractional N spans). The prescaler, fractional N divider, and the postscaler are preset at power-on.

The PLL operates to produce an output frequency in the range of 60 MHz to 96 MHz selectable in 1 Hz increments. The output frequency can be swept (increasing or decreasing) over a selectable 100 Hz to 2 MHz range.

To determine the fractional N frequency for any given center frequency, press (CAL],

MORE 1 OF 2, F’RE’,q DIAGNOSE, and FEAC N FREP . The FRAC N FREQ frequency displayed is the frequency that will be measured at A14J304 with the HP 85633 in zero span.

Confirming an Unlocked Condition

1. Set the spectrum analyzer to the following settings:

Center frequency . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . ...300 MHz Span . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . OHz

2.Connect A14J304 FRAC N TEST to the input of a synthesized spectrum analyzer and view the fractional N PLL output at 66.7 MHz.

Note	If a synthesized spectrum analyzer is not	available, connect A145304 to the
	input of a 20 dB gain amplifier, such as an HP 84473. Connect the output of
	the amplifier to the input of a frequency	counter.

3.If the fractional N oscillator measures a stable 66.7 MHz, the fractional N PLL is probably locked.

4.Check the two LEDs that are visible through the shield on A14. If either LED is lit, the fractional N PLL is not locked.

5.If either LED on Al4 is lit, and no error message is displayed, check FC MUX U305. Refer to function block AH of Al4 frequency control schematic.

6.If neither LED is lit, but the output frequency is wrong by more than 1 MHz, check the postscaler, function block AV.

7.Check that the postscaler is dividing properly. The frequency at A14J304 should be equal to the frequency at A14TP4 divided by either 5, 6, or 7. Refer to Table 11-10. To keep the divide number at a constant value set the spectrum analyzer to:

Span . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . OHz

Trigger . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . ..Single.EXT

(with no external trigger connected)

Synthesizer Section 11-31