HP 8562E, Service Manual

Page: 405 / 482

Unlocked Fractional N PLL
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],
OF 2, DIAGNOSE, and N . The FRAC N FREQ frequency displayed
is the frequency that will be measured at with the HP 85633 in zero span.
1. Set the spectrum analyzer to the following settings:
Center frequency . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . ...300 MHz
Span . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
2. Connect FRAC N TEST to the input of a synthesized spectrum analyzer and
view the fractional N PLL output at 66.7 MHz.
If a synthesized spectrum analyzer is not available, connect A145304 to the
input of a 20
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 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 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 should be equal
to the frequency at 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 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Trigger . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . ..Single.EXT
(with no external trigger connected)
Synthesizer Section 11-31