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