560 Chapter11
Synthesizer Section
Unlocked Fractional N PLL
Unlocked Fractional N PLL
Operation
The fractional N oscillator is used as a reference for the 1st LO phase
locked loop. It provides the 1 Hz start-frequency resolution for the 1st
LO, and is the means by which the 1st 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, FREQ DIAGNOSE, and FRAC N FREQ. The FRAC
N FREQ frequency displayed is the frequency that will be measured at
A14J304 with the spectrum analyzer in zero span.
Confirming an Unlocked Condition
1. Set the spectrum analyzer to the following settings:
Center frequency .................................................. 300MHz
Span ..............................................................................0Hz
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 A14J304 to
the input of a 20 dB gain amplifier, such as an 8447E. 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 visible through the shield on A14. If either LED
is lit, the fractional N PLL is not locked.
5. If either LED on A14 is lit, and no error message is displayed, check
FC MUX U305. Refer to function block AH of A14 frequency control
schematic (sheet 4 of 5).
6. If neither LED is lit, but the output frequency is wrong by more than
1 MHz, check the postscaler, function block AV.
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