Unlocked Fractional N Pll; Operation; Confirming An Unlocked Condition - Agilent Technologies 8564EC Service Manual

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
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:
2. Connect A14J304 FRAC N TEST to the input of a synthesized
If a synthesized spectrum analyzer is not available, connect A14J304 to
NOTE
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
4. Check the two LEDs visible through the shield on A14. If either LED
5. If either LED on A14 is lit, and no error message is displayed, check
6. If neither LED is lit, but the output frequency is wrong by more than
560
, ,
CAL MORE 1 OF 2
Center frequency .................................................. 300 MHz
Span .............................................................................. 0 Hz
spectrum analyzer and view the fractional N PLL output at
66.7 MHz.
fractional N PLL is probably locked.
is lit, the fractional N PLL is not locked.
FC MUX U305. Refer to function block AH of A14 frequency control
schematic (sheet 4 of 5).
1 MHz, check the postscaler, function block AV.
, and
FREQ DIAGNOSE
. The FRAC
FRAC N FREQ
Chapter 11