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Fractional
N
versus
Offset
PLL
or
YTO
PLL
Phase
Noise
If the spectrum analyzer has excessive noise at
>l
kHz offset, measure the noise with center
frequencies of 100 MHz and 2.5 GHz.
If the measurements are equal, suspect the fractional N circuitry and the YTO loop circuitry
on the Al4 frequency control assembly.
If the measurements differ by 2 dB to 5 dB, with the 2.5 GHz measurement at a higher noise
level, suspect the offset lock loop circuitry.
Fractional
N
PLL
Phase
Noise
Check the noise on the 5 V regulators on A14, particularly the regulator in the reference
divider circuitry A14U121. Refer to function block AN on the Al4 frequency control assembly
schematic.
The noise level of the voltage regulator should be
<l
mV.
The typical noise level is 40 PV
RMS between 10 Hz and 100 kHz.
A coaxial probe with very little unshielded tip area should be used to avoid picking up
radiated 60 Hz. Check that your measurement is valid by probing ground on the circuit
and verifying that the measured value is well under the 1
mV
threshold that indicates a
defective regulator.
There can also be phase noise problems if the loop gain is incorrect. See function block AP for
loop gain troubleshooting information.
11-44 Synthesizer Section