HP 8560E - Fractional N PLL Phase Noise; Fractional N Versus Offset PLL or YTO PLL Phase Noise; Reference Versus Reference PLL Phase Noise

562 Chapter10
Synthesizer Section
Phase Noise Problems
Reference versus Reference PLL Phase Noise
If the problem seems to be in the frequency reference or reference PLL
circuitry, measure the noise with internal and external references. If
there is no difference, suspect the circuitry associated with the 100
MHz VCXO (or the SAWR A15U701 on earlier A15 RF assemblies). If
there is a difference, look at 10 MHz distribution, OCXO, or TCXO.
Fractional N versus Offset PLL or YTO PLL Phase
Noise
If the spectrum analyzer has excessive noise at >1 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 A14 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 (08560-60062 and below
only)
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 A14 frequency control assembly schematic.
• The noise level of the voltage regulator should be <1 mV. The typical
noise level is 40 µV 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.