HP 8560A, Service Manual

Page: 420 / 450

3. Check for excessive LO feedthrough. Use the LO Feedthrough performance test in the
Installation and Verification Manual, but check over a Center Frequency range of 300
to 100 MHz. The LO Feedthrough will be 3.9107
greater than the Center Frequency
setting.
Harmonic/Spurious Outputs Too High
Harmonic and spurious outputs may be generated by A10 itself or may be present on the
either the 600 MHz drive or 1st LO drive signal. There is a direct relationship between
spurious signals on the 1st LO and spurious signals on the TG output. There is a five-to-one
relationship between spurious signals on the 600 MHz drive and the spurious signals on the
TG output. For example, if the 600 MHz signal moves 1 MHz, the TG output signal will
move 5 MHz. This is due to the multiplication in the pentupler.
1. If the Harmonic Spurious Responses performance test failed, connect another spectrum
analyzer, such as an HP to the HP 8560A LO OUTPUT connector. Set the
HP 8560A to each frequency as indicated in the performance test, with the Span set to
0 Hz. At each frequency
MORE OF 2, FREQ DIAGNOSE,
LO FREQ
. The frequency displayed will be the fundamental frequency of the LO
OUTPUT. Use the HP to measure the level of the second and third harmonics of
the 1st LO signal.
Note The 1st LO typically has a higher harmonic content than the tracking
generator output. For the purposes of this check, it is the variation in
harmonic content versus frequency which is important.
If the variation of the harmonic level of the 1st LO versus frequency tracks the harmonic
level variation of the tracking generator output, repeat this step while measuring the 1st
LO signal at the end of W43 nearest
If there is little variation in the 1st LO harmonic
level between the LO OUTPUT connector and W43, and the relative variation in
harmonic level tracks with the TG output harmonic level, suspect either the A7 LODA or
A11 YTO.
If the harmonic level variation of the 1st LO versus frequency does not track the harmonic
level variation of the TG output, suspect
2. If sidebands are present at the same frequency offset at every output frequency, check
the spectral purity of the 1st LO and the 600 MHz drive signals using another spectrum
analyzer. When checking the 1st LO, the HP 8560A must be set to zero span. Press
MORE OF 2, FREQ DIAGNOSE, LO FREQ
to determine the 1st LO frequency. A 1 MHz
sideband on the 1st LO will appear as a 1 MHz sideband on the output signal.
To verify that the 600 MHz drive or 1st LO signal is responsible for the sidebands,
substitute a clean signal for the 600 MHz drive or 1st LO signal. If the sidebands on the
output disappear when using the clean signal, the substituted signal was responsible for the
sidebands.
Note The 600 MHz drive signal should be -8 +/-3.5 The 1st LO signal
should be
11-22 RF Section