Spectral Purity/Harmonics (17,18 - HP 8340B Manual

2.
Press [INSTR PRESET] and check for the following sweep
indications:
a.
Check
front panel
SWEEP
LED. The LED should be
blinking.
The sweep LED, which is turned on at start of sweep, off at each bandcross, and at
end-of-
sweep,
is
controlled by LSPLD (low sweep
LED). The
sweep LED
is
turned on when HSP (high
start sweep)
is
low,
LRSP (low reset sweep)
is high,
and LBX (low bandcross)
is high.
b.
Check the
SWEEP OUT
signal.
The sweep outputs come from the A57 marker/bandcross assembly, and are generated from
the
MKR RMP.
For
a
detailed description of the
SWEEP OUT
or
MKR RAMP waveform,
refer
to the sweep generator/YO loop functional group.
If the
sweep out waveform
is normal, the
ASS
assembly ramp generator
is good, the
WSPTM
(write
sweep
time) strobe
is
generated, and LRESET (low reset)
is
working.
c.
Check rear panel O.SV/QHz
waveform.
The 0.5V/GHz signal comes from the A28 SYTM
driver assembly, and
is
generated by the
PRETUNE
signal, which
is generated from VSWP.
If
the rear panel O.SV/GHz signal is
normal,
VSWP
and
PRETUNE
are working.
NOTE:
For sweep widths <5
MHz (20/30
sweeps), 0.5V/GHZ is
fixed.
SPECTRAL PURITY
/
HARMONICS
(17,18)
NOTE:
In this section, spectral
purity
is
considered
a
phase noise problem and spurious responses
are considered to be discrete harmonics.
Phase Noise
This procedure
assumes the
instrument
Failed
the Single Sideband Phase Noise
test,
in Section
IV.
1.
If
the instrument failed
the test at offset frequencies less than
300
Hz, the problem is
most likely
the 100
MHz
Reference section. Refer to the
reference loop
-
M/N
loop.
a.
Replace the
400 MHz
input
to A31 M/N
phase detector
(J1)
using
a
very stable source. Use the
HP 8662A frequency standard as the HP 8340B/41B frequency standard EXT input.
b.
Repeat
the failed test.
If
the HP
8340B/41B now passes the
test, the problem is in the
HP
8340B/41B
reference
loop.
2.
If
the instrument failed the phase noise test at offset Frequencies from
300 Hz to 50 kHz,
the
problem
is most
likely the M/N
loop.
a.
You can
use an external
source
to
replace the
M/N input to the YO loop, but the external
source phase noise would have to be much better than the
HP 8340B/41B phase noise
to
eliminate the
YO
Loop.
b.
Replace
the reference loop
400
MHz input to the
M/N
loop
as described
in
step
1.
HP 8340B/41B
Assembly-Level Service Overall Instrument Troubleshooting
A-45