HP 8340B Manual page 52

c.
If
the instrument continues to fail the phase noise test, the problem is
most likely the M/N
VCO.
3.
If
the instrument failed the phase noise test
at offset frequencies greater than
50 KHz,
the problem
is
most
likely the YO Loop.
Spurious Response
This procedure assumes that the instrument failed the Spurious Response test in Section IV. Spu¬
rious
responses can be divided
into Four categories:
Harmonics/Sub-harmonics
The HP 8340B/41B
is essentially three instruments In
one:
1.
A
heterodyning (low band) source
2.
An unmodified
YIG
oscillator source (band 1)
3.
A synthesized source (full band sweep).
In
low band (10
MHz
to 2.3
GHz), the 2.3 to 7.0 GHz YO output is
mixed
with
a
fixed 3.7
GHz
oscillator
to produce the
RF
frequency.
In
high band, the YO output
Is
used directly or multiplied (by the
band
number) to produce the RF
frequency.
In
high band, the
SYTM
is
designed to pass
only the
RF output frequency
and reject
all
other frequen¬
cies.
In low band, the output of the low band mixer passes
directly through
the
SYTM.
To troubleshoot for harmonics and sub-harmonics
in low band
(10 MHz to 2.3 GHz),
use
a
spectrum
analyzer and
troubleshoot the low band circuitry.
If
the instrument does not meet its
harmonic/sub-harmonic specification
in
high band, suspect the
SYTM.
Line Related Side Bands
Line
related spurs result when magnetic radiation from the
power transformer
is
coupled
into the
M/N
loop, the
reference
oscillator, or the SYTM.
The amplitude of
a
line related spur coupled into the
M/N
loop
is greatest at 6.9 GHz (maximum
M/N
loop frequency); the amplitude of
a
line related spur coupled
into the
SYTM
is
greatest at 2.3
GHz
(minimum
SYTM
drive current).
An
increase
in line voltage, or the line
voltage selector PC can incorrectly installed can increase the
radiation
and, consequently,
the
spur amplitude.
1.
Check the waveform on the unregulated
power supplies.
These
supplies have full-wave bridge rectifiers. If
one diode is open, the supply operates similar to
a
half-wave rectifier.
The
output of the regulated supplies can be normal, but the transformer
current is
unbalanced, and the
magnetic
radiation can
increase.
Magnetic radiation coupled into the
M/N
Loop
can sometimes
be reduced
by replacing
A29U1 (The
amplifier limiter on the reference phase detector assembly).
A-46
Assembly-Level Service
Overall Instrument Troubleshooting
HP 8340B/41B