HP 8340B Manual page 54

Type
B,
caused by the
20/30 mixing with the Ith harmonic
of
10
MHz
in the reference
phase detector.
Type
B, crossing frequency whenever:
10*Flf/10
=
I
or
Fit
=
I
where
FIf
=
20/30
loop output frequency
ORDER
of the spur
=
10
Type
C1 and C3 are both due to
the
20/30
output and its
harmonics mixing
with the
M/N VCO.
Given:
Fmn
=
M/N
Output frequency
=
200
-
10*M/N (MHz)
Fit
=
20/30
Output Frequency
=
20 to 30 (MHz)
M,
N
are
the divider numbers for the
M/N
Loop
I,
J,
K
are integers
Type
C1,
crossing frequency when:
Fmn/Flf
=
1/2 or
Fit
=
20
• (20-M/N)/!
ORDER of
Spur
=
1/2
This type of spur can be caused by several factors,
such
as the A48 sampler assembly 70 MHz
low pass
filter,
or the
A46
low pass
filter
assembly.
Type C3, crossing frequency when:
Fmn/Flf
=
20*1
±
J/(2*K)
or
Fit
=
20
•
K
•
(20-M/N)/20*1
±
J
ORDER
of spur
=
20*1
±
J/(2*K)
This type is caused by the Ith
harmonic of the
PLL1
VCO sampled
by the Kth
harmonic of the
M/N
VCO.
For any
CW frequency,
selecting
the
appropriate SHIFT
functions displays the
M/N
frequency,
20/30
frequency, and YO frequency.
A synthesized spurious response must be
a
function of these signals.
Any coupling from one
signal path into
another can
result
in
a spurious response
(i.e.,
loose
connec¬
tors, poor shielding, cable routing, etc.).
Low band spurs are caused by
mixing
products
in the low band mixer. Because these are not crossing
spurs
(they
are
not
YO
loop sidebands), they
are
added to the output as part of the down-conversion
process. The YO output should mix with the 3.7
GHz oscillator output and produce
a
single mixer
output at Fyo
—
3.7 GHz,
but,
harmonics of both oscillators are present, or are generated
in
the mixer.
These mix to form spurs on the output.
Troubleshooting synthesized spurs.
1.
To
eliminate various RF paths, determine
the YO frequency
at which the spur occurs and select
a
CW
frequency
in both low band and band
1
that uses that
YO frequency.
If
the spur occurs
only
in low band, troubleshoot the low band
RF path. If the spur occurs only
in
band
1,
troubleshoot the
high band RF path.
2.
Using the equations above, change the CW
frequency so
that the
M/N
output remains constant
and only the
20/30
output changes.
A-48
Assembly-Level Service Overall Instrument Troubleshooting
HP 8340B/41B