HP 8340B Manual page 53

Squegging
(^7 GHz)
1.
While changing the instrument output power
level,
observe the spurious response on
a spectrum
analyzer.
If
the frequency of the spur changes with power
level,
suspect squegging.
Refer to the
3RD
Bias
adjustments in Section V.
Synthesized Spurs
NOTE:
For
proper
shielding, the screws on the
20/30
and
M/N
section covers must
be tight.
The HP 8340B/41B uses several internal oscillators to generate the desired output frequency.
All
the
possible output frequencies can be described by the following equation:
Fout
=
k1*F1
+
k2*F2
+
k3*F3
k1, k2,
and k3
are
integers (positive or negative) and
F1, F2,
and F3 are the
frequencies of the
internal
oscillators. Because
F1,
F2, and F3 are
phase locked to the internal reference (10 MHz
standard),
they
are related to the reference frequency
by:
F1
=
Ref*l1/J1 F2
=
Ref*l2/J2
F3
=
Ref*l3/J3
The I's and J's are integers. The combination of these shows the relationship of the output frequency
to the reference frequency:
Fout
=
Ref*(k1*l1/J1
•+-
k2*l2/J2
+
k3*l3/J3
...)
The
intended output frequency is
the result of only one set of integers in the
above equation. Spurs
are possible at all other integer choices. These choices are normally
eliminated through careful use of
filtering, attention to signal levels,
shielding, etc.
A
Spur Family.
A spur family
is
characterized by having the same mixing path through the
instru¬
ment. For example,
if
the 5th harmonic of the
M/N VCO
is
mixing with the 9th
harmonic of
the
20/30
output, the location of the spur can be predicted as the
20/30
frequency
is
changed. More
spurs
in
this
family can be
hypothesized,
such as the 9th
20/30
harmonic with the 6th M/N harmonic. The common
thread
is that the
M/N VCO
mixes
with the
20/30
to
cause a
spur.
Synthesized spurs normally show
up as
phase modulations of the YO frequency. As the spur
fre¬
quency
is
changed (by changing the carrier frequency),
its amplitude remains constant as
long
as the
offset
from carrier remains less than the YO loop bandwidth (50 kHz). Beyond the YO loop
bandwidth,
the
amplitude decreases until the spur is
gone.
Synthesized spurs are called crossing spurs, and are possible when the harmonic frequencies of any
two
oscillators are equal (5th harmonic of 20 MHz
=
4th harmonic of 25 MHz).
A
characteristic of
crossing spurs
is that the offset of the spur
from
the carrier changes as the carrier is
moved. Because
of
this,
there
is a
frequency at
which the offset must be
zero
(assuming
the
sources of the
spurs can
be tuned to this
frequency).
This frequency
is
called the crossing frequency of the spur. The
ratio of
the change in
spur offset to the change in
carrier frequency
is called the order.
Names
can be assigned to the different spur families, such as type
A, B, C1,
C2, C3, etc. Each of these
have
a
set
of defining conditions to determine the
crossing frequencies.
HP 8340B/41B
Assembly-Level Service Overall Instrument Troubleshooting
A-47