HP 8340A Service Manual page 53

The key is to treat the spurs as FAMILIES of spurs. A spur family is characterized by having
the same mixing path through the instrument. For example, if it can be determined that the
5th harmonic of the M/N VCO is mixing with the 9th harmonic of the 20/30 output, then the
location of the spur can be predicted as the 20/30 frequency is changed. Also more spurs in
this family can be hypothesized such as the 9th 20/30 harmonic with the 6th M/N harmonic.
The common thread between all of these is that somehow the M/N VCO is allowed to mix
with the 20/30 to cause a spur.
The spurs normally show up as phase modulations of the YO frequency. As the spur
frequency is changed (by changing the carrier frequency), its amplitude will remain 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.
These spurs are called CROSSING SPURS, and are possible whenever the harmonic
frequencies of any two oscillators are equal (5th harmonic of20 MHz= 4th harmonic of25
MHz). A characteristic of crossing spurs is that the offset of the spur from the carrier changes
as the carrier is moved; therefore, there is some frequency that the offset must be zero
(Assuming the sources of the spurs can be tuned to this frequency). This frequency is called
the CR OS S I NG F REQUENCY 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, Cl, C2, C3, etc. Each
of these have a set of defining conditions to determine the crossing frequencies.
Type B, crossing frequency whenever:
lO* F l f /10
=
where F l f
=
ORDER of the spur
This type is caused by the 20/30 mixing with the Ith harmonic oflO MHz in the Reference
Phase Detector.
Type Cl and C3 are both due to the 20/30 output and its harmonics mixing with the M/N
vco.
Given:
Fmn = M/N Output freuency
F l f = 20/30 Output Frequency = 20 to 30 (MHz)
M, N are the divider numbers for the M/N Loop
I, J, Kare integers
then the conditions for the two spurs are:
Type Cl, crossing frequency whenever:
Fmn/Flf
=
ORDER of spur= I/2
This type can be caused by several factors. Some possibilities are, the A48 Sampler board
70 MHz Low Pass Filter, or the A46 Low Pass Filter Assembly.
8-5 2
Model 8340A - Service
I or F l f = I
20/30 loop output frequency
10
=
=
20 * ( 20-M/N)/I
Scans by HB9HCA and HB9FSX
200 - lO*M/N (MHz)