Passband Tuning - Swann CUBIC ASTRO 102 BX Service Manual

OCUBIC
COMMUNICA"TIONS
SIMAltI
DIVISIDN
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Comparator
BASIC MULTIPLIER PLL
PRACTICAL MULTPLIER PLL
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THE SIN IlATIO AND
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It is a fact of communications life
that the narrower the bandwidth of a re-
PASSBAND TUNING
Digi
tal circuitry lent i tself
quite handily to the implementation
of the frequency divider, in that a
number of available chips could be
used as programmable dividers, or in
,the present terminology, "divide-by-
N" counters,where N is the dividing
integer.
veo'
s or' various configur-.'
ations were easily achieved, using
varicaps as the tuning element.
Initial PLL designs used phase
comparators constructed out of dis-
crete components, and were often
very complex and difficult to "tame".
The development of specially designed integrated circuit phase compar-
ators that were basically digital mixers greatly alleviated many problems,
but created another. In such a mixer, leak-through of the reference freq-
uency components to the error, or tuning line to the
veo
is common unless
specific steps are taken to prevent it. Such leak-through results in freq-
uency modulation of the
veo
at the reference frequency, and if held low
enough, will be urÎnoticable in the reception of AM or FM signaIs, but is
quite difficult to reduce to a level where SSB or
ew
signals can be received
without objectionable distortion caused by this FMing. The means taken to
reduce this distortion is the includion in the loop, between the phase com-
parator and the
veo,
of a filtering arrangement to reduce the amplitude of
the reference frequency feedthrough to the
veo.
Usually a low pass filter
is used for this purpose.
Since the loop filter contains
considerable capacity to ground, of
necessity, and its cutoff frequency
1s below the reference frequency,
it controls the time required for
the loop to reachieve lock
af
ter a
change has been introduced, such as
when the divide ratio of the count-
er has been changed. At low ref-
erence frequencies, lockup time can
become objectionably long. In the
Astro I02BX, a synthesizer is used to generate the different band frequen-
cies which are mixed with the PTO output to produce the local oscillator
signal. Thereference frequency is sufficiently high that a relatively sim-
ple loop filter is all that is required.
Setting the division ratio into the pro-
grammable counter is accomp],ished by the
BAND switch, in conjunction with a diode
matrix and programmabIe read-only mem-
ory integrated circuit.