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AlOA2
AlOA2
Phase
Lock
Loop
1
(PLLI)
Divider,
Circuit
Description
AlOA2 functions as a programmable frequency divider and a phase/frequency detector. The
divider is programmed by the Al5 Controller to divide the input frequency by the
PLLl
divide number
Nl
down to 5 MHz. This is then compared with a 5 MHz reference in the
phase/frequency detector. The detector output, after amplification and integration, tunes the
frequency of the
PLLl
voltage-controlled oscillator (VCO).
The
PLLl
divide number
Nl
always falls in the range between 3.60 and 13.97. The integer
part is coded in four binary bits while the fractional part (0.00 to 0.99) is coded in two 4-bit
BCD bytes.
Divide
by
2
@
U5 constitutes a 3-stage limiting amplifier. U5 is an ECL triple-line receiver which changes
the input to the proper amplitude and dc level (approximately
j-3
V low and
+4
V high) for
driving the subsequent divider. U4A is a D-type flip-flop which divides the limiting amplifier
output by 2.
Input
Latch
@
U9
and UlO are latches which store the divider programming number. The number is clocked
into the latches from the instrument bus with LCK4.
Divide
by
N
@
U12 is a 4-bit binary counter which is programmed with the integer part (3 to 13) of the
PLLl
divide number
Nl.
In operation it is clocked by the output of U4A and counts down to
1. On the next clock pulse, the programming number is loaded into the counter, and the cycle
repeats. Loading time is sensed by U14B when the count is at 2. The next clock pulse causes
U14B to change states which enables the loading of U12 on the subsequent clock pulse.
Fractional
Divide
6J
The fractional divide works on the pulse-swallowing principle. Consider a divider that divides
by N for P-A output pulses and by N
+
1 for A output pulses. Note that the total output
pulses is P. The number of input pulses to get P output pulses is
N(P-A)
+
(N + l)A = NP
+
A.
Therefore, the divide ratio is the number of input pulses divided by the number of output
pulses which is equal to N •l- A/P. In this case P is 100, so N represents the integer portion of
the divide number while A represents the fractional part times 100.
An example best illustrates how the fractional divide works. If the PLL2 divide number
Nl
is 8.57, then
N=8
and
A=57.
The integer divider normally divides by N each divide cycle.
AlOA2
1