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IMI145158 FREQUENCY SYNTHESIZER
Preliminary
CMOS LSI
SERIAL PROGRAMMED PLL FREQUENCY SYNTHESIZER
INTERNATIONAL MICROCIRCUITS, INC. 525 LOS COCHES ST. Rev 1.2 9/3/98
MILPITAS, CA 95035 TEL: 408-263-6300 ext. 276 FAX 408-263-6571 Page 7 of 10
DUAL MODULUS PRESCALING
Dual Modulus prescaling is a wide spread method used to
effectively extend the operating frequency of a digital counter
without sacrificing any frequency resolution. The key to
understanding this method is to remember the basics of division:
W hen any t wo integers are divi ded, a quot ient and a rem ai nder will
result.
W hen used here in a PLL, the numerator is the required PLL total
feedback divider ratio, called Ntot. The denominator is the base
modulus of the dual modulus prescal er, P. The quot ient is applied
directly to the N counter, and the remainder is applied directly to
the A counter. Both counters count down together toward zero.
While the A counter counts, the MC (modulus control) output
signal is LOW , setti ng the prescaler to di vide by P + 1. W hen t he
A counter reaches zero, the MC output is set HIGH while the N
counter continues to count down to zero. When the N counter
reaches zero, both counters are reset to the programmed inputs
and the cycl e i s repeated.
Two particular things should be noticed about this process. First,
the remainder counts are spread among an equal number of
quotient c ounts by the use of t he prescaler modulus P +1. W hen
the remainder has been counted, any rem aining quot ient count s are
handled normal ly by prescaling by m odulus P. This c ounter is thus
performing
Ntot = A(P+1) + (N-A)P
Some al gebra on this relation yields
Ntot = AP+A + NP-AP
= NP + A
which is just the definition of integer division. Second, for this to
work, there must be m ore quotient count s t han rem ainder c ounts for
all poss ible values of Ntot i n the synthesizer design. I f this were not
true, then the N count er will reach zero and cause the entire divider
to be reset before the A counter is finished. There is a minimum
value for Ntot for which this requirement will always hold:
Ntot > P2 - P.
PROGRAMMING GUIDELINES APPLICABLE TO THE IMI145158
The system total divide value (Ntotal) will be dictated by the
application: frequency i nto the pres caler
frequency int o the phase detector
N is the num ber programm ed into the ÷ N counter; A i s the number
programmed into ÷ A counter. P and P + 1 are two selectable
divide ratios available in the two modulus prescalers. To have a
range of Ntotal values in sequence, the ÷ A counter is programmed
from zero through P-1 for a parti cular value N i n the ÷ N counter. N
is then increm ented to N + 1k, and the ÷ A is s equenced from zero
through P - 1 again.
To maximize system frequency capability, the dual modulus
prescaler’ s output mus t go from low to high aft er each group of P or
P + 1 input cycles. The prescaler should divide by P when its
modulus control line i s high, and by P + 1 when it s m odulus cont rol
is low.
For the maximum frequency into the prescaler (FVCO max), the
value used for P must be large enough so t hat :
A. FVCO max divided by P may not exceed the frequency
capability of P in 8 of the IMI145158.
B. The period of FVCO divided by P m ust be greater than the sum
of the ti mes:
a. Propagation delay t hrough the dual m odul us prescal er.
b. Prescaler setup or release time relative to its modulus
control signal.
c. Propagati on tim e from f in to the modulus control s i gnal .
A useful simplification in the IMI145158 programming code can be
achieved by choosing the values for P or 8, 16, 32, or 64, or 128.
For these c ases, the des ired value for Ntotal in binary is used as the
program c ode t o the ÷ A counters in t he following manner:
A. Assume the ÷ N counter and ÷ A counter contains “b” bits
where 2b = P.
B. Always program all higher order ÷ A counter bits above “b” to
zero.
C. Assume the ÷ N counter and ÷ A counter (with all the higher
order bits above “b” ignored) combined into a single binary
counter of 10+b bits in length. The MSB of this hypothetical
counter is to correspond o t he LSB of ÷ A. The system divide
value, Ntotal, now results when the value of Ntotal in binary is
used to program the “new” 10+b bit counter.
Ntotal == N*P+A