NJ88C33

8

APPLICATION CIRCUITS

Single Modulus

In this mode, the NJ88C33 synthesiser can be used with

or without a fixed modulus prescaler. The R counter is

programmed with a value to produce a comparison frequency

fc. When the N counter is changed by 1 the loop is no longer

in lock and the phase detector output produces current pulses

to bring the loop back into lock. These pulses are integrated by

the loop fiiter to produce the VCO voltage drive. When the

VCO loop is locked, Fl/N=f

Using a prescaler with a division ratio P, the smallest VCO

output frequency step is Pf

If a low pass filter is connected to the lock detect output as

shown and sampled by the microprocessor, the proximity of

the synthesiser loop to lock can be evaluated.

The A counter is not used in this mode.

Fig. 7 Single modulus application

Fig. 8 Dual modulus application

Dual Modulus

This mode allows much higher frequencies to be used in

conjunction with a prescaler but maintains the step size, fc. In

this mode, a dual modulus prescaler (with ratios P and P + 1)

must be used with the NJ88C33. The A counter controls the

MOD output, which is used to select the division ratio of the

prescaler.

When the A counter is non-zero, the MOD output is low and

goes high when the A counter has counted down to zero.

MOD remains high until the N counter reaches zero, when

both counters are re-loaded. Thus, the prescaler divides by P

for N-A cycles and by P + 1 for A cycles of Fl. The VCO

frequency is given by PNf

Note that programming A = 0 produces a count of 128 cycles.