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NJ88C33MA Datasheet(PDF) 8 Page - Mitel Networks Corporation |
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NJ88C33MA Datasheet(HTML) 8 Page - Mitel Networks Corporation |
8 / 11 page 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 C i.e., the VCO frequency is N x fC. Using a prescaler with a division ratio P, the smallest VCO output frequency step is Pf C and the VCO frequency is PNfC. 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 C + AfC. Note that programming A = 0 produces a count of 128 cycles. |
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