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NJ88C25

FREQUENCY SYNTHESISER (MICROPROCESSOR SERIAL INTERFACE)

Zarlink Semiconductor Inc 

NJ88C25

2·0

VDD = 5V

OSC IN, FIN = 0V TO 5V SQUARE WAVE

1·5

OSC IN

1·0

FIN

0·5

TOTAL SUPPLY CURRENT IS

THE SUM OF THAT DUE TO FIN

AND OSC IN

12

3 4 5 6 7 8 9 10

INPUT FREQUENCY (MHz)

Fig. 3 Typical supply current v. input frequency

PROGRAMMING

Reference Divider Chain

The comparison frequency depends upon the crystal

oscillator frequency and the division ratio of th ‘R’ counter,

which can be programmed in the range 3 to 2047, and a fixed

divide by two stage.

R = fosc

23fcomp

where fosc = oscillator frequency,

fcomp = comparison frequency,

R = ‘R’ counter ratio

For example, where the crystal frequency = 10MHz and a

channel spacing comparison frequency of 12·5kHz is required,

R=

107

= 400

2312·53103

Thus, the ‘R’ register would be programmed to 400 expressed

in binary. The total division ratio would then be 23400 = 800

since the total division ratio of the ‘R’ counter plus the 42 stage

is from 6 to 4094 in steps of 2.

VCO Divider Chain

The synthesised frequency of the voltage controlled oscillator

(VCO) will depend on the division ratios of the ‘M’ and ‘A’

counters, the ratio of the external two-modulus prescaler

(P/P11)and the comparison frequency .

8

VDD = 5V

7

FIN = LOW FREQUENCY

0V TO 5V SQUARE WAVE

6

5

10MHz

4

3

1MHz

2

1

0·2 0·4 0·6 0·8 1·0 1·2 1·4 1·6

INPUT LEVEL (V RMS)

Fig. 4 Typical supply current v. input level, OSC IN

The division ratio N = MP1A,

where M is the ratio of the ‘M’ counter in the range 8 to 1023

and A is the ratio of the ‘A’ counter in the range 0 to 127.

Note that M>A and

N

=

fVCO

fcomp

For example, if the desired VCO frequency = 275MHz, the

comparison frequency is 12·5kHz and a two-modulus prescaler

of 464/65 is being used, then

N = 2753106 = 223103

12·53103

Now, N = MP1A, which can be rearranged as N/P = M1A/P.

In our example we have P = 64, therefore

223103=

64

M1

A

64

such that M = 343 and A /64 = 0·75.

Now, M is programmed to the integer part = 343 and A is

programmed to the fractional part364 i.e., A = 0·75364 = 48.

NB The minimum ratio N that can be used is P 22P (=4032 in

our example) for all contiguous channels to be available.

To check: N = 343364148 = 22000, which is the required

division ratio and is greater than 4032 ( = P 22P ).

CLOCK

tCH

tCL

ENABLE

tEH

tES

tEH

tES

tDS

tDH

DATA

Fig. 5 Timing diagram showing timing periods required for correct operation

4

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