SP8855D Просмотр технического описания (PDF) - Zarlink Semiconductor Inc

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SP8855D

1.7GHz PARALLEL LOAD PROFESSIONAL SYNTHESISER

Zarlink Semiconductor Inc 

FROM

CHARGE

PUMP

OUTPUT

FROM

CHARGE

PUMP

REFERENCE

C1 C2

–

R2

+

TO

VCO

Fig. 8 Third order loop filter circuit diagram

Loop Filter Design

Generally the third order filter configuration shown in Fig.7

gives better results than the more commonly used second

order because the reference sidebands are reduced. Three

equations are required to determine values for the three

constants where;

t1 = C1

t2 = R2 (C1 + C2)

t3 = C2 R2

The equations are;

+ ƪ )) ƫ 1

t1

w t Kf K0 1

n2

2

2

Ă½

w w t N n2 1

n2

2

3

2

+ t2

1

wn

t2

3

3

+ * ) t3

tan

F0

wn

F1

cos 0

Where;

Kf is the phase detector gain factor in mA/radian

K0 is the VCO gain factor in radian/second/Volt

N is the total division ratio from VCO to reference

frequency

wn is the natural loop bandwidth

F0 is the phase margin normally set to 45°

p Since the phase detector is linear over a range of 2 radian,

Kf can be calculated from

p Kf = Phase comparator current setting/2 mA/radian

These values can now be substituted in equation 1 to obtain

a value for C1 and equation 2 and 3 used to determine values

for C2 and R2

EXAMPLE

Calculate values for a loop with the following parameters

Frequency to be synthesised:

Reference frequency

Division ratio

wn natural loop frequency

FK0 VCO gain factor

0 phase margin

Phase comparator current

1000MHz

10MHz

1000MHz/10MHz = 100

p100kHz

2 x 10MHz/Volt

45°

6.3mA

p The phase detector gain factor Kf

= 6.3mA /2 = 1mA/radian

SP8855D

From equation 3:

t +Ă * ) + 3

p tan 45°

1

cos 45°

100kHz 2

0 . 4142

628319

+ t 659 10*9

3

From equation 2:

t + p 2

(100kHz

1

2 )2

659

10*9

t +2 3 . 844 10*6

Using these values in equation 1:

+ ń t 1

p 1 x 10 *3

p 100 (2

2

10MHz

100kHz)2

V

[A]½

Where A is :

) ) 1

) + ) 1

t Ă wn2

2

2

t Ă wn2

2

3

1 (2p

1 (2p

100kHz)2

100kHz)2

(3 . 844 x 10*6)2

(659 x 10 *9)2

t + ƪ ƫ½

62832

6 . 833

1Ă

39 . 48Ă

1012 1 . 1714

+ t Ă 1 1 . 59 10*9 x 2 . 415

+ t1 3 . 84 10*9

+ N + t Ă Ă Now 1 C1 C1 3 . 84nF

+ ) t Ă 2 R2 (C1 C2)

+ t Ă 3 C2 R2

+ ƪ ) ƫ Substituting for C2

t2

R2

C

1

t

3

R2

N + ) t t Ă 2 R2 C1

3

N + t * t + * * * R2

2

3

C

1

3 . 844

10 6

9 . 61

659

10 9

* 10 9

+ W R2

829 . 4

t * 3

t + N + + 3

C 2ĂR 2Ă

Ă C2

R2

659

10 9

829 . 4

+ C2 0 . 794nF

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