tza3015hw NXP Semiconductors, tza3015hw Datasheet - Page 57

• Manufacturer Part Number: tza3015hw
• Description: 30 Mbit/s To 3.2 Gbit/s A-rate 4-bit Fibre Optic Transceiver
• Manufacturer: NXP Semiconductors

Philips Semiconductors
APPLICATION INFORMATION
Calculations on the clean-up PLL
The important specifications of the clean-up PLL are the
bandwidth (f
the component parameters can be calculated. First
assume that the bandwidth of the VCXO control input
(f
simplifies the loop into a second order, type II PLL. In a
second order PLL, the damping factor determines the
amount of peaking. To obtain peaking of less than 0.1 dB,
0.05 dB,
example.
Now R and C1 may be calculated with the following
formulas:
Where:
These formulas are valid if:
2003 Dec 16
R
C
must be higher than 4.3. For peaking of less than
1
RDIV = reference divider ratio (1, 2, 4, 8, 16 or 32)
f
K
I
depending on I
f
3dB(vcxo)
30 Mbit/s to 3.2 Gbit/s A-rate
4-bit fibre optic transceiver
--------------------------------------------------- -
2
CP
=
VCXO
3dB
3dB(VCXO)
= damping factor.
>> 1 and
=
RDIV 2
----------------------------------------------------- -
= charge pump current in A (100 A or 1 mA,
------------------------------------------------------
RDIV
= clean-up PLL bandwidth in Hz
C1
K
= VCXO gain in Hz/V
K
R C1 C2
) is much higher than f
VCXO
must be higher than 6. See Fig.30 for an
VCXO
+
3dB
> 2
C2
) and the jitter peaking. If these are known,
2
2
I
C-bus bit CLUPPLLHG)
CP
I
f
CP
f
3dB
f
–
–
3dB
3dB
and
2
2 f
2
–
3dB
3dB
.
and C2 is left out. This
57
The transfer has a first order roll-off (i.e. 20 dB/decade), up
to the bandwidth of the VCXO control input. If a second
order roll-off is required C2 may be added, as long as
Example: The clean-up PLL uses a VCXO with a
frequency of 20 MHz and has a gain K
The bandwidth of the control input is f
Since the reference frequency is 20 MHz, the reference
divider ratio RDIV = 1. According to the specification, the
maximum allowed jitter peaking is 0.1 dB. To add some
margin the design is for less than 0.05 dB peaking, so
less than 100 kHz. To satisfy the conditions as previously
described, f
component tolerances, f
Choosing I
C1 = 1.167 F.
To calculate f
following formulas:
-----------------------------------------
2
R
C
f
–
= 6. Also according to the specification, f
3dB
1
=
=
C1
=
R
--- -
2
-----------------------------------------------
2000 100 10
=
2000 100 F 6
------------------------------------------------- -
1 2
+
C1 C2
1
K
------------------------------------------ -
C2
VCXO
2
K
---------------------------------------------- -
CP
VCXO
3 dB
-3dB
2
= 1 mA yields R = 7854
< 0.5
RDIV
2500
2500
RDIV
and , if R and C1 are known, use the
I
2 f
CP
I
CP
–
2
6
–
R
3dB
f
2
3dB(VCXO)
C1
3 dB(VCXO)
=
=
78540
116.7nF
Preliminary specification
= 2.5 kHz is chosen.
TZA3015HW
< 5 kHz. To cope with
3dB(VCXO)
VCXO
and
3dB
= 2000 Hz/V.
should be
= 10 kHz.