cxa3516r Sony Electronics, cxa3516r Datasheet - Page 60

• Manufacturer Part Number: cxa3516r
• Description: 3-channel 8-bit 165msps A/d Converter Amplifier Pll
• Manufacturer: Sony Electronics

The loop filter F(S) is described below.
The loop filter smoothes the output pulse from the phase detector (PD) and inputs it as the DC component to
the VCO. In addition to this, however, the loop filter also functions as an important element in determining the
PLL response characteristics.
Typical examples of loop filters include lag filters, lag-lead filters, active filters, etc. However, the CXA3516R's
LPF is a current input type active filter as shown below, so the following calculations show an actual example
of deriving the PLL closed loop transmittance when using this type of filter and then using this transmittance to
create a formula for setting the filter constants.
Current input type active filter
The filter transmittance is as follows.
Here, assuming A > 1, then:
Next, substituting (2) into (1) and obtaining the overall closed loop transmittance for the PLL:
F (S) =
F (S) =
Vo
A
o/N
i
i
r
+ Vo = (R +
=
n =
1 + SRC
1 + S
1 + S
=
=
=
SC
SC
SC
C
–A
= RC
S
S
1
2
2
KPD · KVCO
2
R
+
2
KPD · KVCO ·
+ 2
SC
–Vo
·
........................ (2)
·
1
NC
KPD · KVCO ·
nS + n
n ..................................................................... (6)
1 + A
1 + A
) · i
A
A
nS + n
NC
–1
i
NC
2
2
Vo
· S +
................................................. (4)
................................................. (5)
· S +
KPD · KVCO
KPD · KVCO
NC
NC
– 60 –
The Bode diagram for formula (2) is as follows.
... (3)
–90
0
1
–45deg
log
log
CXA3516R