CXA3106Q
Charge Pump and Loop Filter Settings
The CXA3106Q's charge pump is a constant-current output type as shown below.
When a constant-current output charge pump circuit is used inside the
PLL, the phase detector output acts as a current source, and the
dimension of its transmittance KPD is A/rad. Also, when considering the
VCO input as a voltage, the LPF transmittance dimension must be
expressed in ohms (Ω = V/A).
VCC
S1
S2
To LPF
Therefore, the PLL transmittance when a constant-current output charge
pump circuit is used is as follows.
LPF
VCO
PD
ω /N
ωr
θr
0
KPD
(A/rad)
F (S)
(Ω)
KVCO
(rad/S/V)
1/S
+
–
θo
N
counter
1/N
1/S
ω /N
0
The PLL closed loop transmittance is obtained by the following formula.
θo/N
θr
KPD · F (S) · KVCO · 1/N · 1/S
1 + KPD · F (S) · KVCO · 1/N · 1/S
=
... (1)
Here, KPD, F (S), and KVCO are:
KPD: Phase comparator gain (A/rad)
F (S): Loop filter transmittance (Ω)
KVCO: VCO gain
(rad/s/V)
1 The reason for the 1/S inside the phase detector is as follows.
t
θo (t)/N = o ω0 (t)/Ndt + θo (t = 0)/N ... (a)
∫
θo (t = 0) = 0
t
θo (t)/N =
∫
o ω0 (t)/Ndt ... (b)
Performing Laplace conversion:
1
S
θo (S)/N =
W0 (S)/N ... (c)
– 26 –
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