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RF2514
Rev A5 DS040115
7628 Thorndike Road, Greensboro, NC 27409-9421 · For sales or technical
support, contact RFMD at (+1) 336-678-5570 or sales-support@rfmd.com.
The transfer function is
where the time constants are defined as
and
The frequency at which unity gain occurs is given by
This is defined as the loop bandwidth.
Once the desired phase margin (PM) and loop bandwidth (
ωLBW) are chosen, it is possible to calculate the time constants.
These are found using the equations
and
The phase detector gain, KPD, is calculated by dividing the charge pump current by 2π. For the RF2514, the charge pump cur-
rent is 100
μA.
With these known, it is then possible to determine the values of the filter components.
As an example, consider a loop bandwidth of 300kHz, a phase margin of 60°, a divide ratio of 64, a KVCO of 33MHz/V, and a
KPD of 100μA/2πrad. Time constant τ1 is 142.15ns, time constant τ2 is 1.98ms, C1 is 0.62pF, C2 is 8.0pF, and R2 is
247.5k
Ω.
The control lines provide an interface for connecting the device to a microcontroller or other signal generating mechanism. The
designer can treat pin 5 (MOD IN), pin 14 (DIV CTRL), and pin 2 (PD) as control pins whose voltage level can be set. The lock
detect voltage at pin 13 (LD FLT) is an output that can be monitored by the microcontroller.
Fs
()
R
2
s
τ
2
1
+
⋅
s
τ
2
s
τ
1
1
)
+
⋅
(
⋅⋅
-------------------------------------------
⋅
=
τ
2
R
2
C
2
⋅
=
τ
1
R
2
C
1
C
2
⋅
C
1
C
2
+
-------------------⎠
⎞
⎝
⎛
⋅
=
ω
LBW
1
τ
1
τ
2
⋅
-------------------
=
τ
1
PM
()
sec
PM
()
tan
–
ω
LBW
--------------------------------------------------
=
τ
2
1
ω
LBW
2
τ
1
⋅
------------------------
=
C
1
τ
1
τ
2
-----
K
PD
K
VCO
⋅
ω
LBW
2
N
⋅
-----------------------------
1
ω
LBW
τ
2
⋅
()
2
+
1
ω
LBW
τ
1
⋅
()
2
+
----------------------------------------
⋅⋅
=
C
2
C
1
τ
2
τ
1
-----1
–
⎝⎠
⎛⎞
⋅
=
R
2
τ
2
C
2
------
=
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