Preliminary
11-42
RF2516
Rev A10 010613
11
The setup of theVCO canbesummarizedasfollows.
First, open the loop. Next, get the VCO to run on the
desired frequency by selecting the proper inductor and
capacitor values. The capacitor value will need to
include the varactor and circuit parasitics.
After the VCO is running at the desired frequency, set
the VCO sensitivity. The sensitivity is determined by
connecting the control voltage input point to ground
and noting the frequency.
Connect the same point to the supply, and again note
the frequency. The difference between these two fre-
quencies divided by the supply voltage is the VCO sen-
sitivity expressed in Hz/V. Increasing the inductor value
while decreasing the capacitor value will increase the
sensitivity.
Decreasing
the
inductor
value
while
increasing the capacitor value will lower the sensitivity.
When increasing or decreasing component values,
make sure that the center frequency remains constant.
Finally, close the loop.
External to the part, the designer needs to implement a
loop filter to complete the PLL. The loop filter converts
the output of the charge pump into a voltage that is
used to control the VCO. Internally, the VCO is con-
nected to the charge pump output through a 4k
Ω resis-
tor. Theloopfilteristhen connectedinparalleltothis
point at pin 14 (LOOP FLT). This limits the loop filter
topology to a second order filter usually consisting of a
shunt capacitor and a shunt series RC. A passive filter
is most common, as it is a low-cost and low-noise
design. An additional pole could be used for reducing
the reference spurs, however there is not a way to add
the series resistor. However, this should not be a rea-
son for concern.
The schematic of the loop filter is:
The transfer function is:
where the time constants are defined as:
and
The frequency at which unity gain occurs is given by:
This is also the loop bandwidth.
If the phase margin (PM) and the loop bandwidth
(
ω
LBW) are known, it is possible to calculate the time
constants. These are found using the equations4:
and
With these known, it is then possible to determine the
values of the filter components.4
As an example, consider a loop bandwidth of 50 kHz, a
phase margin of 45°, a divide ratio of 64, a KVCO of
20 MHz/V, and a KPD of 0.01592mA/2
πrad. Time con-
stant
τ1 is 1.31848µs, time constant τ2 is 7.68468µs,
C1 is 131.15pF, C2 is 633.26pF, and R2 is 12.14kΩ.
In order to perform these calculations, one will need to
know the value of two constants, KVCO and KPD.KPD is
calculated by dividing the charge pump current by 2
π.
For the RF2516, the charge pump current is 100
µA.
KVCO is best found empirically as it will change with
frequency and board parasitics. By briefly connecting
pin 14 (LOOP FLT) to VCC and then to ground, the fre-
quency tuning range of the VCO can be seen. Dividing
the difference between these two frequencies by the
difference in the voltage gives KVCO in MHz/V.
V
CC
R2
C2
C1
VCO
Charge Pump
Loop Filter
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
+
-------------------ø
ö
è
æ
⋅
=
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
LB W
2
N
⋅
-----------------------------
1
ω
LBW
τ
2
⋅
()
2
+
1
ω
LBW
τ
1
⋅
()
2
+
----------------------------------------
⋅⋅
=
C
2
C
1
τ
2
τ
1
-----
1
–
èø
æö
⋅
=
R
2
τ
2
C
2
------
=
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