RF2516
RESNTR+
L
RESNTR-
L
LOOP FLT
4 kΩ
As mentioned earlier, the inductors and the varactors are tuning a differential amplifier. To tune the VCO the designer only
needs to calculate the value of the inductors connected to pins 12 and 13 (RESNTR- and RESNTR+). The inductor value is
determined by the equation:
2
1
1 1
--- --
⎛
⎝
⎞
⎠
----------------
L =
⋅
⋅
2 ⋅ π ⋅ f
C 2
In this equation, f is the desired operating frequency and L is the value of the inductor required. The value C is the amount of
capacitance presented by the varactors and parasitics. For calculation purposes 1.5pF should be used. The factor of one-half
is due to the inductors being in each leg. As an example, assume an operating frequency of 433MHz. The calculated value of
each inductor is 45nH. A 47nH inductor would be appropriate as the closest available value.
The setup of the VCO can be summarized as follows. 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 con-
trol 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 frequencies divided by
the supply voltage is the VCO sensitivity 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 connected to the charge pump output
through a 4kΩ resistor. The loop filter is then connected in parallel to this 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 reason for concern.
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Rev A17 DS060712
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RFMD [ RF MICRO DEVICES ]
