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 fre-
quency 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 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 out-
put 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 pas-
sive filter is most common, as it is a low-cost and low-noise design. An additional pole could be used for reducing the ref-
erence spurs, however there is not a way to add the series resistor. However, this should not be a reason for concern.
11-8
Rev A17 060712
RFMD [ RF MICRO DEVICES ]
