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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.
An important part of the overall design is the voltage controlled oscillator. The VCO is configured as a differential amplifier. The
VCO range is set by the external inductor(s) and is fine-tuned via internal varactor diodes. The varactors are tuned by the loop
filter output voltage through a 4k
Ω resistor. (Refer to the internal schematic for RESNTR- in the pin description table.) To tune
the VCO the designer only needs to calculate the value of the inductor(s) connected to RESNTR- and RESNTR+. The inductor
value is determined by the equation:
In this equation, f is the desired operating frequency and L is the value of the inductor required. In the case of a two-inductor
resonator configuration, the value of L is halved due to the inductors being in each leg. The value C is the amount of capaci-
tance presented by the varactors and parasitics. For calculation purposes, 1.5pF should be used. As an example, assume an
operating frequency of 868MHz. The calculated inductor value is 22.4nH. A 22nH inductor (two 10nH inductors for the two-
inductor configuration) would be appropriate as the closest available value. Be aware that any inductance in the traces con-
necting the inductor(s) to the VCO pins will contribute to the overall resonator inductance and should be subtracted from the
calculated value of L.
A parameter of the VCO that is necessary for calculating the loop filter values is the VCO sensitivity, KVCO (sometimes referred
to as VCO gain). To determine the VCO sensitivity, first connect the control voltage input point (LOOP FLT pin) to ground and
note the frequency. (The frequency can be observed at the output if the LD FLT pin is connected to VCC.) Then 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. There is little that the designer can do to increase the VCO sensitivity since it
is largely determined by the tuning capacitance of the on-chip varactors. While increasing the inductor value will increase the
tuning sensitivity, it will also lower the center frequency of the VCO's tuning range. A very small capacitance (1pF or less) may
be added across the VCO pins, which will have the effect of lowering the VCO center frequency and decreasing VCO sensitivity,
but this is likely to be neither necessary nor desirable in most applications.
Should adequate centering of the VCO range be unachievable with standard inductor values, two options are available for
proper centering. First, a two-inductor resonator may be used with one inductor being one standard value higher than the
other. Second, the tuning range of the VCO may be extended at the upper limit of the control voltage by increasing the VCO bias
resistor(s). This allows the internal varactor diodes to be slightly forward biased, further increasing the resonator capacitance
and thereby extending the lower frequency operation. Care should be taken not to reduce the VCO bias so much that the circuit
ceases operation at the minimum required supply voltage.
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 4k
Ω resistor. The loop filter is then connected in parallel with this point at pin 12 (LOOP FLT). This limits the loop fil-
ter topology to a second order filter usually consisting of a shunt capacitor and a shunt series RC, as shown in the following
schematic.
L
1
2
π f
⋅⋅
----------------
⎝⎠
⎛⎞ 2 1
C
----
⋅
=
VCC
R2
C2
C1
VCO
Charge Pump
Loop Filter
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