RF2516
Designing With the RF2516
The reference oscillator is built around the onboard transistor at pins 1 and 2. The intended topology is that of a Colpitts oscil-
lator. The Colpitts oscillator is quite common and requires few external components, making it ideal for low-cost solutions. The
topology of this type of oscillator is as seen in the following figure.
VCC
X1
C2
C1
This type of oscillator is a parallel resonant circuit for a fundamental mode crystal. The transistor amplifier is an emitter fol-
lower and the voltage gain is developed by the tapped capacitor impedance transformer. The series combination of C1 and C2
act in parallel with the input capacitance of the transistor to capacitively load the crystal.
The nominal capacitor values can be calculated with the following equations6:
60 ⋅ Cload
C1 = ----------------------- and C2 = --------------------------
freqMHz
1
1
1
------------- – -----
Cload C1
The load capacitance is usually 32pF. The variable freq is the oscillator frequency in MHz. The frequency can be adjusted by
either changing C2 or by placing a variable capacitor in series with the crystal. As an example, assume a desired frequency of
14MHz and a load capacitance of 32pF. C1=137.1pF and C2=41.7pF.
These capacitor values provide a starting point. The drive level of the oscillator should be checked by looking at the signal at
pin 2 (OSC E). It has been found that the level at this pin should generally be around 500mVPP or less. This will reduce the ref-
erence spur levels and reduce noise from distortion. If this level is higher than 500mVPP then increase the value of C1. The val-
ues of these capacitors are usually tweaked during design to meet performance goals, such as minimizing the start-up time.
Additionally, by placing a variable capacitor in series with the crystal, one is able to adjust the frequency. This will also alter the
drive level, so it should be checked again.
An important part of the overall design is the voltage controlled oscillator. The VCO is configured as a differential amplifier. The
VCO is tuned via internal varactors. The varactors are tuned by the loop filter output voltage through a 4kΩ resistor.
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Rev A17 DS060712
RFMD [ RF MICRO DEVICES ]
