RF2516
The RF2516 contains onboard band gap reference voltage circuitry which provides a stable DC bias over varying tem-
perature and supply voltages. Additionally, the device features a power-down mode, eliminating battery disconnect
switches.
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
oscillator. 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
follower 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
-----------------------
freqMHz
1
--------------------------
and C2 =
C1 =
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 sig-
nal 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 reference spur levels and reduce noise from distortion. If this level is higher than 500mVPP then increase the
value of C1. The values of these capacitors are usually tweaked during design to meet performance goals, such as mini-
mizing 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.
Rev A17 060712
11-7
RFMD [ RF MICRO DEVICES ]
