RF2514
The lock protection circuit in the RF2514 is intended to stabilize quickly after power is applied to the chip and to disable the
base drive to the transmit amplifier. This attenuates the output to levels that will be generally acceptable to regulatory boards
as spurious emissions. Once the phase detector has locked the oscillators, then the lock circuit enables the MOD IN pin for
transmission of the desired data. There is no need for an external microprocessor to monitor the lock status, although that can
be done with a low current A/D converter in a system micro, if needed. The lock detect circuitry contains an internal 1kΩ resis-
tor which, combined with a designer-chosen capacitor for a particular RC time constant, filters the lock detect signal. This sig-
nal is then passed through an internal Schmitt trigger and used to enable or disable the transmit amplifier.
If the oscillator unlocks, even momentarily, the protection circuit quickly disables the output until lock is achieved. These
unlocks can be caused by low battery voltage, poor power supply regulation, severe shock of the crystal or VCO, antenna load-
ing, component failure, or a myriad of unexpected single-point failures.
The RF2514 contains onboard band gap reference voltage circuitry which provides a stable DC bias over varying temperature
and supply voltages. Additionally, the device features a power-down mode, eliminating battery disconnect switches.
Designing with the RF2514
The reference oscillator is built around the onboard transistor at pins 15 and 16. 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 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 equations
60 ⋅ Cload
C1 = ----------------------- and C2 = --------------------------
freqMHz
1
1
1
------------- – -----
Cload C1
The load capacitance, Cload, is a characteristic of the crystal used; freqMHz 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 oscillator 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
the OSC E pin. 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 produced by distortion. If this level is higher than 500mVPP then increase the value of C1.
The values of these capacitors are usually adjusted during design to meet performance goals, such as minimizing the start-up
time.
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RFMD [ RF MICRO DEVICES ]
