11-6
RF2516
Rev A17 060712
The RF2516 contains an onboard phase detector and charge pump. The phase detector compares the phase of the ref-
erence oscillator to the phase of the VCO. The phase detector is implemented using flip-flops in a topology referred to as
either “digital phase/frequency detector” or “digital tri-state comparator”. The circuit consists of two D flip-flops whose
outputs are combined with a NAND gate which is then tied to the reset on each flip-flop. The outputs of the flip-flops are
also connected to the charge pump. Each flip-flop output signal is a series of pulses whose frequency is related to the
flip-flop input frequency.
When both inputs of the flip-flops are identical, the signals are both frequency- and phase-locked. If they are different,
they will provide signals to the charge pump which will either charge or discharge the loop filter, or enter into a high
impedance state. The name “tri-state comparator” comes from this.
The main benefit of this type of detector is the ability to correct for errors in both phase and frequency. When locked, the
detector uses phase error for correction. When unlocked, it uses frequency error for correction. This type of detector will
lock under all conditions.
The charge pump consists of two transistors, one for charging the loop filter and the other for discharging the loop filter.
Its inputs are the outputs of the phase detector flip-flops. Since there are two flip-flops, there are four possible states. If
both amplifier inputs are low, then the amplifier pair goes into a high impedance state, maintaining the charge on the loop
filter. The state where both inputs are high will not occur. The other states are either charging or discharging the loop fil-
ter. The loop filter integrates the pulses coming from the charge pump to create a control voltage for the voltage con-
trolled oscillator.
The VCO is a tuned differential amplifier with the bases and collectors cross-coupled to provide positive feedback and a
360° phase shift. The tuned circuit is located in the collectors, and is comprised of internal varactors and external induc-
tors. The designer selects the inductors for the desired frequency of operation. These inductors also provide DC bias for
the VCO.
The output of the VCO is buffered and applied to the prescaler circuit, where it is divided by either 32 or 64, as selected
by the designer, and compared to the reference oscillator frequency.
The transmit amplifier is a two-stage amplifier consisting of a driver and an open collector final stage. It is capable of
providing 10dBm of output power into a 50
Ω load while operating from a 3.6V power supply.
The lock-detect circuitry connects to the output of the phase detector circuitry and is used to disable the transmitter
when the VCO is not phase-locked to the reference oscillator. This is necessary to avoid unwanted out-of-band transmis-
sion and to provide compliance with regulatory limits during an unlocked condition.
There are many possible reasons for the PLL not to be locked. For instance, there is a short period during the start of any
VCO in which the VCO begins oscillating and the reference oscillator builds up to full amplitude. During this period, the
frequency will likely be outside the authorized band. Typically, the VCO starts much faster than the reference oscillator.
Once both VCO and reference oscillators are running, the phase detector can start slewing the VCO to the correct fre-
quency, slowly sliding across 200MHz of occupied spectrum. In competitive devices, the VCO radiates at full power
under all of these conditions.
The lock protection circuit in the RF2516 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 con-
tains an internal resistor which, combined with a designer-chosen capacitor for a particular RC time constant, filters the
lock-detect signal. This signal is then passed through an internal Schmitt trigger and used to enable or disable the trans-
mit amplifier.
If the oscillator unlocks, even momentarily, the protection circuit quickly disables the output until the lock is stable. These
unlocks can be caused by low battery voltage, poor power supply regulation, severe shock of the crystal or VCO,
antenna loading, component failure, or a myriad of unexpected single-point failures.
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