RF2516 is a monolithic AM / ASK VHF / UHF launch chip launched by RF Micro Devices, which can work from 100MHz to 500MHz and uses AM / ASK modulation. It integrates PLL, VCO and reference oscillator on-chip. The operating voltage is 2.25V ~ 3.6V, which can provide ± 10dB output power for 50Ω load.
The RF2516 can be used in remote keyless access systems, wireless security systems and other remote control equipment from 315 MHZ to 433 MHZ. Due to its integrated VCO, phase discriminator, prescaler and reference oscillator, a complete phase-locked loop can be formed as long as an external crystal resonator is connected. In addition to the standard power-down mode, the RF2516 also has automatic phase-lock detection. When the PL loses lock, the transmit output is invalid.
1 pin function
RF2516 adopts 16-pin QSOP package. The following is the function description of each pin:
Pin 1 (OSCB): This pin is directly connected to the base of the reference oscillator transistor. A 68pF capacitor is connected between this pin and pin 2;
Pin 2 (OSCE): This pin is directly connected to the emitter of the reference oscillator transistor. A 33pF capacitor is connected between this pin and ground;
Pin 3 (PD): When this pin is low, all circuits are closed; when this pin is high, the circuit works normally;
Pin 4 (GND): The ground terminal of the TX OUT amplifier, the lead wire should be as short as possible and directly connected to the ground, in order to obtain the best working performance;
Pin 5 (TXOUT): Transmitter output terminal, this pin is an open collector output terminal, you need to connect a feeding inductor and matching capacitor to form a collector resonance matching circuit? At the same time feed the collector of the amplifier; 
Pin 6 (GND1): TX output buffer amplifier ground;
Pin 7 (VCC1): This pin provides bias to the buffer amplifier;
Pin 8 (MOD IN): AM and digital modulation signals are added to the carrier through this pin. An external resistor provides bias to the output amplifier through this pin, and its voltage cannot exceed 1.1V. Excessively high voltage may damage the device;
Pin 9 (VCC2): This pin is used to provide DC bias to VCO, crystal oscillator, and phase detector. The intermediate frequency bypass capacitor should be directly connected between this pin and ground;
Pin 10 (GND2): digital PLC ground terminal;
Pin 11 (VREF P): Bypass bias reference voltage pin. The bypass capacitor should be selected to an appropriate size to filter the reference crystal oscillator. The bypass capacitor is directly connected to this pin;
Pin 12 (RESNTR-): This pin is used to provide DC control voltage to VCO and adjust the center frequency of VCO;
Pin 13 (RESNTR +): same as pin 12;
Pin 14 (LOOP FLT): the output end of the pump circuit, the RC network from this pin to ground is used to set the PLL bandwidth;
Pin 15 (LD FLT): This pin is used to set the threshold of the lock detection circuit, and the bypass capacitor is used to set the RC time constant with the on-chip 1kΩ resistor. This signal is used to control the MOD IN circuit. The time constant should be 10 times the reference period;
Pin 16 (DIV CTL): preset divider control terminal, high level selects 64 frequency division, low level selects 32 frequency division.
2 Internal structure
The internal structure of RF2516 is shown in Figure 1. The reference oscillator in RF2516 is a COLPITS (Cobbitz) type oscillator (capacitive feedback three-point oscillation circuit). Pin 1 (OSC B) and pin 2 (OSC E) are usually connected to the oscillating transistor. The two capacitors are 68pF and 33pF respectively. The COLPTTS type oscillator has the characteristics of high reliability and suitable phase noise.
The function of the prescaler is to divide the output frequency of VCO by 64 or 32 by setting the logic level on the DIV CTL pin. High level selects 64 frequency division, low level selects 32 frequency division. The frequency-divided signal will be fed back into the phase detector and compared with the reference frequency. 
The phase detector and charge pump are integrated in the RF2516. The phase detector can compare the phase of the reference oscillator and VCO. Its output is generally connected to the charge pump. The phase discriminator is a bistable trigger digital phase discriminator, also known as a digital phase discriminator / frequency discriminator or digital tri-state comparator. The circuit is mainly composed of two D flip-flops, and their output is added to the NAND gate. The output of the NAND gate is connected to the reset terminal of the flip-flop. The output of the trigger is also connected to the charge pump. The output of each trigger is a pulse signal whose frequency is related to the frequency of the input signal of the trigger. When the inputs of the two flip-flops are the same, their outputs are frequency-locked and phase-locked. When the input is different, the trigger will provide a signal to the charge pump, and the charge pump charges, discharges, or enters the high-impedance state of the loop filter. This is also the origin of the tristate digital comparator. The main advantage of this phase discriminator is the ability to correct phase and frequency errors. When the phase is locked, the phase discriminator can be corrected with phase error; when the phase lock is lost, the phase discriminator is corrected with frequency error. This phase detector can enter the phase-locked state under any conditions.
The charge pump in the device consists of two transistors, one to charge the loop filter and the other to discharge the loop filter. Its input is the trigger output signal of the phase detector. Because there are two D flip-flops, there are four states. The loop filter can integrate the pulse from the charge pump to obtain the VCO control voltage.
The output of VCO is added to the prescaler circuit through the buffer amplifier, and the frequency division of 64 or 32 can be selected and compared with the reference oscillator frequency.
The transmitter amplifier is composed of a two-stage amplifier circuit with open driver and collector. It can provide + 10dB output power to 50Ω load under the action of 3.6V voltage.
The phase-lock detection circuit is connected to the output of the phase discriminator. When the VCO and the reference oscillator are not locked, the phase-lock detection circuit can be controlled so that the transmitting circuit does not work.
3 Application circuit
Figure 2 shows the RF transmitter circuit of RF2516 operating at 315MHz. When the circuit works, the audio signal controls the capacitance of the varactor diode D1, which is a capacitor of the crystal resonant circuit and can be used for direct frequency modulation of the audio signal to the carrier. In the circuit of Figure 2, pin 5 of RF2516 is connected to a 50Ω microstrip transmission line, which is only resistive but not inductive. The inverted L-shaped resonant matching circuit formed by the capacitor C3 and the inductor L1 can resonate at 315MHz and match with the wireless radiation impedance. Resistor R3 and capacitor C4 are the feed loop of the power supply. The RC network composed of resistor R5 and capacitor C7 is used to set the bandwidth of PLL.
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