The principle and application of single chip RF transceiver chip TRF6901

Abstract: TRF6901 is a monolithic RF transceiver launched by TI. The chip contains a complete transmit and receive circuit, so it is particularly suitable for half-duplex two-way wireless transmission of data in the ISM band. Compared with TRF6900, it has the advantages of large transmission power, low price, wide applicable voltage range, simple use, and few peripheral devices. The paper introduces the structure, principle, characteristics and application circuit of TRF6901.

1 Overview

TRF6901 is a monolithic low-cost RF transceiver chip launched by TI. It integrates a complete transmit circuit and receive circuit, and can easily form a half-duplex RF connector. TRF6901 has multiple channels, the operating frequency can be programmed, the frequency range is 860 ~ 930MHz. The chip's power supply voltage ranges from 1.8V to 3.6V, and the RF output power is up to + 9dBm.

The main features of TRF6901 are as follows:

● The working frequency range is 860MHz ~ 930MHz;

● Low power consumption, the maximum emission current is 40mA, the reception current is 20mA, and the standby current is 4μA;

● You can use OOK and FSK two modulation methods;

● Integrated frequency synthesizer and voltage controlled oscillator;

● Contains PLL and reference oscillator;

● With a typical output power of 9dB;

● Contains programmable battery detection circuit;

● With received signal strength detector;

● With a flexible 3-wire serial interface, it can be easily connected with a microcontroller;

● Programmable fine-tuning of the reference oscillator frequency;

● High degree of integration and few peripheral components;

● Adopt 48 feet PQFP package, small size.

figure 1

Between TRF6901 and the microcontroller can be connected through 11 pins. If you want to form a low-power system, you can choose TI's MSP430 series single-chip microcomputer as the microcontroller.

2 Pin function

3 Internal structure

The internal structure of the TRF6901 chip is shown in Figure 1. The functional principles of each part are as follows:

3.1 Low noise amplifier and mixer

The integrated low-noise RF amplifier inside TRF6901 uses differential input. The external network of the chip has the function of 50Ω impedance matching and 180 ° phase shift. It can convert the input signal into two signals with a phase difference of 180 ° and input them to low noise. Two pins of the amplifier. The input impedance of this differential circuit is about 500Ω.

The output signal of the low-noise amplifier is a high-frequency signal. In order to facilitate further amplification and demodulation, the mixer should be used to turn it into an intermediate frequency signal. The local oscillator signal of this mixer is provided by VCO controlled by on-chip PL. The local oscillator frequency is equal to the received signal frequency minus the intermediate frequency, and the local oscillator frequency can be programmed and controlled according to the received signal frequency.

3.2 Filter and intermediate frequency amplifier

In order to make the entire receiving circuit have good selectivity, the output signal of the mixer needs to be filtered by an external intermediate frequency filter. The intermediate frequency can be selected as 10.7MHz, and the corresponding filter can also be selected as a three-terminal ceramic filter of 10.7MHz. The output of the filter is then input to an intermediate frequency amplifier for amplification.

IF amplifiers also use differential inputs. In order to turn the output signal of the three-terminal filter into a differential mode signal, an inductor can be connected to the two input terminals of the differential circuit to achieve a 180 ° phase shift. The IF amplifier contains a limiting amplifier with a gain of 68dB to achieve high gain IF signal amplification.

3.3 Received signal strength indication circuit (RSSI)

The output voltage of the received signal strength indicator is a DC voltage obtained by rectifying the input signal of the intermediate frequency limiting amplifier, and its voltage is proportional to the strength of the received signal. The RSSI signal can be used as a fault detection, a handshake signal in receiving and sending, and a selection signal of the radio frequency channel; in some applications, it can also be used as a demodulation signal for ASK and OK.

figure 2

3.4 FSK demodulation circuit

The output signal of the limiting amplifier is input to the demodulation circuit for demodulation. The demodulation circuit is an integral FSK demodulation circuit. The outside of the demodulator must be connected to a ceramic discriminator or LC resonant circuit that is consistent with the intermediate frequency. It is best to use a ceramic discriminator. The internal of the chip is connected in parallel with a programmable resistor and an external discriminator, so that the center frequency of the discriminator can be finely adjusted by changing the value of the resistor. The value of the programmable resistor is controlled by the 12 to 14 bits of the D register in the chip. The recommended value is 110. If the external circuit uses an LC circuit, the values ​​of L and C can be calculated according to the formula fres = 1 / 2π (LC) 1/2. Both ends of the LC circuit can also be connected in parallel with a resistor as needed to reduce the Q value of the circuit.

3.5 Data detector

The data detector consists of a comparator. Its output is determined by the received signal and the threshold voltage, which is stored on the external capacitor of pin 34. When the learning / holding control signal of pin 39 is high, the storage capacitor is charged by the received signal, and the charging value is the average of the high and low levels; and if the learning / holding control signal of pin 39 is low, the voltage on the capacitor Will drop slowly, so the storage capacitor must be charged regularly. Usually, before each data reception, a capacitor is charged with a training signal. The capacity of the external capacitor is about one ten thousandth of the signal period. However, if the signal at the time of modulation does not have a DC signal, then the TRF 6901 can always work in the learning mode, so that no training signal is required.

3.6 Frequency synthesizer

The frequency synthesizer contains a main frequency divider, which can be used to divide the frequency of the VCO signal. The main divider consists of a 5-bit A counter, a 9-bit B counter, and a prescaler. The frequency divider of the prescaler can be 32 or 33. When the A counter overflows, the frequency divider of the prescaler is 33. Until the B counter overflows, it becomes 32 again, so the total divider of the main divider The frequency is:

N1 = 33 & TImes; A + 32 (B-A)

In addition, there is a reference frequency divider in the frequency synthesizer that can be used to divide the oscillation signal of the reference crystal oscillator. The frequency divider is 8 bits and the maximum frequency division number (N2) is 255.

The signals of the above two frequency dividers can be input to the phase detector at the same time. The output of the phase discriminator is used to control the oscillation frequency of VCO. The 31 pin of the integrated circuit is connected with a capacitor connected in series with the crystal. The capacitor is grounded through an internal switch. At the same time, the ground switch is controlled by the digital signal to be sent. In this way, the digital signal to be sent can control the oscillation frequency of the reference oscillator. , And then realize FSK modulation.

3.7 Serial control interface

TRF6901 has a serial interface to connect with the microcontroller. The microcontroller uses the serial interface to program the four 24-bit registers inside the TRF6901.

TRF6901 has three serial interfaces, namely data (DATA), clock (CLOK) and gate control (STROBEE). The gate control signal is used to control the start and end of each register programming. The first 2 bits of data are used to select the register. The programming sequence of the register is shown in Figure 2.

image 3

3.8 Main auxiliary circuits

In addition to the above circuits, there are some auxiliary circuits inside TRF 6901, such as DC-DC, battery detection and clock output. Among them the internal DC-DC converter is used to achieve boost. When the system power supply is lower than 1.8V, it can be used to boost to power VCO and PLL; and the on-chip integrated battery detector can be used for battery power detection, and the detection threshold can be set by bits 1 and 2 of B register; The clock output circuit can divide the reference oscillation signal and output it to provide a clock signal for the microcontroller.

4 Application circuit

Figure 3 shows the actual application circuit of TRF6901 working in the 860 ~ 870MHz band. In order to overcome the mutual interference of each unit circuit inside the chip, when power is supplied, each unit should be connected with a resistance-capacitance decoupling circuit. A 10Ω resistor and a 0.1μF capacitor can be used for design.

The control port of TRF6901 can be directly connected to the general-purpose port line of MPS430 series MCUs; if the selected MCU has a hardware serial port, you can directly connect the TX_DATA and RX_DATA of TRF6901 to the serial port of the MCU, which can bring greater programming and debugging Convenience.

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