Low power VHF wireless communication receiver MICRF004 and its application

    Abstract: MICRF004 is the latest small monolithic wireless communication receiver integrated circuit launched by Micrel. It can be used to truly realize the "wireless input and data output" functions of the microcontroller. In addition, MICRF004 has very high efficiency and reliability, and is currently the lowest-cost single-chip solution in wireless communication applications. The article introduces its main features, pin functions, working principles and typical application circuits. At the same time, the actual circuit of the 150MHz, 1kb/s receiver/decoder composed of MICRF004 and the specific parameters of its peripheral components are also given.

    Keywords: wireless communication receiver low power MICRF004

1 Overview

MICRF004 is the latest small monolithic wireless communication receiver integrated circuit launched by Micrel. It is a wireless receiving device specially designed for wireless communication applications in the very high frequency band (VHF band). Using MICRF004 can truly realize the single-chip "wireless input and data output" functions. All radio frequency (RF) and intermediate frequency (IF) adjustments can be completed automatically within the device, thus greatly reducing development costs and shortening the product marketization process. In addition, because the efficiency and reliability of MICRF004 are very high. Therefore, it is currently the lowest-cost single-chip solution in wireless communication applications.

MICRF004 is very easy to use. It can provide two basic operating machines: fixed and scanning. In fixed operating mode, the MICRF004 functions like a traditional superheterodyne receiver with a fixed local oscillator frequency, with the internal local oscillator operating frequency provided by an external crystal oscillator. Fixed mode is typically used where the transmitter can be precisely controlled by a crystal or SAW (surface acoustic wave) device; during scanning operation, the MICRF004 will scan the local oscillator frequency within a larger range of this baseband data ratio, effectively broadening the receiver RF bandwidth. Therefore, MICRF004 can receive signals from simple and cheap LC transmitters without the need for special tuning circuits, and can even extend the reception range to the superheterodyne range. In this mode, the system can replace expensive quartz crystals with ±0.5% cheap ceramic devices as a reference.

Shutdown control is an important feature of MICRF004. This feature can provide logical instructions when RF signals appear in duty-cycled and wake-up outputs, and enable MICRF004 to adapt to low-power and very low-power applications. , such as RKE (remote keyless entry) and RFID (RF identification) systems.

The MICRF004 provides all data filtering functions for the system demodulator, eliminating the need for external filters. The MICRF004 contains four filters, one of which is programmed within the data rate bandwidth. When MICRF004 is in scanning mode, its binary bandwidth range is 0.55kHz ~ 4.4kHz, while in fixed mode, its bandwidth range is 1.1kHz ~ 8.8kHz.

In addition, MICRF004 has the following main features:

●Integrate all VHF receiving functions on a single chip;

●The frequency range can reach 140MHz～200MHz;

●When receiving with a monopole antenna, its typical receiving distance is 200 meters;

●Can be adjusted automatically without manual tuning;

●No external filter required;

●At 150kHz, its power supply operating current is as low as 240μA;

●Microprocessor or enable encoder can be used to wake up the operation;

●Has standard Ics CMOS logic interface;

●Has extremely low RF antenna re-emission characteristics;

MICRF004 is an antenna receiving application-specific integrated circuit specially designed for very high frequency band (VHF band) wireless communication applications. Therefore, it can be widely used in car remote control, remote radio frequency reception, remote fan and light control, and automatic opening and control of car doors.

2 Pinout and functions

MICRF004 adopts 16-pin DIP or SOP package. Figure 1 shows its pin arrangement. The function description of each pin is shown in Table 1.

Table 1 Pin function description of MICRF004

3 Working principle

The internal working principle block diagram of MICRF004 is shown in Figure 2. As can be seen from the figure: MICRF004 consists of UHF downconverter, OOK demodulator, reference and control, and call distribution circuit. At the same time, the figure also shows the capacitors of the CTH and CAGC pins and the reference oscillator outside the REFOSC pin. These external components are the components required to construct a UHF receiver using the MICRF004. MICRF004 has 4 control input pins such as SEL0, SEL1, SWEN and SHUT. Using these four pins, the user can set and select the operating mode of MICRF004. These pins have internal pull-up resistors, and their inputs are CMOS level compatible.

3.1 Bandwidth selection

The demodulator band of MICRF004 can be selected and set using the SEL0 and SEL1 pins. In scanning mode, its selectable frequency bandwidth range is 550Hz ~ 4400Hz, and in fixed mode, its selectable frequency bandwidth range is 1100Hz ~ 8800Hz. Table 2 shows the demodulator bandwidth selection method for MICRF004.

Table 2 Bandwidth selection method of MICRF004

3.2 Mode selection

The operating mode of MICRF004 can be controlled by using the input level of SWEN pin. When the input of the SWEN pin is high level, the device selects the scan mode; and when the SWEN pin is low level, the device selects the fixed mode. In fixed mode, MICRF004 uses the usual superheterodyne reception method to work.

●Fixed operating mode

If the transmitting system uses a relatively stable transmitting device like a SAW transmitter, then the MICRF004 can choose a general superheterodyne fixed operation mode for receiving work. The signal frequency range that the fixed mode can receive is narrow, but the data rate is relatively high, usually reaching more than 10kb/s.

●Scan operation mode

In scan mode, MICRF004 still uses the extended superheterodyne method to work, but it needs to be operated under LO local oscillator modulation. At this time, the scan rate is higher than the ordinary data rate. The scanning method can effectively broaden the RF bandwidth of MICRF004. And it can make the device work normally when the external transmitting frequency and receiving frequency are not very accurate. The use of the LO local oscillator modulation method does not affect the IF bandwidth of the system, and compared with the fixed operation mode, the performance of the scan mode in terms of IF noise immunity is not greatly weakened. Therefore, their IF bandwidth is 500kHz whether in fixed or scanning operating mode. However, LO local oscillator modulation will have an impact on the rate. In scan mode, the upper limit of the rate is approximately 2.5kb/s.

3.3 Reference oscillation

All timing and tuning of the MICRF004 is based on the reference oscillator internal to the device. These tuning operations can be performed on the REFOSC pin in one of three ways:

(1) Connect a ceramic resonator;

(2) Connect a quartz crystal oscillator;

(3) Use an external timing signal to drive the REFOSC pin.

If a microprocessor controlled by a crystal oscillator or ceramic resonator is used to generate an oscillation signal, that is to say, when the signal frequency of the system is relatively accurate, the above three methods are very useful for reducing system costs.

3.4 Wake-up and shutdown

The main function of the wake-up circuit in MICRF004 is to reduce the power consumption of the entire system. WAKEB is a logic signal output terminal. When the system detects an RF carrier in the detection output signal, the WAKEB terminal outputs a low-level signal. This output can be used to enable external circuitry such as data decoders and microprocessors when an RF signal is detected. Of course, the wake-up feature can only be used when the system is in shutdown mode.

The wake-up function is contained in a recoverable counter inside the device, which uses the 2.34kHz internal clock. This clock is generated from an internal 6.0MHz reference frequency. When this 23.4kHz clock is monitored, the system will make the WAKEB end enter a low level after 5.12ms (128 clock cycles are required at 25kHz) and continue until the data starts. This 5ms duration is very precious, it can greatly reduce the error rate of system monitoring. And there is no need to perform conversion compensation on the signal. Therefore, it is very convenient to use this method to complete the wake-up function.

The shutdown function is controlled by the logic state of the SHUT pin. When V SHUT is high, the system enters low-power standby mode, and the standby current at this time is less than 1μA.

4 application circuit

4.1 Typical application circuit

MICRF004 can be used in a variety of wireless communication applications. Figure 3 is a typical application circuit of MICRF004 for 150MHz 1200b/s switch keying receiver. This typical application circuit is very simple and uses few peripheral components. It uses a 4.85MHz ceramic resonator as an external oscillator. The 4.7μF capacitor outside the CAGC pin is used to automatically control the gain of MICRF004 together with the AGC circuit inside the device. The 0.047μF capacitor on the CTH pin is used to maintain the demodulation waveform on the DC level of the internal comparator input. This circuit operates with a +5V supply voltage.

4.2 Practical application circuit

Figure 4 is a practical application circuit using MICRF004 to design a 150MHz 1kb/s switch keying receiver/decoder. This circuit can continuously receive scans during off-duty cycles. It is designed in scan mode with 6 address decoding bits and 2 output encoding bits.

In the circuit of Figure 4, U1 is a small single-chip wireless communication receiver MICRF004 from MICREL Company, and U2 uses the HT-12D logic decoder from HOLTEK Company. The circuit operating frequency is 150MHz, so a cheaper ceramic resonator is used to generate the reference Oscillation frequency signal. Adjusting the length of the ceramic resonator, local oscillator capacitor C4 and antenna can make the circuit achieve the best connection capacity and choose an 8.2pF ceramic capacitor. It should be noted that the power supply filter capacitor C1 has two grounding methods: if the grounded end of C1 is connected to the analog ground, the capacitor will work in the RF frequency range; and if the grounded end of C1 is connected to the digital ground, the circuit will Operates in the baseband frequency range.

In fact, the data rate of this circuit can be adjusted through R1. When the data rate is 1kb/s, the value of R1 is 68kΩ. The function of R2 in the circuit is mainly to provide current limiting bias for the red indicator light-emitting diode, and an ordinary resistor of 1kΩ can be used.