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2022-08-23
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Using the concept of mixed signal design to improve the performance of short-range wireless transmission system

we have been talking about wireless transmission for several years, but if you look carefully, you will find that we have actually lived in a world of wireless transmission. In the early 1990s, mobile was mainly used by business people, but now almost everyone, from young students to the elderly, moves. The massive popularity of mobile is just one of the more obvious examples of our transition to a wireless transmission world. Another important and obvious example is the popularity of wireless network applications with the emerging wireless network hotspots in more and more bookstores, cafes and other public places. What makes us more surprised is that such technology has gradually spread to some application fields that we originally thought were not obviously wireless transmission, such as remote door switch (RKE) and remote control applied to TV set-top box and audio equipment

many of this type of wireless transmission applications are located in the frequency band between 260~470mhz, which belongs to the ultra-high frequency (UHF) band without license plate. The main reason why this type of devices and their applications can gradually integrate into our lives is that they provide more convenience for our lives. Many such short-range wireless transmission devices use traditional analog RF technology, and then operate in the UHF band without license plate. After applying the latest mixed signal design technology, they will be able to significantly improve some characteristics. Although each short-distance transmission system has its own advantages and disadvantages, generally speaking, they have been improved in extending the transmission distance, increasing the battery life and reducing the size of portable devices after using the latest technologies and concepts. This paper will briefly introduce the current short-range wireless transmission systems, and introduce the design concept of mixed signals to improve these systems

reference system

short distance wireless transmission technology has been widely used in many daily life applications, such as remote control door switch (RKE), tire pressure monitoring system (TPMS), car anti-theft device, remote controller, home security and automation, 20V standard voltage garage door remote control switch and many other products that are remotely controlled by radio. Although these applications are different, the basic module diagram of short-range wireless transmission system is very similar, as shown in Figure 1

Figure 1: typical block diagram of short-range wireless transmission system

generally speaking, the transmitter is a portable device that works by battery, and has some buttons or keyboards as input tools. For example, in the remote door switch (RKE) system, the entire transmitter system is a remote key composed of a CR2032 battery and a button as an input device to open and close the door and the rear compartment. The button input device is connected to a microcontroller (MCU) that sends a series of digital signals to the RF transmitter. This RF transmitter is a typical amplitude shift keying (ask) regulator, which uses an external power transistor to turn on and off the SAW resonator. At the receiver end, the system includes an analog RF receiver, a microcontroller, and some exciters that can be used as control output driven by batteries or other power supplies. Continuing with the previous example of remote door switch (RKE), the receiver uses a linear regulated car battery as the power, and an RF receiver demodulates the signal of amplitude shift keying (ask) into a series of digital signals, which are successively translated into output signals through the microcontroller, so as to achieve the purpose of locking or unlocking the door. This type of wireless transmission represents many existing short-range wireless transmission applications, which will be used as a reference in this paper

improve wireless transmission distance

in wireless transmission system, one of the most expected features is long-distance transmission. The two most practical methods are to increase the power of the transmitter and improve the sensitivity of the receiver. However, the government's laws and regulations limit the transmission power of the transmission system, which aims to enable different systems to share the same frequency band at the same time with the least interference. The Federal Communications Commission (FCC) of the United States and the European Telecommunications Standards Institute (ETSI) have formulated radiated power standards in their respective regions, and then press the confirm key to enter and restrict the signal transmission of radio devices, whether intentional or unintentional. These limits determine the maximum transmission power, so in fact, the only feasible way to increase the wireless transmission distance is to increase the sensitivity of the receiver

if we carefully study our RF reference system, we can find that the initial frequency accuracy of the RF transmitter built on the surface acoustic wave (SAW) resonator is very poor, and its frequency error range is about ± 150 kHz. At the same time, we also find that its frequency stability is very poor due to temperature. This leads to large carrier frequency compensation of the transmitter, which forces the receiver to have a wide channel filter. Large bandwidth makes redundant noise enter the system, which reduces the overall sensitivity and transmission distance

a possible solution is to use phase locked loop (PLL) based on crystal oscillator to replace the transmitter based on surface acoustic wave (SAW). This solution can significantly improve the frequency accuracy of the transmitter, and then improve the transmission distance by reducing the bandwidth of the received channel filter. Another option is to replace the standard analog RF receiver with a mixed signal RF receiver with an integrated DSP or a demodulator with digital processing capability. The advantage of this mixed signal receiver method is to track the transmitter frequency compensation based on surface acoustic wave (SAW) by using a digital processing capable filter that minimizes the bandwidth, and thus reduce the noise. Due to the continuous improvement of CMOS technology and economies of scale, the cost of mixed signal RF receivers is lower than that of analog RF receivers. Another way to improve receiver sensitivity is to use antenna diversity. These techniques use additional amplitude and/or phase information from the RF signals of different antennas to improve the sensitivity of the receiver. Mixed signal integrated circuits are widely used in these receivers because of their ability to process a large amount of information from all antennas

reduce power consumption

in any communication system, how to increase battery life or reduce power consumption has always been the focus of research. In the case of our remote door switch (RKE) system, reducing the power consumption of the transmitter is equal to increasing the battery life of the key. Reducing the power consumption of the receiver means that less car battery energy is consumed, which is particularly important when the car is parked or idle. For the remote door switch (RKE) receiving system in the car, the average current required by most automobile manufacturers is defined as less than 2 mA. At present, the existing solution is to set the receiver in a low duty cycle polling or sampling mode to meet the requirements of low average current

Figure 2 shows the basic concept of polling by allowing the receiver to reduce the average consumed current in a low duty cycle mode. Most of the time, the receiver is in sleep mode and only maintains the minimum current required to record the sleep period (zone 1). It will periodically enter a sampling mode (area 2), where the analog RF receiver will start working, and determine whether there is an incoming transmission signal by observing the input received signal strength indication (RSSI) level and comparing it with the preset threshold value. In this example, the transmitter transmits the same packet twice (the transmission reception rate is two), and adjusts the selection of sampling and sleep time of the receiver at the same time to ensure that the receiver can sample at least once during the two transmission packets, so as to avoid losing any packets

Figure 2: reduce the average current consumption by adjusting the duty cycle polling mode

if the input received signal strength indication (RSSI) exceeds the preset standard, the analog RF receiver will start to run the micro control unit to further process the incoming signal (area 3). But there is a problem with this method. Every time the received signal strength indication (RSSI) exceeds the preset standard, it will not check whether these incoming signals are from the expected transmitter, and it will directly wake up the external micro control unit. The mixed signal IC has a packet or address proof, so before interrupting and waking up the micro control unit (area 4), he will confirm whether the transmitted signal is expected for the receiver? In this way, some power will be saved. This home appliance industry is an important field of engineering plastics utilization. For applications that require long-term parking in busy and crowded parking lots such as airports, it can effectively save power

in the transmitter part, the main goal is to maximize the battery life, which can be achieved by using a mixed signal IC with low operating voltage and low leakage current. Table 1 is an example of calculating the battery life of a hypothetical RKE transmitter. This transmitter has a transmission current of 15 mA, a packet size of 136 bits at a data rate of 2.4 Kbps, a transmission repetition rate of 2, and it is assumed that the button will be pressed 20 times a day, a CR2032 button battery with a capacity of 210ma/hr will be used, and the total leakage rate is 2ua/h (17.52 mah/yr). In this example, the battery life is more than 10 years, and is limited by the leakage current from the sleep mode, the leakage current of the transmitter, and the leakage current of the battery itself. In order to reduce leakage, the transmitting circuit can only be started after the key is pressed, and the power must be cut off automatically after the transmission. Using mixed signal technology can easily realize the wake-up characteristic of pressing this key. Using large CMOS switches can substantially connect and disconnect the transmitter voltage to the battery or ground, so as to reduce the leakage current

Table 1: an example of battery life calculation for RKE transmitter

another key factor in designing low-power transmitter is to maximize the efficiency of power amplifier. One of the techniques especially applicable to mixed signal technology is to achieve the resonance of PCB loop antenna by using varactor, so as to maximize the efficiency. In IC, this transformer is generally a capacitor of weighted binary array, which is activated or deactivated by a series of CMOS switches to provide digital control and/or programmable capability. The analog transformer cannot be programmed, so some circuits and currents are needed to realize the bias voltage of this transformer

reduce system cost

compared with traditional analog RF design, one of the main advantages of mixed signal integration design is its high integration ability, thus saving the demand for external components. In many cases, as shown by silicon laboratories products, hybrid signal design can significantly reduce the circuit board area and external material cost of wireless systems compared with traditional analog RF solutions, and achieve the best performance at the same time. The current mixed signal architecture integrates all front-end analog circuits including high-performance analog-to-digital converters (ADCs) and digital back-end processors, thereby reducing the need for external components

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