RF front-end design for short-range wireless consumer electronics

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As system cost and power consumption continue to decrease and product performance improves, the wireless short-range consumer electronics market is growing rapidly. This article will introduce some new trends and solutions in the market to design engineers, and introduce the features and applications of the RFW102 chipset.

Short-range wireless applications refer to solutions that can transmit data within a range of up to 100 meters. The main types of wireless short-range applications are as follows:

Wireless Local Area Network (WLAN) is a high-speed transmission solution (11~56Mbps) that can replace wired LAN connections. WLAN solutions are common in office applications and are becoming increasingly popular in small office/home office (SOHO) and private residence applications. Its indoor effective distance is usually 100 meters.

Personal Area Network (PAN) is suitable for medium transmission rate applications within a range of 10 meters (some PANs sometimes require longer distances). This solution is mainly aimed at open environment applications and products (Bluetooth).

Figure 1: Functional block diagram based on the RFW102 transceiver

Wireless short-range consumer products are used for low-speed data transmission, with an effective range of 30 meters. These products may overlap with some PAN applications. This type of solution is generally targeted at closed application environments and products. Currently, there are many solutions for this type of product.

RF Front-End Technology Trends

As the demand for wireless communication functions in consumer products continues to grow, various technical solutions currently exist to meet these needs. The superiority of various technical solutions can be measured by the following five key indicators:

Cost: The lower the cost, the wider the application and the greater the market potential. It can be said that when the cost is reduced by 10%, the market potential will expand by 100%.

Transmission range: The applicable range of short-range wireless solutions is generally within 30 meters indoors.

Power efficiency: Wireless devices are in many cases battery-powered, so battery life is a key metric. This issue will be indirectly reflected in costs because consumers will have to replace batteries more frequently due to high power consumption. Even when using rechargeable batteries, consumers want more powerful batteries so that each charge can last as long as possible.

Quality of Service (QoS): Provides reliable data links while meeting data transfer rate requirements.

High data rate: This parameter can open up new application areas in the wireless consumer product market. Applications that were not possible in the past due to cost issues can now be realized with proper design.

A wireless solution generally consists of two parts: the RF front end and the media access layer (MAC). The RF front end is mainly measured by the following performance indicators:

Link budget: Link budget [dB] = output power [dBm] - sensitivity [dBm]. Output power refers to the power output from the transmitter during transmission, and sensitivity refers to the sensitivity of the receiver. A 10-meter indoor distance requires a link budget of at least 70dB, and many current products meet this requirement. Output power is generally 0~10dBm, and sensitivity is -80~-90dBm.

Figure 2: RFW102 front-end implementation circuit diagram.

Operating frequency: Consumer products cannot afford the cost of frequency occupation, and it is best to operate in the ISM band that does not require a license. 2.4GHz is the only ISM band used worldwide, and other bands used by suppliers include 900-928MHz (only applicable to the American continent) and 868-870MHz (most of Europe).

Modulation scheme: Many current products use amplitude shift keying (ASK) or frequency shift keying (FSK).

Spread spectrum scheme: Spread spectrum can provide stability and security for wireless applications operating in the ISM band. Two commonly used spread spectrum schemes are frequency hopping spread spectrum (FHSS) and direct sequence spread spectrum (DSSS), two completely different spread spectrum schemes that have the same stability.

Receiver/Transmitter/Transceiver: A simplex link means that each side (node) can either transmit (transmitter) or receive (receiver) at a given time, but not both at the same time; a duplex link means that each node can transmit and receive (transceiver) at the same time.

RF Front End for Consumer Products

The wireless short-range consumer market is a large market with a wide variety of products (see Table 2). However, from the consumer's perspective, current products cannot provide satisfactory solutions at a reasonable price.

When choosing a technical solution, wireless product manufacturers generally pay more attention to cost issues, and even believe that cost is more important than quality. However, consumers cannot accept this idea. They hope that wireless products can at least be as good as low-priced wired products. Therefore, no matter how good a wireless solution is, it cannot be applied to consumer products as long as it cannot achieve a reasonable price; similarly, if the wireless solution used cannot provide data link performance as good as the wired solution, consumers will not be satisfied. Table 1 shows the technical requirements for improving the five basic parameters discussed above.

Therefore, an important issue is to find a wireless solution with ultra-low price and high performance. Since all wireless solutions face the problems mentioned above, the following will take the wireless solution of RFWaves as an example to discuss.

RFW102 transceiver chipset implementation

Figure 1 depicts a typical application using the RFW102 transceiver chipset from RFWaves. The solution consists of two main parts: the RF front end and the digital back end.

Table 1: Requirements for improving system cost and performance.

RF Front End

The RF front end uses the RFW102 transceiver chipset, which includes three chips: RFW24, RFW488C and RFW488R.

RFW24--Performs all timing, amplification, switching, transmission and reception functions.

RFW488C is a 4-pin SAW correlator implemented on a quartz substrate. This chip is a completely non-differential passive device and is the basic unit for DSSS spread and despread. It can be used as a lateral 13-bit BPSK Barker code correlator (matched filter).

RFW488R--Single-port SAW resonator, resonant frequency is 488MHz, can be used as the CW oscillation source of the system.

In addition to the RFW102 chipset, only a few passive components are needed to implement a complete solution, as shown in Figure 2.

RFWaves modems have the following features:

Working frequency: 2.4GHz, a global license-free frequency band, using DSSS technology.

Low receiving power consumption: 1kbps@40μA; 10kbps@380μA; 100kbps@3.8mA; 1Mbps@38mA;

Low transmit power consumption: 1kbps@25μA; 10kbps@220μA; 100kbps@2.2mA; 1Mbps@21mA.

Standby current: 2.6μA; wake-up time: 20μS; synchronization time: 1μS. RFWaves transceiver only needs one bit to achieve synchronization (synchronization time is 1.2μS);

Output power: +2dBm; Sensitivity: -80dBm.

Data transfer rate: up to 1Mbps.

The RF front end has a simple three-wire digital interface with the following basic functions:

Tx/Rx--determine the working mode;

TxD/RxD--bidirectional line that can send and receive digital signals;

ACT--Enables standby mode to save power.

Digital backend

The RF front-end interface data line is an asynchronous serial bit pipeline. Since the data uses an RF link, an error detection/correction scheme must be provided. In order to use a low-end MCU to reduce the total cost, the MCU must support MAC/protocol functions. Therefore, there is a hardware-implemented interface between the RF front-end and the MCU.

As mentioned earlier, the system uses a low-end MCU that runs the MAC/protocol and application. The MCU can be configured with various special interfaces such as USB, SPI, and I/O depending on the application requirements. Since only one MCU is used, costs can be reduced and system power consumption can be reduced. In addition to the digital backend provided by RFWaves partners, developers can also obtain protocol files and source code reference designs.

RFWaves Overall Solution

Consumer product developers all hope to use easy-to-integrate design units to simplify design and reduce risks, and use as many off-the-shelf units as possible. However, the RF front end is generally very complex and requires extensive communication knowledge and design skills. Based on the above facts, design engineers urgently need an easy-to-integrate solution that treats RF and communication circuits as a "black box". With this black box, designers only need to understand how it connects to the application system. RFWaves' overall solution takes these design requirements and QoS issues into consideration. The front end uses a 13-bit barker DSSS solution, duplex link, error detection solution, and protocol conversion based on different applications.

Ultra-low power: Low power consumption when receiving and transmitting, very low standby current, very short wake-up and fast acquisition times, and a protocol that can take advantage of system idle time to enter standby mode.

Table 2: Major application markets for wireless consumer products.

Data transfer rate: The total bandwidth of the RF link is currently 1Mbps.

Distance: With a link budget of over 80dB, the effective indoor distance is up to 25m, and adding a power amplifier to the RF front end can extend the effective range to 100m.

Low total system cost: The total system cost of the RFWaves solution is comparable to that of low-end solutions. To further reduce costs, RFWaves also provides reference designs for antenna layout on PCB.

Application Design Examples

The following will describe the design requirements and steps using the design of a wireless gaming keyboard/controller using an RFWaves RF front end as an example.

A gaming keyboard is a gaming peripheral that is attached to a gaming platform (PS2, Xbox, or Gamecube) or PC. In recent years, wireless gaming keyboards and operating handles have been very popular.

Basic Requirements

Low cost – every consumer product has this requirement;

Low latency – extremely fast response time, as it will be used for high-speed gaming;

Transfer rate - relatively high data transfer rate;

Multi-device coexistence - up to 16 devices can work simultaneously in the same area;

One gaming platform can support 4 control keyboards;

Duplex link - returns the feedback signal sent by the game platform;

QoS is particularly important in this instance, as any operation that is unacceptable to the gaming platform will affect the gamer's entertainment experience.

Design Steps

1. Select a suitable digital backend (I/O ports, RAM, ROM, etc.) provided by one of RFWaves partners. Selecting a different digital backend depends on the application.

2. Understand the data transmission method between the game control keyboard and the game platform.

3. After understanding the data transmission method, adopt the correct protocol provided by RFWaves as a reference.

4. Implement MAC/protocol and application software on the MCU.

5. Debugging

6. Build a prototype.

Summary:

Wireless consumer applications are a huge market, and they are growing at a rapid rate every year. However, many developers have experienced failures in this area and lack application experience. In addition, compared with wired products, many current wireless solutions are still insufficient in terms of price and performance. With the introduction of more low-cost, high-performance short-range wireless transmission solutions, it is believed that there will be more and better wireless consumer products to choose from, and the wireless consumer product market will continue to expand.

author:

David Ben-Bassat

Ari Mizrachi