Designing Wheel-Tuned, Digital-Display Radios with Next-Generation Radio ICs

Transcription

1 Designing Wheel-Tuned, Digital-Display Radios with Next-Generation Radio ICs Radio technology has been around for more than a century, and traditional wheel-tuned radio products have been used for decades by countless listeners around the world. They provide a simple user interface based on a tuning wheel to dial the frequency and a moving needle with a frequency mark to show the tuned station. During the past decade, high-performance DSP-based radio designs have enabled sophisticated new user interfaces with buttons for auto seek/tune capabilities and liquid crystal displays (LCDs) that display the frequency. This article will discuss how to design a mid-level wheel-tuned radio with an LCD to display frequencies, and it will explore the practical considerations in selecting audio ICs and display drivers, as well as how to leverage these technologies to deliver the best end-user radio tuning experience. As a growing number of portable applications such as mobile phones and portable media players integrate the FM radio function, there s a misconception in the market that traditional radios are no longer needed. The reality is, wheel-tuned radios have remained immensely popular for a number of reasons. For instance, it can be technically challenging to integrate the AM and shortwave (SW) radio feature in portable multimedia devices due to interference and size constraints. Many consumers still prefer to listen to sports news and other audio broadcast content through AM and SW radios such as boom boxes, smart phone docking stations and other portable radio products. Traditionally, these radio products have adopted the appearance of a tuning wheel and a needle with frequency marking to show the tuned frequency. In recent years, DSP-based radios have attracted consumer interest by offering convenient LCD/LED frequency displays and pushbuttons designed to auto-seek the frequency. However, while many radio users appreciate the convenience of displaying frequencies on an LCD or LED panel, they still prefer to use the intuitive tuning wheel, as shown in Figure 1. For simplification, let s call this market the wheel-tuned, digital-display radio, also known as the analog-tuned, digital-display (ATDD) market. (Note that the analog designation is no longer accurate since digital radio ICs predominate in this market; however, we still use the popular industry acronym, ATDD.) There are multiple ways to design an ATDD radio. Let s consider each design approach from a system level including RF performance, level of complexity, feature differentiation and finally bill of materials (BOM). We ll start by examining the traditional approach of using analog ICs to design an ATDD radio, then look at creative designs such as click-wheel radios using DSPbased radio ICs, and conclude with an overview of new multi-band radio IC technology optimized for the ATTD market. Silicon Laboratories, Inc. Rev 1.0 1

2 Figure 1. Example of a Typical Wheel-Tuned, Digital-Display Radio Traditional Analog IC for ATDD Radios Traditional analog radio ICs can be used in wheel-tuned digital-display radio designs. However, due to the limitation of the AM/FM receiver s analog architecture, the receiver IC requires a large BOM because much of the signal processing is performed off chip by other components. In addition, the analog IC does not provide the tuned frequency information for the display driver. Thus, in these traditional radio solutions, an intermediate-frequency (IF) counter IC is needed to interpret the local oscillator pulses as tuned frequency and translate these pulses to a display driver, which then displays the calculated tuned frequency, as shown in Figure 2. Figure 2. Simplified System Schematic Using a Traditional Analog Receiver IC Traditional radio ICs have served the wheel-tuned radio market for several decades and have made significant contributions to the evolution of the radio. However, these traditional solutions pose a number of limitations for both manufacturing processes and achieving a high-quality radio experience: Silicon Laboratories, Inc. Rev 1.0 2

3 Traditional solutions have poor RF performance due to the inherent limitations of analog radio ICs. Traditional analog solutions have poor sensitivity and low selectivity. The resulting radio products are sometimes unable to receive radio stations in rural areas that have weak signals. In addition, with analog radio designs, it can be difficult to listen to a preferred station in cities with a crowded spectrum with interference from neighboring stations. The digital display is a key selling feature of ATDD radios compared to traditional analogtuned, analog-display (ATAD) radios. However, the displayed frequency is often inaccurate due to tuning errors caused by analog ICs. In fact, the actual tuner frequency may be off by as much as four or five channels from the displayed frequency, resulting in a frustrating user experience. Because of the limitations of analog technology, systems based on analog ICs require numerous discrete components for signal processing such as inductors and IF filters. The resulting radio designs have large BOMs with component counts as high as 70 discrete components. This high number of components is only part of the story. Although the cost of analog IC is very low, because there are so many components in the traditional solutions, they add up to a high total BOM cost. To make these radios work effectively requires extensive hands-on human involvement during the assembly, testing and tuning phases of manufacturing. As labor costs soar while component prices stabilize, the cost of manufacturing analog radios based on traditional solutions will continue to rise over time. The system design and board layout for a single radio product are complicated by the high number of components and resulting electromagnetic interference (EMI) among these components. For multiple radio models with different frequency band limits, designers must create multiple designs since the analog IC cannot support a universal frequency band. Furthermore, radios based on traditional analog IC solutions cannot pass the European emissions compliance test (EN55020), limiting the opportunity to sell these radios in the European market. DSP ICs for Click-Wheel ATDD Radio Designs Modified-wheel ATDD radios, known as click-wheel radios, have emerged in today s radio market. The tuning wheel for these radios can be tuned like a traditional wheel but with unlimited turns. Unlike protruding wheels used in traditional wheel-tuned designs, click-wheels are recessed or embedded, similar to what is used in many portable media players. The radio receiver IC in click-wheel designs down-converts the RF frequency to IF frequency, then processes the signal in the digital domain through an analog-to-digital converter (ADC), and then finally restores the signal for speaker output using a digital-to-analog converter (DAC). The clickwheel design eliminates external BOM components such as IF filters and transformers required by traditional solutions, resulting in lower cost and superior performance. Today s radio ICs include both digital inputs and digital outputs. The digital input for the userselected frequency is converted through digital processing to digital output for the LCD driver. To work with a frequency tuning wheel, an MCU encoder is used at the front end to interpret the wheel tuning to a digital signal and feed the signal to the radio receiver. Then the receiver handles the digital processing and outputs the frequency to an LCD/LED driver for displaying on the screen. See Figure 3 for an example of a simplified click-wheel radio system schematic. Silicon Laboratories, Inc. Rev 1.0 3

4 Figure 3. Simplified Schematic for a Click-Wheel Radio Currently, the most popular radio ICs used in this type of click-wheel system is Silicon Labs Si473x multi-band receiver. The Si473x device s digital low-if architecture handles all of the audio signal processing at the digital level, providing superior RF performance. The Si473x has powerful functions, supports advanced features such as auto-scan, stores favorite station settings, and even displays the signal strength or signal-to-noise ratio (SNR) on the LCD screen. However, there are two design considerations in using this solution to build an ATDD radio: Since the ICs are designed for digital-tuned radios, an additional MCU encoder is required at the front end to work as an ADC, which actually increases the BOM. The encoder wheel is different from the traditional tuning wheel. A traditional wheel uses a potentiometer or variable capacitor, which has minimum and maximum physical stops, but the encoder wheel has no stops. This is less intuitive as a frequency band does have minimum and maximum limits. Given these two issues, radio manufacturers are still looking for new ways to design ATDD radios that offer superior performance without higher cost. Multi-band Radio IC with Optimized Features for the ATDD Market Silicon Labs recently introduced the Si484x AM/FM/SW receiver family to meet this ATDD radio market need. The Si484x ICs will help revolutionize the ATDD market by enabling exceptional performance, higher integration, superior tuning accuracy and modern, streamlined manufacturing techniques. The Si484x family is based on Silicon Labs proven, patented digital low-if architecture, which provides a full radio from a very simple antenna interface to L/R analog audio out. The Si484x ICs feature a built-in ADC that can directly interpret the analog tuning of the wheel to frequency changes while providing I 2 C-compatible 2-wire control to a combined MCU and LED/LCD driver. Unlike a traditional analog IC that cannot output the tuned frequency, the Si484x not only outputs the actual tuned frequency, it also supports indicators for valid stations and mono/stereo signals to display on the LCD/LED. The Si484x also provides digital volume control, soft mute and bass/treble audio enhancements. Additionally, the Si484x provides advanced audio conditioning for all signal environments, removing pops, clicks and loud static in variable signal conditions. Traditional solutions do not provide any of these key features. Silicon Laboratories, Inc. Rev 1.0 4

5 Figure 4. Si484x Multiband Radio IC Architecture New ATDD radios using solutions such as the Si484x family bring several important benefits of modern digital radios to this traditional analog market. Let s examine each of these benefits. Reduced BOM and Labor Costs Compared to traditional analog radio ICs, the highly integrated Si484x solution requires minimal external components and reduces BOM cost by more than 70 percent. In contrast, traditional solutions require several steps of manual tuning and testing, which increases labor cost and manufacturing time. The Si484x requires no manual tuning, enabling reduced manufacturing cost and faster time to market. In addition, the Si484x requires only a single test of RF to analog. Compared to click-wheel radio solutions, the Si484x eliminates the need for the encoder while providing the advanced features comparable to click-wheel radio designs. Superior RF Performance The Si484x family s advanced digital architecture enables superior RF performance compared to traditional solutions. Table 1 summarizes the Si484x family s superior performance using realworld radiated test data. For instance, the selectivity parameter of a radio determines how well it can detect a target radio station in the presence of many other radio stations, a common scenario in crowded cities. Traditional analog radios use a wide channel filter with 800 khz to 1 MHz bandwidth for FM band, which means radio stations within this bandwidth will interfere with one another and degrade the sound quality of the desired station. The Si484x devices have a superior digital selectivity filter with narrow bandwidth that enables flawless reception of the targeted station even in the presence of 50 db stronger interfering radio stations as close as 200 khz away. Table 1. Real-World RF Test Comparison between Si484x and Traditional Radios (dbuv) Si484x Traditional Radio 1 Traditional Radio 2 FM FM SNR AM AMSNR Silicon Laboratories, Inc. Rev 1.0 5

6 Figure 5 presents the Si484x family s superior selectivity compared to traditional solutions. The selectivity value shown in Figure 5 is the minimum amount of delta required for blockers to interfere with the reception of the desired signal. 200KHz away Figure 5. Si484x Selectivity Compared to Traditional Analog ICs Accurate Tuned Frequency Display Traditional ATDD solutions use frequency counter ICs to approximate the tuned frequency of legacy analog ICs. This can frequently lead to the actual tuned frequency being significantly different than the displayed frequency, resulting in a poor user experience. The Si484x tuning experience is precise, leaving no room for error when the Si484x provides its tuned frequency to the display driver via the I 2 C interface. The user hears precisely the station shown on the LCD/LED display. Easy to Design and Build Digital-based solutions are more highly integrated than traditional analog solutions, and therefore generally easier to design onto the printed circuit board. For example, the Si484x family is based on a proven Silicon Labs architecture and builds on Silicon Labs leading market share in other broadcast receiver markets. The architecture s very small front-end matching network, voltage supply isolation and functional configuration are implemented on a single-layer board, resulting in a simple system BOM that is cost-effective to manage and manufacture. There are no manuallytuned parts in the Si484x BOM, allowing manufacturers to eliminate labor involved with manual placement, testing and tweaking from their assembly lines. Radio makers thus benefit from faster time to market, streamlined manufacturing processes and lower labor costs, and they can also differentiate their radio products based on advanced features. Silicon Laboratories, Inc. Rev 1.0 6

7 Summary Fierce global competition in the radio market challenges radio system designers to consider all factors in their wheel-tuned, digital-display radio designs including RF performance, BOM cost and manufacturing flow. Table 2 summarizes key ATDD radio design considerations and how the various solutions compare. After examining traditional analog solutions, click-wheel solutions and newer ATDD solutions, Silicon Labs next-generation Si484x family stands out as an optimal multi-band receiver solution for ATDD radios in all categories. By using Si484x family-based solutions, radio manufacturers can significantly reduce BOM and manufacturing costs while designing differentiated radio products with unique features that will gain new share in today s competitive market. Table 2. Summary Table of All Three Solutions Design considerations Wheel tuning Digital display Radio performance BOM and labor Si484x solution Precise with intuitive min and max limits Accurate display of frequency, band, stereo, tone control, etc. Industry-leading performance with superior selectivity, sensitivity, etc, good reception under all signal conditions Low BOM count, standard manufacturing flow, reduced labor cost and rework rate Click-wheel radio solution with Si473x Precise but without required min and max limits Accurate display of frequency, band, stereo, tone control, etc. Industry-leading performance with superior selectivity, sensitivity, etc, good reception under all signal conditions Encoder required for analog tuning feel Traditional analog solution Imprecise with intuitive max and min limits Inaccuracies in displayed frequency; limited information on display for stereo, etc. Inferior performance to digitally-based solutions results in poor weak signal reception and blocker immunity. High BOM count, high labor cost, high rework rate Silicon Laboratories, Inc. Rev 1.0 7