How to design a traditional radio

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.

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 DSP-based radio ICs, and conclude with an overview of new multi-band radio IC technology optimized for the ATTD market.


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:
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 analog-tuned, 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 click-wheel 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 user-selected 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.




Figure 3. Simplified schematic for a click-wheel radio.

Single-chip multi-band receiver ICs such as Silicon Labs Si473x device work well in click-wheel systems. The Si473x device’s digital low-IF architecture handles all audio signal processing at the digital level. The Si473x supports advanced features such as auto-scan, stores favorite station settings, and 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 radio 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.