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.
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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:
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•
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.
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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
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the wheel to frequency changes while providing I 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.
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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)
FM
FM SNR
AM
AMSNR
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Si484x
28
54
82
54
Traditional Radio 1
60
43
108
36
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Traditional Radio 2
33
52
93
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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
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the display driver via the I 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.
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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
Si484x solution
Digital display
Accurate display of
frequency, band, stereo,
tone control, etc.
Radio
performance
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
BOM and labor
Precise with intuitive
min and max limits
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Click-wheel radio
solution with Si473x
Precise but without
required min and max
limits
Accurate display of
frequency, band, stereo,
tone control, etc.
Traditional analog
solution
Imprecise with intuitive
max and min limits
Industry-leading
performance with
superior selectivity,
sensitivity, etc, good
reception under all signal
conditions
Encoder required for
analog tuning feel
Inferior performance
to digitally-based
solutions results in
poor weak signal
reception and blocker
immunity.
High BOM count, high
labor cost, high rework
rate
Rev 1.0
Inaccuracies in
displayed frequency;
limited information on
display for stereo, etc.
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