The first time frequency control devices were used in an automobile was in the 1930s when car radios from the Galvin Manufacturing Corp. were installed. Galvin Manufacturing was later renamed Motorola (the prefix “motor” was chosen because of the company’s early involvement in the automotive industry) and later the automotive products division was sold to Continental. Then in 1952 Blaupunkt became the first company to offer FM car radios. Twenty years later analogue-based ECUs (Engine Control Units) utilized frequency control devices as the system clock while digital ECUs became a reality around 1986. Since then the utilization of frequency control devices in the transportation industry has exploded. Today there could be over 30 frequency control devices in every new automobile (see Figure 1).
Communications systems for transportation applications have come a long way since those modest beginnings. Each new application seems to have more critical specifications than the last one. A tire pressure monitoring system (TPMS), which includes four RF transmitters and a receiver, provides the safety that would be seriously compromised by false reads. Clearly in this instance, failure cannot be tolerated. With the growth of drive-by-wire applications, frequency controls are also being used as the gateway for communication not only in the wireless devices, but in wired systems as well.
Current examples of in-car frequency control applications include safety and drive train applications, infotainment, security and convenience applications. Common safety and drive train systems employing frequency control devices include: ESC (Electronic Stability Control), TPMS (Tire Pressure Monitor Systems), collision avoidance and smart cruise control radar, ABS (Anti-locking Braking Systems), airbag, and ECU. Infotainment applications include navigation systems, GPS (Global Positioning Systems), satellite radio, TV media centers and hands-free mobile phones. Security and convenience applications include components and subsystems such as RKE (Remote Keyless Entry), immobilizers, DVR (Digital Video Recorder), rearview camera and parking assistance.
Communications outside the vehicle is also playing a big role in improving current and future traffic management systems, helping to ease automobile congestion. Smart vehicles will be much more aware of their environment, alerting drivers about their immediate surroundings (vehicle proximity warning) as well as major congestion and alternate routes. Ideally, these smart vehicles will reduce the time that cars spend stuck in traffic jams and possibly reduce their overall carbon emissions and contribution to greenhouse gases. Current examples of Smart Vehicle Frequency Control Applications include: GPS/navigation systems, VICS (Vehicle Information and Communication System), commercial vehicle support systems, AHS (Automated Highway Systems), IVC (Inter-Vehicle Communication), ETC (Electronic Toll Collection), ASV (Advanced Safety Vehicle), highway video monitoring, automated driving infrastructure, emergency vehicle support systems and road-to-vehicle communication infrastructures.
Considerations When Selecting Frequency Control Devices
The following are leading factors in determining the requirements for a frequency control device in automotive applications. Given the safety liabilities and costs involved in a warranty claim, reliability is the number one concern in transportation applications. If the microprocessor (MPU) is the brain of the automotive application, the crystal would be the heart. The clock keeps the system in check allowing the MPU to perform computations with a stable reference signal. Therefore, it is often critical to include the frequency control device manufacturer in the very early stages of the project. Most frequency device suppliers have many years of experience to guide system integrators. Some can even offer “oscillation margin analysis” where the application is brought to the lab for testing and design tweaks are implemented to help ensure optimal performance. Quartz-based frequency control components offer many advantages critical to demanding automotive applications including stability over temperature range and excellent aging hysteresis (stable repeatable performance over time). Quartz is also hard and can be processed consistently from lot to lot, yet it is not brittle, which gives it excellent shock and vibration resistance.
Environmental conditions must be taken into consideration when specifying the proper frequency control devices for the application. Automotive environments are infamous for their grueling requirements, often looking for –40° to +125°C operating temperature ranges while operating during high shock and vibration in various humidity conditions. Needless to say, special consideration must be taken in the design and manufacturing of devices that can handle these rigorous elements. The products must be beefed up by using properly engineered materials and assembly techniques to meet these demands. This attention to detail can have an impact on the cost of the component. Product specification should be driven by the application of the product. For example, a device used under the hood would require tougher environmental specifications than an infotainment application placed inside of the vehicle.
Safety applications require extra special treatment to avoid situations such as a failure in an airbag deployment or an improper reading from a TPMS leading to an accident. For safety applications the frequency control devices must be designed to higher than normal standards, once again utilizing special materials and production techniques. These components are run in a special batch process marking them and separating them from other standard components. Components for safety applications must receive a 100 percent inspection for physical, electrical and performance characteristics, which ensures their conformance.
Proper package selection is key to ensuring the use of mainstream components that will provide a reliable supply. The largest user of frequency control devices is the mobile phone industry. They drive the volume and package size. Today’s phones keep getting thinner and thinner while delivering more features than ever. This trend will continue to force smaller and smaller components to be developed. What might be a popular size today will not necessarily be mainstream in a few years. The manufacturers of frequency control components continue to make investments in the production tooling of these smaller packages. So as the volume demand for the smaller packages raises the price point drops, and as the volume demand decreases for the larger packages pricing will go up and lead to an end-of life product. Due to their small size and excellent environmental characteristics the 3.2 x 2.5 mm quartz crystal has become a very popular frequency control device in TPMS applications (see Figure 2).
Selecting a crystal or oscillator for the application must also be considered. Oscillators such as XOs (crystal oscillators), TCXOs (temperature-compensated crystal oscillators), VCXOs (voltage-controlled crystal oscillators) and OCXOs (oven-controlled crystal oscillators) deliver an all-in-one device oscillation signal. They provide users with an easy solution to fulfill their clock system requirements. The other method of fulfilling clock system requirements is to utilize a crystal and build your own oscillation circuit. Either method can be effective depending on the application. In higher volume with lower stability requirements developing a PLL could provide an overall unit cost savings when compared to utilizing an oscillator. For lower volume applications, oscillators can be cost effective when calculating design and development time. Oscillators also offer the advantage of being more stable because of their ability to adjust to changing conditions.
The Benefits of Quartz Crystal
A crystal wafer mechanically vibrates at several modes, as shown in Table 1. To pick up desired vibration energy effectively, the system supporting the crystal wafer is very important. An example of the typical internal construction for a thickness-shear mode crystal unit at the minimum displace point of mechanical vibration on the wafer is shown in Figure 3a. The holder is hermetically sealed to prevent deterioration of the crystal unit’s performance, as shown in Figure 3b.
Because crystal units are widely used for their stable oscillation frequency, superior temperature characteristics are required. However, as with ordinary materials, a crystal flake cut as a quartz unit is influenced by temperature change, causing its oscillation frequency to change. The level of change in the oscillation frequency (frequency-temperature characteristics) varies depending on the cutting azimuth.
Cutting angles differ depending upon the applications (oscillation frequencies and electrical characteristics). Table 1 shows vibration modes, frequency ranges and capacity ratios (typical values).
Taking the most popular AT-cut crystal wafer, for example, it operates in a plane, which makes an angle of 35°15' to the Z-axis and the wafer thickness is approximately 0.06 mm in the case of 28 MHz fundamental-wave thickness shear vibration.
Figure 4 shows three different temperature characteristics for different cutting angles. Curve 2 provides the smallest rate of frequency change against temperature change near normal temperatures; therefore, crystal units represented by this curve have excellent characteristics suited for most usual applications. On the other hand, over a wider temperature range of –55° to + 105°C, curve 1 shows better characteristics. It is necessary to determine the most appropriate temperature characteristics taking into consideration applications and required operating temperature ranges. Cutting angle allowance is determined by operating temperature range and allowable frequency tolerance.
A quartz crystal unit’s high Q and high stiffness (small C1) make it the primary frequency and frequency-stability determining element in a crystal oscillator. The Q values of crystal units are much higher than those attainable with other circuit elements. In general-purpose crystal units, Qs are generally in the range of 104 to 106. A high stability 5 MHz crystal unit’s Q is typically in the range of two to three million. The intrinsic Q, limited by internal losses in the crystal, has been determined experimentally to be inversely proportional to frequency (that is, the Qf product is a constant for a given resonator type). For AT- and SC-cut resonators, the maximum Qf = 16 million when f is in MHz. An oscillator built from a quartz crystal resonator has an advantage over one designed with a tank circuit built from discrete Rs, Cs and Ls in that the crystal is far stiffer and has a far higher Q than can be achieved using normal discrete components.
The use of frequency control devices in automotive applications is just beginning to hit its stride. Looking back at car radios and early ECUs, no one could have imagined today’s applications. These same frequency control devices will ensure that GM’s OnStar system will be able to give authorities the location of a stolen vehicle and slow the vehicle before shutting the engine down. This system incorporates external communication with internal communication. Infotainment options are available on all vehicles, with some of these components found as standard equipment. Soon you will be able to download movies via satellite selecting programming that will keep passengers entertained. Concepts to enhance safety and ease road traffic such as Advanced Cruise-assist Highway Systems (AHS) and Advanced Safety Vehicle (ASV) seem futuristic now, but engineers are working diligently on these applications. These programs look to develop technology that will lead to safer driving, aiming to reduce accidents, enhance transport efficiency, improve environmental conditions, and reduce burdens on drivers by enhancing their convenience and comfort. Over the past decade, research and development has been promoted jointly with the AHS Research Association, formed by the 21 enterprises possessing leading-edge technologies. Soon these systems will be as common as air bags. The future looms large for increased use of crystals and oscillators in the entire transportation industry.