Determining the Proper Oscillator for Wireless Applications
By LeRoy Sutter, Product Manager, Fox Electronics

As WCDMA, WiMAX, Telecom, 3G, SONET and other telecom, wired and wireless systems have become more sophisticated, it is necessary for the components used in these systems to develop also. Crystal oscillators, which are used to ensure accurate time measurement and coordination, have undergone several significant technological advancements. This article provides a basic overview of crystal oscillators and discusses the newer design elements now available to assist designers in selecting the best, most cost-effective solution for the system being developed.

Oscillator Basics
Oscillators stabilize time-frequency generators, which in turn provide carrier and pilot signals for electronic communication and navigation systems. Oscillators also provide the reference signals for other special-purpose systems and the clock signals used by data processing equipment. The specific application dictates the required accuracy and stability of the oscillator’s output frequency, typically ranging from less than ±5 PPB for extremely accurate frequency control applications to ±1,000 PPM for simple microprocessor clocks.

Oscillators are made up of two basic components: an amplifier section and a feedback section containing a phase correction network. An oscillator will start to operate when the gain around the circuit becomes greater than unity and when the signal leaving the phase correction network is in phase with the signal applied to the amplifier network.

Because of changes in the phase of the signal, oscillator frequency is constantly changing and may occur in any or all of the sections of the oscillator. These changes must be corrected to sustain a specific frequency. This is done through the phase correction network, a cost-effective alternative to incorporating a means of phase correction into all of the various systems within an oscillator circuit. Although there are several ways to correct the phase, quartz crystals are preferred because their reactance changes so dramatically with changes in phase that all other components in the circuit may be considered constant and invariable.

Choosing the Right Oscillator
With demand constantly increasing for smaller and more cost-efficient components that consume less power, oscillator technology now provides new types and designs to meet these advanced requirements.

Clock Crystal Oscillators
The most basic oscillator is a Clock Crystal Oscillator (XO). In an XO, the crystal is the major component that determines the overall stability of the part. Ambient and aging performance are controlled by quartz material used for the crystal. The voltage and load stabilities are closely tied to the variation in capacitance instabilities in the oscillator application specific integrated circuit (ASIC).

XOs offer a frequency range of 1 MHz to 1.3 GHz. The designer must determine the package, output wave shape, input voltage and current requirements that best fit his needs. The most common operating temperature ranges for an XO are -40°C to + 85°C and-20°C to +70°C, with total stabilities of ± 100 PPM down to ± 20 PPM.

One of the newest types of XOs available is the XpressO, which features exceptionally low jitter and phase noise at an extremely affordable price per unit, making them ideal for SONET, SDH, ATM, WAN, WiFi, and WLAN applications. By combining a proprietary ASIC architecture with advanced quartz technologies, these XpressO oscillators are able to generate very clean output frequencies. Typically housed in an industry standard packaging of 7.5 mm x 5 mm x 1.4 mm or 5.15 mm x 3.35 mm x 1.4 mm, the 3.3-volt oscillators are available in HCMOS, LVPECL and LVDS waveform versions.

Oven Controlled Crystal Oscillators
Oven Controlled Crystal Oscillators (OCXOs) are often used as precision frequency standards and in navigation systems as timing control devices. Because they can be designed to meet stratum level accuracy requirements for telecommunications applications, these OCXOs are extremely useful in base stations, telecom switching, GPS and LAN/WAN applications, as well as in test and satellite equipment. OCXOs eliminate the effects of changes in the ambient temperature by maintaining the frequency-controlling element at a steady temperature. This is accomplished in one of two ways:

• Through a temperature-regulated chamber that houses both the crystal and the
  oscillator circuitry, or
• Using an oven that maintains the crystal at a constant temperature.

When used for frequency control in precision radio applications, OCXOs are superb; however, they are expensive, consume a lot of power, and may be quite large.

OCXOs are capable of producing frequency accuracies on the order of 10-10 to 10-8, and for some special applications, even tighter. For example, the Fox FTS501AH offers an overall accuracy of ±4.6 PPM, a frequency stability of ±250 PPB over a standard operating temperature range of 0°C to 70°C (32°F to 158°F) and a frequency range of 10 to 40 MHz. Although OCXOs provide the tightest frequency stability of all oscillator types, they are specialty items, not generally available from most frequency control product suppliers.

Temperature Controlled Crystal Oscillators
Because Temperature Controlled Crystal Oscillators (TCXOs) are available in a low voltage format (3.0 VDC), they are ideal for hand-held, battery powered communications devices. They are also used in cellular phones and radios, and in aerospace and two-way radio communications. TCXOs correct output frequency against the effects of temperature. Temperature compensation is usually effected by a temperature-sensing device that regulates a variable capacitor, with the result that at any temperature within the design range of the oscillator, the output frequency remains nearly constant.

TCXOs, available in through-hole or surface mount device (SMD) configurations, are capable of frequency accuracies on the order of 10-7 to 10-6; available frequencies range from 1 MHz to 60 MHz. These oscillators usually have tight frequency stabilities from ±1 PPM to ±2.5 PPM.
TCXOs that incorporate both voltage control and temperature compensation are known as VCTCXOs (Voltage Controlled, Temperature Compensated Oscillators). These oscillators are now available in packages as small as 3.2 mm x 2.5 mm x 1.2 mm with frequencies that range from 13 MHz to 26 MHz.

Voltage Controlled Crystal Oscillators
Voltage Controlled Crystal Oscillators (VCXOs) provide a means of controlling the output frequency over a narrow range, typically by using a varactor diode as a tuning capacitor. VCXOs are housed in an industry standard packaging of 7.5 mm x 5 mm x 1.4 mm or 5.15 mm x 3.35 mm x 1.4 mm. Offering 3.3 volts, these oscillators are available in HCMOS, LVPECL and LVDS waveform versions and have a frequency range of 1 MHz to 1.3 GHz. They are often used in PLL (Phase Lock Loop) applications and in RF applications when data needs to be transmitted.

In comparison with XOs, several more specification requirements must be considered when sourcing a VCXO for an application. The center frequency, input current, supply voltage, output wave form, package size and pin configuration are all typical items that the designer will need to specify. But ambient performance, voltage control/frequency control range, voltage stabilities, load stabilities and aging are all addressed differently with a VCXO as compared to a normal XO.

Absolute Pull Range (APR), an important term to understand, is the amount of frequency change above all other instabilities the VCXO may encounter. A specified voltage range is applied to the voltage control input port to achieve the frequency change versus voltage change. Inherent frequency instabilities in the VCXO, such as maximum temperature versus frequency change, load changes versus frequency change, voltage change versus frequency change and total change in frequency because of component aging, are determined for the product platform. Then the APR value is added to these instabilities.

By using APR as a specification in applications where the primary function of the VCXO is to track external frequency control sources and to phase lock the system or module to those external sources, it covers most of the customer needs for specifying voltage control versus frequency change.

There are some applications, such as transmitting devices that have voice or data components added to the output waveform of the VCXO, where modulation information is superimposed on the voltage control input port. In these applications, the designer may have to specify additional requirements about the voltage versus frequency transfer function. Requirements such as specific center voltage needs and upper and lower limits for frequency change versus voltage may need to be added in order for the designer to maintain input and output modulation symmetry.

Other specifications, such as linearity and modulation bandwidth, will also be important to the designer in voltage changes versus frequency change cases. Because today’s frequency control devices are sub-systems, the designer must understand how the control system will interact with the transfer characteristics on the VCXO. Typical modulation bandwidths fall into the audio range of less than 20 kHz, but there can be situations where higher bandwidths are needed, based on system locking requirements. In these cases, it becomes important for the designer to work closely with the supplier to determine the best solution to fit their application.

A Final Note
In this article, we have introduced the basic oscillator types used in telecommunications, wireless and other electronic applications. While a number of configurations and modifications are possible for each basic design, the rationale for selecting the precise oscillator type for a specific application can be based on the general attributes discussed here.

Fox Electronics
www.foxonline.com
TXTLINX.COM76
Email this article to a friend!