Thursday, 7 April 2011

Oscillators for Communications Applications: Basic Operation and Available Types

Oscillators lie at the heart of many electronic and communications systems that depend on accurate time measurement and coordination. They are used to stabilize time-frequency generators, which in turn provide carrier and pilot signals for electronic communication and navigation systems. Oscillators also provide the clock signals used by data processing equipment, as well as the reference signals for other special-purpose systems. The required accuracy and stability of oscillator output frequency depend on the application, ranging from about ±1000 PPM in the case of simple microprocessor clocks, to less than ±5 PPM for applications requiring very precise frequency control. In this article, we explore basic oscillator operation and oscillator types used in communications applications.
Basic Oscillator Operation
Fundamentally, oscillators consist of two sections: an amplifier section, and a feedback section containing a phase correction network.
fx3142fig1.gif - 2665 Bytes
Figure 1.0 - Basic Oscillator Configuration
For an oscillator to start and operate, two conditions must be met: (1), the gain around the circuit must be greater than unity; and (2), the signal leaving the phase correction network must be in phase with the signal applied to the amplifier network.
When the oscillator circuit is first energized, the only "signal" within the circuit is electrical noise. A component of this noise, at a frequency that satisfies the phase requirement for oscillation, propagates through the amplifier and feedback sections with increasing amplitude. This process is repeated until the gain of the phase-corrected signal is sufficient for steady-state oscillation. The exact frequency of oscillation depends on the phase condition, i.e., the phase must be equal to 2npi.gif - 872 Byteswhere n=0,1,2,3.
The frequency of an oscillator changes from one moment to the next because of changes in the phase of the signal. Such changes may occur in any or all of the sections of the oscillator, and must be corrected to sustain a specific frequency. In theory, it is possible to incorporate some means of phase correction in all of the various systems within an oscillator circuit. Such an approach, however, would be costly and would increase the size of the oscillator package. The more common approach is to rely solely on the phase correction network. While there are several means by which the phase might be corrected, quartz crystals have proven superior. The reactance of a quartz crystal changes so dramatically with changes in phase that all other components in an oscillator circuit may be considered to be constant and invariable.

Types of Oscillators
The possibilities for oscillator type and design are nearly limitless; and in fact, crystal controlled oscillators are available in a multitude of types and sizes employing many different circuit designs. However, for the purposes of this article, crystal controlled oscillators can be grouped into four general categories:

Simple Packaged Crystal Oscillators (SPXOs) are commonly used as the clocking device for microprocessors. These are also common in automotive applications, where they are used to provide clock signals for the various computer controlled functions used in newer vehicles.
SPXOs are capable of providing frequency accuracy on the order of 10-5 to 10-4. They are generally available in standard frequencies from 1 MHz to 100 MHz, with frequency stability of ±100PPM. SPXOs do not incorporate any means to minimize the effects of temperature on the output frequency of the device. However, their low cost makes them an ideal choice for applications, including most computers, in which temperature variation is minimal or for which greater accuracy and stability are not required.
SPXOs are available with a wide range of output capabilities. They are available in both pin-through and SMD configurations; they range in size from 21mm x 14mm x 6mm to 5mm x 3.2mm x 1.5mm. Typically, pricing for SPXOs is in the range of $0.90 each, in quantities of 5000.
Voltage Controlled Crystal Oscillators (VCXOs) are often used in PLL (Phase Lock Loop) applications. They provide a means of controlling the output frequency over a narrow range, typically by using a varactor diode as a tuning capacitor. VCXOs come in packages sized 14mm x 10mm x 3mm and in through-hole and SMD configurations.
VCXOs offer frequency accuracies on the order of 10-6 to 10-5. Frequency range is generally from 1 MHz to 30 MHz. These are tight tolerance oscillators whose frequency stability is ±50 PPM and frequency pullability is ±100 PPM. VCXOs are available in HCMOS and TTL outputs, and their phase noise is usually -125 to -130 dBc/Hz. Pricing is typically around $4.00 each, in quantities of 5000.

Temperature Controlled Crystal Oscillators (TCXOs) are used in portable telephones, cellular phones, communications equipment, cellular radios, and in aerospace and two-way radio communications. Because they are available in low voltage (3.0 VDC format), TCXOs are ideal for use in hand held, battery powered communications devices. These oscillators correct the 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 are available with voltage control as well as temperature compensation; those incorporating both are known as VCTCXOs (Voltage Controlled Temperature Compensated Oscillator).
TCXOs are capable of frequency accuracies on the order of 10-7 to 10-6; available frequencies range from 1 MHz to 60 MHz. TCXOs usually have tight frequency stabilities, from ±1PPM to ±2.5PPM. They are available in through-hole or SMD configurations; package sizes range from 30mm x 30mm x 15mm to 11.4mm x 9.6mm x 3.9mm. Pricing for TCXOs usually starts at $4.00 each, in quantities of 5000.
Oven Controlled Crystal Oscillators (OCXOs) are often used in navigation systems as timing control devices, and also as precision frequency standards. These devices incorporate an oven that maintains the crystal at a constant temperature, or a temperature-regulated chamber that houses both the crystal and the oscillator circuitry. By maintaining the frequency-controlling element at a steady temperature, the effects of changes in the ambient temperature are eliminated. 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. They also provide the tightest frequency stability of all oscillator types. However, OCXOs are specialty items, not generally available from most frequency control product suppliers. Pricing for OCXOs is roughly $250 each, in quantities of 10.
In this article, we have introduced the four basic oscillator types used in communications and other electronic applications. While a number of configurations and modifications are possible for each of the basic designs, the rationale for selecting a specific oscillator type for a specific application is based on the general attributes discussed here.

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