Oscillator

An ideal oscillator is an active device that produces a single frequency output for a DC input. Moreover, the signal exists as a purely sinusoidal waveform with a fixed, with respect to time, output power and frequency. However, in practice the output signal is not fixed or confined to a single frequency. If one views the spectrum of an oscillator, it appears as a continuum of frequencies with decreasing amplitude distributed about the desired frequency. This "sideband" noise is referred to as phase noise. In a typical application one specifies the phase noise from the desired frequency in dBc per Hz. For example, at 1 KHz offset the phase noise must be greater than 90 dBc per Hz. The "cleaner" an oscillator the lower the phase noise.

Another deviation from the ideal oscillator occurs in the frequency stability with respect to time. This is due to aging and thermal variation of the resonator and surrounding circuitry. Thus, one usually specifies an aging limit and temperature stability factor. Frequency stability, over a broad range of temperature, is usually accomplished by employing a biasing network of thermistors that compensate the oscillator circuit with a quadratic least-squares fit. This class of oscillator is termed temperature compensated crystal oscillator or TCXO. However, the best stability obtainable is usually on the order of 1 ppm. Higher stability, on the order of 1 ppb, is achievable using ovenized crystal oscillators, or OCXO’s. These oscillators have a built-in oven that keeps the crystal at a constant temperature typically equal to the turning point of the crystal.

Most engineering applications require a frequency source that is electronically tunable. This is a necessity for any modulation scheme used in information transfer. Tuning is typically accomplished by the use a varactor diode in the resonant circuit. The diode capacitance is a function of the applied voltage. Thus, the applied voltage controls the output frequency of the oscillator. The prefix for such a device is a VC, or voltage controlled. Therefore for example a voltage controlled crystal oscillator is called a VCXO. When the voltage tuning and temperature compensating features are combined it is called a VCTCXO or TCVCXO. If an inductor or microstrip is used as the resonator the nomenclature VCO is used. Similarly a dielectric oscillator is termed a DRO. In the case of no temperature compensation or voltage control the device is called a clock oscillator. These are primarily used as fixed reference sources for digital circuits, where stability is not as critical.

Filtronetics, Inc. offers a wide variety of crystal, dielectric, LC, and microstrip oscillators. Frequency ranges, stability, and phase noise differ for each technology. LC oscillators are great for low stability, broad tuning, and inexpensive applications. The frequency range is confined to below 3 GHz with noise floors near 120 dBc per Hz. Microstrip is similar to LC but spans from 1 GHz and up with much better stability. DRO’s have a similar frequency range as microstrip but offer better noise characteristics. However, tuning range is much less. Crystal oscillators or XO’s offer the greatest stability but suffer from the lowest tuning range. Phase noise is typically the best for this group of oscillators, particularly for the OCXO’s. The frequency range is limited to below 600 MHz.

Filtronetics Inc can supply oscillators in through hole, surface mount (SMD), and connector packages.