Quartz crystal oscillators are often used in microprocessor and microcontroller applications to generate the clock pulses upon which all subsequent timing is based. Specialty oscillators, such as TCXOs and VCXOs are found in communication applications as the local oscillator of synthesizers, mixers and phase lock loops. In all cases, crystal oscillator specs use terms that may be unfamiliar to many design engineers, but are nevertheless essential to selecting the most appropriate oscillator model. This article will "demystify" some of those useful terms.
A good place to start when selecting a crystal oscillator is the type of load the oscillator is intended to drive in the application. Most oscillator designs are optimized to drive a particular type of load, whether they are TTL, CMOS, ECL or analog loads. For CMOS and HCMOS devices, the clock-input pin typically has an input capacitance specification, and this is the load that the oscillator must drive.
The most common digital applications call for 5 Volt HCMOS clock inputs. If the intention is to drive several loads in parallel, the capacitance values are added together. A good rule-of-thumb for an HCMOS load is 5pF per gate.
Typical crystal oscillator frequencies range between 1.8 and 70 MHz, with frequencies outside this range available for certain models. Stability is how far an oscillator will drift from the desired frequency over a specified temperature range. Stability measures, expressed in parts-per-million (ppm), are inclusive of room temperature tolerance, temperature, load change, supply voltage change, aging, shock and vibration.
The most common stability is ±100ppm from 0° to 70 °C (for example, a 10MHz oscillator with a ±100ppm stability will have a frequency variation of ± 1000 Hz). Tighter stabilities (± 50ppm and ± 25ppm) for crystal oscillators are also available, depending on the desired frequency and model.
The output form of an oscillator intended for digital applications is a trapezoidal wave. The wave shape is specified with the following terms:
- Rise time, from the logic-zero threshold (Vol) to the logic-one threshold (Voh);
- Symmetry, the ratio of time at logic-one relative to the total period; and
- Fall time, from Voh to Vol.
The rising and falling edges are fastest with very light loads. This is an important factor to consider when dealing with radiated emissions.
The output of some oscillator models can be tri-stated, which means the output can be turned on or off using pin one for control. When disabled, the output goes to a high impedance. This allows, for example, automatic test equipment to use its internal clock source without loading down the signal. For tri-state models, the output is asynchronous with the pulse on the control pin, because the oscillator is still running and only the output buffer is turned off.
The package holds the oscillator in its final form. The 14-pin, metal DIP remains the most popular model. Surface mount oscillators, however, such as the miniature ceramic (5 x 7mm), are expected to overtake through-hole styles in the near future.
These styles, along with the half-size, 8-pin DIP are shown in Figure 1. There are a variety of other SMD crystal oscillator packages to choose from as well.
A VCXO is a voltage-controlled crystal oscillator. Its input pin allows the output frequency to be shifted in response to a voltage. The range over which it can be tuned is called the pullability. VCXOs have a much smaller pullability than VCOs. The speed at which the frequency can be varied over this range is the modulation bandwidth. Ideally, a plot of pullability versus control voltage is perfectly linear. In practical application, there is always some variation from this ideal. A typical specification might indicate a linearity of ± 10%.
TCXOs are temperature compensated crystal oscillators. A typical stability for these oscillators for cellular phone applications would be ± 2.5ppm over -30 to +75°C. A typical output for TCXOs would be 1.0 Vp-p clipped sine wave into a parallel load of 10 KW and 15pF. Some TCXO modules are also available with a voltage control feature and/or mechanical frequency adjustments. These allow fine-tuning of the frequency to null out the effects of loading, aging, etc.
Specifying for Portable Applications
Low power oscillators are optimized for a supply voltage of 3.0V and are often used in portable applications, such as lap top computers and their plug-in peripherals. These oscillators also feature a standby function whereby the oscillator can be "put to sleep," drawing as little as 10mA when not in use and 2mA max at full load. Emerging logic technologies may create a future demand for oscillators with even lower supply voltages. The input current for crystal oscillators tends to go up as a function of frequency and load.
We have discussed only the most basic terms used in crystal oscillator specifications.
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