Pay attention to some parameters, the design engineer can choose the oscillator suitable for the application.
Today, countless electronic circuits and applications require precise timing or clock reference signals. Crystal clock oscillators are ideal for many applications in this area.
The clock oscillator is available in a variety of packages and is characterized by a wide range of electrical performance specifications. There are several different types: voltage controlled crystal oscillator (VCXO), temperature compensated crystal oscillator (TCXO), oven crystal oscillator (OCXO), and digitally compensated crystal oscillator (DCXO). Each type has its own unique properties.
----frequency stability considerations ----
One of the main characteristics of a crystal oscillator is stability over the operating temperature, which is an important factor in determining the price of the oscillator. The higher the stability or the wider the temperature range, the higher the price of the device.
The design engineer must carefully determine the actual needs for a particular application and then specify the stability of the oscillator. Too high a target means more money.
For applications with a frequency stability requirement of ±20ppm or higher, a normal uncompensated crystal oscillator can be used. For stability from ±1 to ±20 ppm, TCXO should be considered. For stability below ±1 ppm, OCXO or DCXO should be considered.
----Output----
Other parameters that must be considered are output type, phase noise, jitter, voltage stability, load stability, power consumption, package form, shock and vibration, and electromagnetic interference (EMI). The crystal oscillator is HCMOS/TTL compatible, ACMOS compatible, ECL and sine wave output. Each output type has its unique waveform characteristics and uses. Attention should be paid to the requirements of tri-state or complementary output. Symmetry, rise and fall times, and logic levels are also specified for some applications. Many DSP
And communication chipsets often require strict symmetry (45% to 55%) and fast rise and fall times (less than 5ns).
----Phase noise and jitter----
The phase noise obtained in the frequency domain measurement is a true measure of short-term stability. It measures up to 1 Hz of the center frequency and typically measures 1 MHz.
The phase noise of the oscillator is improved at frequencies away from the center frequency. TCXO and OCXO oscillators, as well as other crystal oscillators using fundamental or harmonic methods, have the best phase noise performance. An oscillator that uses a phase-locked loop synthesizer to produce an output frequency generally exhibits poor phase noise performance than an oscillator that uses a non-phase-locked loop technique.
The jitter is related to phase noise, but it is measured in the time domain. Jitter expressed in picoseconds can be measured with an effective value or a peak-peak value. Many applications, such as communication networks, wireless data transmission, ATM and SONET requirements, must meet stringent saturation specifications. It is important to pay close attention to the jitter and phase noise characteristics of the oscillators used in these systems.
----Power and load impact -----
The frequency stability of the oscillator is also affected by oscillator supply voltage variations and oscillator load variations. Proper selection of the oscillator minimizes these effects. The designer should verify the performance of the oscillator under the recommended supply voltage tolerances and load. It is not expected that an oscillator that can only be rated for 15pF will perform well when driving 50pF. Oscillators operating above the recommended supply voltage will also exhibit poor waveform and stability.
For devices that require battery power, power must be considered. Introducing a 3.3V product is bound to develop an oscillator that operates at 3.3V. The lower voltage allows the product to operate at low power. Most commercially available surface mount oscillators today operate at 3.3V. Many perforated oscillators using conventional 5V devices are being redesigned to operate at 3.3V.
----Package----
Similar to other electronic components, clock oscillators are also available in smaller and smaller packages. For example, M-tron's M3L/M5L series of surface mount oscillators are now available in a 3.2 x 5.0 x 1.0 mm package. In general, smaller devices are more expensive than larger surface mount or perforated package devices. Small packages often have a trade-off between performance, output selection, and frequency selection.
----working environment----
The environment in which the oscillator is actually used needs to be carefully considered. For example, high vibration or shock levels can cause problems for the oscillator.
In addition to possible physical damage, vibration or shock can cause erroneous actions at certain frequencies. These externally induced disturbances can cause frequency jitter, increased noise footprint, and intermittent oscillator failure. EMI is another priority for applications that require special EMI compatibility. In addition to using the appropriate PC motherboard layout techniques, it is important to choose a clock oscillator that provides the least amount of radiation. In general, oscillators with slower rise/fall times are better.
EMI characteristics.
For frequencies below 70MHz, an HCMOS oscillator is recommended. For higher frequencies, an ECL type oscillator can be used. ECL type oscillators usually have the best total noise rejection, and even at lower frequencies of 10 to 100 MHz, the ECL type is slightly better than other types of oscillators.
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