[SOLVED] Questions about oscillator

[Sunny]

Dear all,

I would like to ask what is the difference between crystall and crystall oscillator.

and also what is the difference between 2 pins crsytal oscillator and 4 pins crystal oscillator?

Thnx very much

Sunny CHU

 

[barrybear]

Hi A crystal is able to produce very accurate frequencys by the way it is manufactured duoto its own resonance.
The 2 pin is just a single crystal you have to provide the circuit that makes it osilate IE two resistors and a cap. IE found on pics like 16c84. The 4 pin hase its own circuit built in two the package
Barrybear

 

[james]

Even if some model of four pins crystal built the oscillator and (some time) a frequency divider, often they are simple cristals where two pins of them are simply connected to internal case. Please see the attached datasheet.

The crystal, is a thin slice of natural or sinthetic quartz cutted on particulars axes and a free to oscillates mechanically. It follow the phisical fenomena, usually called piezoelectricity. If a force compression is exercised accross a couple of sides, electricals charges will be on the opposite sides of the crystal. Vicecersa, if electrical charges have applied, a mechanical force will be exercided at natural frequency that is directly proportional to its phisical characteristics (cutting axes, thickness etc). Temperature and atmospheric pressure may have a little influence.

The elctrical equivalent circuit of a crystal is a resistor Rs, inductor L, and capacitor Cs. This oscillating circuit has, near to natural frequency, a great variation of its impedance Z, with a very good efficiency factor Q.

If inserting this component into a positive feedback of an amplifier (linear or not) it will start to oscillate with the same frequency of the natural frequency of the crystal quartz.

Frequency stability is some hundred ppm as well as its long term stability.

james




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[Pim]

Another remark re crystals; They can be somewhat tuned, either by applying capacitance/inductance in series or capacitance in parallel. The tuning range is however very very limited. What method to prefer for finetuning depends on whether one wants to tune the frequency up or down.

When byuing crystals they are usually specified (at least in fairly comprehensive datasheets) as having a certain oscillating frequency with a capacitive loading of a certain value, also whether the capacitance should be in series or in parallel. Should one have available a crystal that has a specified oscillating frequency with a certain parallell capacitive loading, the oscillating frequency will be sligthly off if one instead skip the parallell capacitive loading and instead connect the capacitance in series.

Crystals for very high frequencies are operated in an overtone-mode, meaning that e.g. a crystal stated as being a 90 MHz crystal in essence is a 30 MHz crystal but being manufactured so that the internal mechanical resonance is optimized to be strong at a certain odd harmonic of the fundamental frequency, i.e. in this case the third harmonic.

I assume that with some efforts put into the oscillator design the above mentioned crystal could also being brought to use in an oscillator oscillating at the 5:th and higher odd harmonics as well i.e. 150 MHz, 210 MHz etc.

The crystals I have seen only use two connections to the crystal itself, the rest, if any, are for grounding of the case. Take care though when soldering crystals; they are fragile to heat and mechanical stress of the connections. Take extremely care regarding heating the metal enclosure of the crystal if soldering on it directly.

There also exists ceramic resonators that could often very successfully be used when the frequency precision is not of the utmost importance. Such are often used in connection with the clock-circuitry of microprocessors and the like.

/Pim

 

[Pim]

After re-reading the first poster's post I realized that I maybe did not provided any very helpful reply to the intended question.
Sorry ( , but here what I might add to that: An oscillator needs at least ground, power supply and output connection to be meaningful. I.e. three connections at least, of which the result is that there simply could not exist anything like two pin crystal oscillators. If "it" has two pins then "it" is simply a crystal, not an oscillator.

If "it" has four pins then the fourth pin could be used at the manufacturers will, either as another ground connection, an inverted output, an input of any kind to control some characteristic of the oscillator or whatever. The above applies only if the "thing" indeed is a crystal oscillator, not a simple crystal whose metal can is grounded by the extra pins.

In short: a crystal is a very exact frequency-determining component, a crystal oscillator is an oscillator wherein that crystal is used to very precise determine the oscillator's oscillating frequency.

/Pim