Electronic – Why the prevelance of mechanical oscillators in electronic circuits

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The clock sources in modern electronics seem to come invariably from quartz and MEMS oscillators, both of which generate vibrations mechanically. The amplitude and frequency of the vibration are orders of magnitudes different from the everday mechanical vibrations I observe in, say, musical instruments. Nevertheless, it's surprising to me that we don't get clock sources in the electromagnetic domain directly, say using capacitive or inductive elements.

I know that inductors especially are hard to manufacture without parasitic losses. But I would expect mechanical oscillators to be non-ideal as well.

You could use the propagation delay of electricity, but then it would be hard to make a small oscillator that operates at slow frequencies.

Is it really true we can make microscopic vibrating devices more ideally than we can make electrical oscillating components?

Best Answer

Because the mechanical devices are much more stable than their electric counterparts. Let's compare a crystal oscillator to an LC oscillator:

Crystal:

  • Has a very high Q. According to wikipedia, a crystal oscillator has a typical Q of 10,000-1,000,000.
  • Stable with temperature. Many crystals are specified at <50ppm over their temperature range, and temperature compensated or controlled crystals are also available, down to ~1ppm with temperature
  • Manufactured to a tight tolerance. Cheap crystals are usually specified to ~25ppm, but tighter tolerances are available

LC or RC:

  • Not available as an integrated device, so must be assembled from off the shelf components (unless integrated into a mcu or similar)
  • Low Q, it's difficult to make an inductor with a Q higher than a few hundred
  • Temperature sensitive - making temperature stable inductors is difficult

  • Voltage sensitive - the threshold voltage and charging voltage in the feedback circuit is usually voltage dependent.

    However, that doesn't mean that electric oscillators are never used, just that they're not used where great precision is needed. They do however have some advantages over crystal oscillators:

  • They can be easily integrated into another IC. Many microcontrollers now come with an integrated oscillator

  • They (sometimes) use less power. Often times a microcontroller will include a low power oscillator to run the watchdog timer, which uses less power than a high speed (MHz) crystal, and sometimes less power than a low speed (32.768kHz) crystal.
  • Since they can be integrated onto an IC, they can be used in places where a crystal would be far too large
  • They can be tuned fairly easily. A crystal can only really be shifted a few kHz off its calibrated frequency, but by adjusting the capacitance of the LC circuit (like with a varactor diode), the frequency can be adjusted over a fairly wide range. This means that LC oscillators can be used in circuits like PLLs or VCOs, possibly even locked to a crystal reference.

Non-mechanical oscillators are used in many devices, just not in those where accurate timing is required.

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