when reading about the main differences between crystal resonators, oscillators and filters theres not much difference in operation, except that filters seem to have the crystal resonators and capacitors and resistors, And resonators seem to oscillate when the proper frequency is applied, And oscillators oscillate when a voltage is applied but also have amplifiers and feedbacks to the crystal. So my main question is why do you need a crystal filter with capacitors to properly filter when you can use a quartz resonator to filter to the specific frequency? Is it for the narrow bandwidth?
berkeman
Aug18-08, 08:04 PM
I haven't seen crystals or resonators use much in filters, but I suppose there are some special applications where they could be used. Most filters I'm familiar with are RLC passive filters, or RC active filters (with opamps or other amplifier stages).
Crystals have higher Qs than ceramic resonators, so they start up slower in oscillator circuits, but have better frequency accuracy. Ceramic resonators have lower Qs, so they start up faster in oscillator circuits, but have worse frequency accuracy and drift. Oscillator circuits using either crystals or ceramic resonators will have a gain feedback element, to create an instability at the resonant frequency of the crystal or resonator, where the combination oscillates stably at the desired frequency.
Does that help, or do you have follow-up questions?
Idea04
Aug19-08, 06:38 PM
so if I wanted to select a certain frequency from a signal, and the start up time didn't matter, I could use a simple crystal resonator set at the desired frequency as a filter. Without any other electronic components.
berkeman
Aug19-08, 06:55 PM
so if I wanted to select a certain frequency from a signal, and the start up time didn't matter, I could use a simple crystal resonator set at the desired frequency as a filter. Without any other electronic components.
Startup time applies to crystal oscillators, not to filters.
The low power crystals used in watches have very long startup times.
Idea04
Aug23-08, 07:03 PM
okay so on the page on monolithic filters it mentions a typical crystal filter that includes capcitors resistors and crystal oscillators. But doesn't mention the exact reason it needs those other electrical components. But I do know that typical crystal filters use several crystals. So are the other components there to basically direct the signal through the other crystals and to get rid of the unwanted frequencies. Also with the monolithic filter they usually are cut a certain way to get the desired frequency. So do they need external components to operate in an electronic circuit.
NoTime
Aug25-08, 09:04 AM
Chances are you will need some sort of external circuitry for just about any filter.
Just what any internal components might be doing depends on the specific device.
There are SAW (surface acoustic wave) filters also.
These are not cut to a frequency, but depend on antenna like structures deposited on the substrate surface.
http://www.oscilent.com/catalog/Category/saw_filter.htm
Pumblechook
Sep23-08, 07:28 PM
I have build ladder crystal IF filters using 4 or 5 crystals of the same frequency. The make good filters with steep sides and low ripple.
They are used in professional and amateur radio equipment.
http://www.pan-tex.net/usr/r/receivers/cryfil.htm
An off the shelf filter crystal filter for (CW) Morse can cost £80.. $150..