SAW filters

[Kevin Weddle]

I have heard that SAW filters have good attenuation. Has anybody ever used them? I would think their charactersitic is very useful. Could somebody post a circuit with them. I am just curious.

I see now that they have funny response to them. Maybe this is the reason for their lack popularity.

[audioguru]

Hi Kevin,
SAW filters have been popular for awhile for their use in cable TV boxes, TV transmitters and TV sets. In the olden days, those systems used many tuned LC filters to achieve the "funny response" that is required for sharp cutoff of the side of the picture carrier that isn't transmitted (vestigal single-sideband), a flat response for the desired channel and notch filters for adjacent channel picture and sound carriers on both sides of the bandpass in order to avoid interference. The LC filters were difficult to tune because adjusting one would affect the rest.
Now a SAW filter accurately does all that without needing any adjustment.
I can't think of any function that would be useful for a hobbiest, unless they want to make one of the devices (listed above) from scratch.

[trigger]

Saw filter also applied widely in wireless products, it is nice with quite low attenuation.

I also hear that a crystal can act like a filter..... I haven't tried myself.... any idea?

[audioguru]

Hi Andrew,
A crystal is a tuned circuit, that's why a crystal oscillator works. It is a low impedance series LC circuit at one frequency and a high impedance parallel LC circuit at a near frequency.
If you use a crystal as a filter, its bandwidth is so narrow that it wouldn't receive much modulation (just CW morse code).

AM-FM radios use "ceramic filters" in their IF amplifier that are designed with a certain fairly wide, flat bandwidth and steep skirts.
A ceramic filter is similar to a crystal.

[surajbarkale]

SAW filters provide accurate filter characteristics which are not influenced by temperature. Also the tolerance from one filter to another is very small. If RC / RL filter is used in it's place it would be very difficult to tune it (forget about replacing it ;D). Check this out http://www.oki.com/en/otr/html/nf/otr-158-15-2.html