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LF: Re: Help needed, mechanical filters

from [James Moritz] [Permanent Link][Original]
To: <[email protected]>
Subject: LF: Re: Help needed, mechanical filters
From: "James Moritz" <[email protected]>
Date: Sat, 30 Jun 2012 09:14:41 +0100
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Dear Stefan, LF Group,
The mechanical filter consists of an array of metal disks or rods that are mechanically resonant near the centre frequency of the filter. In order to convert the electrical input and output to/from mechanical vibration, there are transducers at the input and output, sometimes piezo-electric, but in this case probably magnetostrictive. These are a coil of wire wound around magnetic material which changes shape slightly depending on the magnetic flux. So electrically, the input and output of your filter are inductors - in order to get efficient transfer of signal power into the transducers and through the filter, they must be tuned to resonance, like any LC filter circuit. If they are not correctly tuned, little power from the mechanical resonators inside the filter will be coupled to the source and load, so the resonators will have very little damping, and the filter will have large peaks in the response, as you saw. Also, the insertion loss will be higher. The same type of behaviour with mis-matched loads occurs with almost any filter - LC, crystal, ceramic, cavity, etc. A similar effect occurs if one resonator is defective - in this case the mis-match occurs inside the filter.
You can probably use the input and output resonating capacitors to match your filter directly to 50R, using two capacitors in a series or shunt matching arrangement, like some have used for loop antennas. But buffer amlifiers might be needed to provide stable input and and output impedances for the filter, and provide some gain to compensate for the insertion loss. 

Cheers, Jim Moritz
73 de M0BMU
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