LF: Re: Help needed, mechanical filters

["James Moritz" <james.moritz@btopenworld.com>]

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