Hi Jim,
Thanks for your comments.
Am 30.06.2012 10:14, schrieb James Moritz:
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.
So, is a microphone effect possible?
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.
Thanks, that help to extend my understanding of such filters.
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.
Good idea about the matching! So it is about like the input filter of
JA8SCD's LF converter:
http://icas.to/idc-136-kit/idc-136-kit-manual-eng.pdf (page 6 of 6).
According to the data sheet, the insertion loss (Durchlassdämpfung) is <
2 dB. So if one uses a well matched 50 Ohm transmit antenna and a RX
with 50 Ohm input, i think no additional gain is needed.
Best 73, Stefan/DK7FC
Cheers, Jim Moritz
73 de M0BMU
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