Message-ID: <4FEEF036.4020702@iup.uni-heidelberg.de>
Date: Sat, 30 Jun 2012 14:25:26 +0200
From: =?ISO-8859-15?Q?Stefan_Sch=E4fer?= <Stefan.Schaefer@iup.uni-heidelberg.de>
User-Agent: Mozilla/5.0 (Windows; U; Windows NT 6.1; de; rv:1.9.1.8) Gecko/20100227 Thunderbird/3.0.3
MIME-Version: 1.0
To: rsgb_lf_group@blacksheep.org
References: <4FEE0F7E.2090109@iup.uni-heidelberg.de> <0F87F4C0AE524B1E9EA631C81D579573@gnat> <4FEE3A73.1090602@iup.uni-heidelberg.de> <E78F8B1D5F0B4BBB99655F1BC9BE6D48@JimPC>
In-Reply-To: <E78F8B1D5F0B4BBB99655F1BC9BE6D48@JimPC>
Subject: Re: LF: Re: Help needed, mechanical filters
Content-Type: text/plain; charset=ISO-8859-15; format=flowed
Content-Transfer-Encoding: quoted-printable
Sender: owner-rsgb_lf_group@blacksheep.org
Precedence: bulk
Reply-To: rsgb_lf_group@blacksheep.org
X-AOL-SCOLL-URL_COUNT: 0  
x-aol-sid: 3039ac1d408d4feef1066054
X-AOL-IP: 195.171.43.25
X-AOL-SPF: domain : blacksheep.org SPF : none


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=20
> 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 =20
> mechanical vibration, there are transducers at the input and output,=20
> sometimes piezo-electric, but in this case probably magnetostrictive.=20
> These are a coil of wire wound around magnetic material which changes=20
> shape slightly depending on the magnetic flux. So electrically, the=20
> input and output of your filter are inductors - in order to get=20
> efficient transfer of signal power into the transducers and through=20
> the filter, they must be tuned to resonance, like any LC filter=20
> circuit. If they are not correctly tuned, little power from the=20
> mechanical resonators inside the filter will be coupled to the source=20
> and load, so the resonators will have very little damping, and the=20
> filter will have large peaks in the response, as you saw. Also, the=20
> insertion loss will be higher. The same type of  behaviour with=20
> mis-matched loads occurs with almost any filter - LC, crystal,=20
> ceramic, cavity, etc. A similar effect occurs if one resonator is=20
> 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=20
> match your filter directly to 50R, using two capacitors in a series or=20
> shunt matching arrangement, like some have used for loop antennas. But=20
> buffer amlifiers might be needed to provide stable input and and=20
> output impedances for the filter, and provide some gain to compensate=20
> for the insertion loss.
Good idea about the matching! So it is about like the input filter of=20
JA8SCD's LF converter:=20
http://icas.to/idc-136-kit/idc-136-kit-manual-eng.pdf (page 6 of 6).

According to the data sheet, the insertion loss (Durchlassd=E4mpfung) is =
<=20
2 dB. So if one uses a well matched 50 Ohm transmit antenna and a RX=20
with 50 Ohm input, i think no additional gain is needed.


Best 73, Stefan/DK7FC


>
> Cheers, Jim Moritz
> 73 de M0BMU