RadioBanter - View Single Post

March 22nd 06, 05:25 PM posted to rec.radio.amateur.homebrew

wrote:

On Wed, 22 Mar 2006 10:29:56 +0000, John Wilkinson
wrote:

Hi,
I am using a couple of 45MHz crystal filters in my first IF amp.
Has anyone any experience of using these, and give me any advice on
problems they encountered.
I think I have a problem with screening, as they require 700 Ohms
termination resisistance.

When the manufacturers spec a termination resistance, do the filters
themselves look like say 700 Ohms, or do I need to set this externally with
a resistance?

Best regards,
John.

It means looking from the filter into the next stage it should see a
700ohm resistive load.

How you get from 700 to the optimum for the stage is up to you
but FETs are higher input impedence for noise and gain and
transistors (BJT) are generally lower. Devices like NE602,
MC1350 and similar differential inputs are around 1.5k.

Allison

It means that the source impedance of the amplifier driving the filter,
_as well as_ the input impedance of the amplifier following need to be
700 ohms, resistive. Get this wrong and the filter shape will be off.

--

Tim Wescott
Wescott Design Services
http://www.wescottdesign.com

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