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Re: [Amps] tuning bandpass filters

To: <dansawyer@earthlink.net>, "Amps" <Amps@contesting.com>
Subject: Re: [Amps] tuning bandpass filters
From: "jeremy-ca" <km1h@jeremy.mv.com>
Date: Mon, 21 Jan 2008 11:23:30 -0500
List-post: <mailto:amps@contesting.com>
----- Original Message ----- 
From: "Dan Sawyer" <dansawyer@earthlink.net>
To: "Amps" <Amps@contesting.com>
Sent: Monday, January 21, 2008 9:36 AM
Subject: [Amps] tuning bandpass filters


> All, I am trying to tune bandpass filters. I have a network analyzer and
> can see both reflected and transmitted signal. My questions are:
>
> If the filter were tuned and terminate properly in the bandpass range is
> the reflected signal a minimum?

The termination should be a broadband one and a pure resistance. A perfect 
filter will have no reflection. In the real world you want at least a 20db 
return loss in the passband.

Im making an assumption here based upon no input info and the name of the 
forum that you want to drive a grounded grid linear amp. If so the common 
matching device is a 3 pole pi network which is a low pass filter and not a 
bandpass.

>
> Should the reflected signal show the same shape as the transfered signal?

I dont understand that. You are looking at a single trace on the NA that 
will show the shape of the filter. Set your markers for the desired start 
and stop frequencies.

>
> What is the best way to tune these filters?

So they work. (;   The input tuning is adjusted for a match to the 
transmission line once you do your cal procedure.
The output tuning is adjusted for filter shape. Once you are in the ballpark 
then filter L and both C's are fine tuned.

Use your filter design program to determine the expected shape. With a 
typical Q of 2 it will be very broad.

Once installed in the amp it will require final tweaking.

Carl
KM1H



>
> Thanks, Dan
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