Path: utzoo!censor!geac!torsqnt!news-server.csri.toronto.edu!cs.utexas.edu!usc!apple!vsi1!indetech!pacbell!osc!jgk
From: jgk@osc.COM (Joe Keane)
Newsgroups: sci.electronics
Subject: Re: Switched Filters
Summary: Keep filter input impedenance fixed.
Message-ID: <4028@osc.COM>
Date: 22 Nov 90 00:37:18 GMT
References: <17100009@inmet>
Reply-To: jgk@osc.COM (Joe Keane)
Organization: Versant Object Technology, Menlo Park, CA
Lines: 22

In article <17100009@inmet> erich@inmet.inmet.com writes:
>  in ______./ .______/\/\/\_______ out
>          switch     res     |
>			    | |cap
>			    | |
>			     |  
>			   ____
>			   /// 

>Will  	Output Signal = Input signal x Duty cycle of control line 
>	with no distortion of the output signal relative to the input
>	except for attenuation?

Almost.  The problem is that you're not really switching voltages, since when
the switch is off the filter input is disconnected rather than grounded.  The
output will just follow the input.  If you switch the filter input to ground
during this time, the circuit will behave as you expect.

It's easier to work with current switching and filtering, since when you open
the switch you know you'll get zero current.  In this case your energy storage
device will be an inductor rather than a capacitor.  This is how most
switching power supplies work.