Relay-Version: version B 2.10 5/3/83; site utzoo.UUCP
Posting-Version: version B 2.10.2 9/18/84; site brl-tgr.ARPA
Path: utzoo!linus!philabs!cmcl2!seismo!brl-tgr!tgr!d3unix!jhs@mitre-bedford.ARPA
From: jhs@mitre-bedford.ARPA
Newsgroups: net.ham-radio
Subject: Re: RF Probes
Message-ID: <1673@brl-tgr.ARPA>
Date: Mon, 23-Sep-85 16:30:06 EDT
Article-I.D.: brl-tgr.1673
Posted: Mon Sep 23 16:30:06 1985
Date-Received: Wed, 25-Sep-85 11:24:15 EDT
Sender: news@brl-tgr.ARPA
Lines: 15

People trying to make voltage dividers etc might make something of the
following observation:  If you have an R shunted with a C as input impedance
of some device like a meter or amplifier, you can make it into a voltage
divider / lowpass filter by putting an R shunted by an L in series with the
original RC combination.  If the R values are equal then choose L so that
sqrt(L/C) = R, and the whole thing will look like EXACTLY R at ALL frequencies
as seen from the input, while giving a 2:1 voltage division ratio at
frequencies well below cutoff.  Often the cutoff frequency comes out
acceptable.  Cutoff frequency is of course 1/(2*pi*sqrt(LC)).

The other approach is the capacitive compensation approach, which avoids the
cutoff problem but which presents a capacitive input impedance.

						73,
						de W3IKG