From: "Terrell W. Fritz"
Date: Sat, 03 Feb 2001 21:59:34 -0700
Subject: Re: [TSSP] 50 ohm Generator hookup
Hi Paul, At 10:40 PM 2/3/2001 +0000, you wrote: > Length of >coax connected to the 'volts' port should be kept to 3 m or less, >because it's shunt capacitance steals some of the measured current, >but for a known length its C can be allowed for. The coax can also be simply disconnected after the voltage is known. Then it will not act like a load for the current measurement. My 777pF long coax did add significant loading :-)) However, I want to point out another nasty problem I had and how to fix it. I used a single core and bobbin ferrite transformer as shown at: http://hot-streamer.com/temp/T1.jpg What I found is that when the primary and secondary of the transformer are wound directly over each other as in this transformer, there is significant primary to secondary capacitance which wreaks havoc on the measurement circuit in this case. http://hot-streamer.com/temp/T1pic.jpg What I did to partially fix it is to reverse the secondary connection so the grounded side of the 100 turn section was next to the 10 turns section. This in effect makes a ground shield between the two. However, it is not a perfect fix and there is still some capacitive current that is leaking between the two windings that is still having small effect. What I really need to do is pull off the winding and insert a copper foil ground shield between the two windings (being careful not to make a shorted turn). Just wanted to point this out to others that may make one of these. Cheers, Terry
Maintainer Paul Nicholson, paul@abelian.demon.co.uk.