From: Paul
Date: Sat, 03 Feb 2001 08:08:00 +0000
Subject: Re: [TSSP] 50 ohm Generator hookup
Terrell W. Fritz wrote: I'm having a polyurethane party this weekend in order to convert a quantity of copper and tea into a transformer. Hence the reason for being up at this hour to catch up on some paperwork! Terry, the new signal generator is really impressive and its frequency setting accuracy opens the door to doing some precision Q measurements. > I tried terminating the coax with 1 and 50 ohms: > ... > I got Qs of 103 and 105 respectively OK. That's reasonable. It looks like a 2% change in effective series resistance against an overall resistance of around 500 ohms, so the coil is seeing a source resistance of around 10 ohms with the 50 ohm termination. Could the generator have a nominal 50 ohms output impedance but in fact be somewhat lower? > I have parts on order for a low Z wide band amp but > that won't be together for about a week. An excellent solution to the problem. > The generator and the HP meter are consist ant but the scope likes > to read high. Yes. Even if the scope was spot on, some calibration would be needed anyway to account for whatever probe or divider arrangement is in use for the top voltage. You may find now that with the new generator the variability of your coil's Q may become troublesome. Do you have any other coils available for testing? Smaller, non-sonotube, perhaps? Measuring Q indoors is a real problem since the resulting Q depends so much on how much energy is absorbed in walls, ceiling and floor. For a test setup capable of doing justice to your new generator you might want to consider switching to a smaller coil on a low-loss former, one which is small enough to completely enclose in shielding so that its Q will be high, stable, and possibly even predictable. As regards the base drive, the method you've been using seems fine, although it's better if the current shunt, etc is moved to the coil base end of the feeder. Something like the arrangement shown in http://www.abelian.demon.co.uk/tssp/tmp/basedrive.gif might have some advantage. I tend to prefer this so that I can get both V and I samples referenced to ground, although the thing has to be calibrated by replacing the secondary coil with a known resistor. The coupling transformer (flyback ferrite) is a step-down ratio to drop the incoming 50 ohms down to something nearer 1 ohm, say 5:1 or 10:1 turns ratio. The whole gubbins can be boxed for stability and shielding, and placed under or near the coil. Regards, -- Paul Nicholson, Manchester, UK. --
Maintainer Paul Nicholson, paul@abelian.demon.co.uk.