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Re: tuning a coil with a freq gen and a scope

Original poster: "Malcolm Watts by way of Terry Fritz <twftesla-at-qwest-dot-net>" <m.j.watts-at-massey.ac.nz>

Hi Greg,

On 7 May 2002, at 8:22, Tesla list wrote:

> Original poster: "Mr Gregory Peters by way of Terry Fritz
<twftesla-at-qwest-dot-net>" <s371034-at-student.uq.edu.au>
> Hello all,
> I'm wondering what the best procedure for tuning a coil with a scope 
> is. Should you grid dip the seondary when it is installed within the 
> primary or when it is free standing? Should you grid dip the primary 
> with the secondary installed, or without? I have previously grid dipped 
> them separately, and then added an extra turn in the primary to 
> compensate for corona loading. I normally base feed the coils through a 
> 1k resistor. Is this method reliable?

Best way is to check the secondary Fr with the coil either distant 
from the primary or with the primary open circuit. The tune the 
primary to the same frequency with the secondary absent. I don't like 
grid dipping much - you have to get too close to the resonator which 
adds to the self-capacitance somewhat. My favourite method is to 
bottom-feed the coil with a lo-Z sig gen and pickup the e-field 
coming off it with a small aerial probe a few feet away. For the 
primary, short the gap so a closed resonant circuit is formed and 
feed a sig gen (connected in series with a reasonably high value of 
resistance) across it, sampling the resulting voltage with a hi-Z 
scope probe directly across the coil.  

    Base-feeding with a 1k resistor is robbing the resonator of 
signal but won't affect Fr too much. It is a total no-no for 
measuring Q.