Scoping of Secondaries


Here is an alternative method of measuring secondary
coil resonance using an oscillator (OSC) and an
oscilloscope (CRO) which I have used and may be of

Instead of connecting the oscillator into the base of
the coil, it is fed into a very loosely coupled primary
consisting of a few turns of thin wire at large diameter.
(eg. 10 turns of .2mm wire bunched together at 500mm radius)
The base of the secondary is earthed via a low value
resistor, and the CRO is strung across the resistor.
The resistor value is a compromise between getting
enough reading on the CRO and it not affecting the
resonance very much.  100 Ohms or less seems about right.
The schematic in ASCII art is:

     loosely        O  Tesla
     coupled        <  secondary
     primary        <
 +------>           <
75 Ohm  >           <
OSC     >           <
 +------>           <
 |                  |  Resistor
earth               +--^v^v^v--+---earth
                    |          |
                              ||| (shielded cable)

The advantages are that the CRO registers a resonance
peak rather than a dip; the off resonance reading is
almost zero; there is virtually no mains hum pickup; the trace
is very clean; and there is nothing affecting the top
of the secondary.

I have a digital frequency meter in
parallel with the OSC to get accurate frequency readings.

Another of my ideas is replacing the OSC with a
home-brew wobbulator which takes synchronisation
from the horizontal timebase of the CRO.
Voila!  Instant spectrum analyser (of sorts, anyway).
The wobbulator is switchable to cover approx.
50-100KHz wide bands from 50Khz up to 2MHz.  It also feeds the
2nd channel of the CRO with what looks like a vertical
line cursor which can be moved using a pot on the
wobbulator to line up with any interesting part of the
sweep response trace on channel 1. Flip the wobbulator
to manual mode, and the frequency meter shows the
frequency of interest.  It is a very fast way of finding
and identifying resonances over a wide band.
Q can also be determined using formulae, and also K if a
real Tesla primary is used as the feeder.

Comments please???

Glenn Baddeley <gbaddele-at-vitgssw.telecom-dot-com.au>
Melbourne, Victoria, Australia