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measurung Tesla-coil resonant frequency
"I was woundering what would be the best way to test for resonant
frequency in the secondary (Toriod included) if one had a variable
function generator and o'scope available to use?"
Given the equipment that you have available the, *best* method is to connect
your function generator to the bottom of the secondary (remember to disconnect
the bottom of the secondary from ground for this). If your function generator
has a 50 ohm output impedance (this is preferred) make this connection with a
50 ohm cable (use a BNC to alligator adapter at the end of the cable). Connect
a 10X oscilloscope probe to the bottom of the secondary with the scope ground
connecting to the 50 ohm cable ground (make sure that the oscilloscope probe is
properly compensated before measuring). Set the function generator to sine-wave
output and maximum amplitude and no DC offset. Now tune the function generator
frequency until you find the *minimum* amplitude as seen on the oscilloscope.
This represents the resonant frequency of the secondary and it works because a
series resonant circuit has minimum impedance (maximum current drain) at the
resonant frequency. You can also easily determine the "Q" of the secondary by
measuring the upper and lower 3dB frequencies (those points where the output
current of the generator is 0.707 times the current at resonance). The "Q" is
then Fc / (Fu - Fl) where Fc is the Center (peak resonant) frequency, Fu is the
Upper 3dB frequency and Fl is the Lower 3dB frequency. This value should fall
in the range of a few hundred to a few thousand.
"And what about the [resonant frequency of] primary?"
Completely remove the secondary. Short the primary spark-gap. Connect the
function generator across the primary LC circuit through a series resistor so
that the load on the generator is the series resistor in series with a parallel
resonant circuit. Since this test involves a parallel resonant circuit rather
than a series resonant circuit the tuning is performed while looking for the
maximum output amplitude. The Fu and Fl frequencies are defined as being at
0.707 of the maximum output amplitude seen at Fc. The Q is calculated exactly
the same way as before. The optimal choice of series resistor depends on the
actual values of L and C for the primary so experimentation might be necessary.
******************************************************************************
I worked for Tektronix for 22 years and part of that time was as a project
leader designing portable oscilloscopes.
Micheal Cranford Resident Skeptic Intel P6 Architecture Labs