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Re: Scoping coils

> Being a great radio hack, I took my
> function generator, freq counter and scope, and made a test of
> what was what with this coil.  At 147. Khz I found a BIG 
> resonance peak on the bare coil that was a PEAK.  While tuning
> off a small amount, the peak diminished greatly.  

> I also noticed that the coil was greatly disturbed in waveform
> amplitude by any object brought near while tuned off peak.  
> When the coil was peaked, it made little difference.  Note that
> this is for the bare coil sitting on my bench without any top 
> capacitance.  My function generator and counter were connected
> to the bottom or the coil, and the scope to the top. 

Another problem with this is that the probe, when physically connected 
to the top of the coil, adds capacitance and throws off your 
measurements of the resonant frequency significantly.

I also tried attaching the scope probe to the top of the coil just to see 
what I would get.  As in your case, the output waveforms from the top of 
the coil were generally very not sinusoidal and the coil peaked at a 
frequency much lower than the resonant frequency that I measured 
using the method described by R. Quick.  When I unhooked the probe 
from the coil winding and just placed it about a foot away from the coil, 
the waveform was clean and the resonant frequency was very close to 
what I expected.  Even by moving the probe a few inches closer, I 
coupled capacitance into the system and shifted the voltage peak down 
in frequency by a measurable amount.

> What I prefer to do is to common ground the function generator,
> scope, and frequency counter. I put a 1K resistor in series with
> the output of the function generator and then feed the signal
> directly into the base of the coil. I make my connection to the
> frequency counter on the function generator side of the resistor.
> On the coil side of the resistor I hook up the O'scope probe (or
> in my case I just connect the center lead of the BIN coax).

I cut apart a piece of coax, soldered a 1K resistor into the line, and 
closed the cable back up again.  My signal generator is T'ed to the freq. 
counter with regular coax and to the scope with the coax that has the 1K 
resistor.  The scope is then T'ed on through to the base of the coil.  
This way, there are no boxes, discrete components, etc.. to fuss with.
 
> With this setup the O'scope shows a drop in voltage when the
> function generator output matches a resonate frequency in the
> coil. The flatter the line in the scope, the more efficient
> (higher Q) the resonate node. This is called "grid dipping".
> Because the coil will show a very high impedance to non-resonate
> frequencies, your dips will be quite noticeable. It is not
> unusual to find several resonate frequencies, or dips, in a high
> Q construction.

Another interesting thing is that while tuned at resonance, you can bring 
your hands close to the base of the coil and not affect the dip all that 
much.  But moving your hands near the top of the coil has a much 
larger effect on the tuning.


Steven Roys (sroys-at-radiology.ab.umd.edu)