So why is with the lower inductance (106uH instead of the 115uH the LITTLE peak at #1 lower in frequency? Or is the new resonance peak the one at #2 (around 7Mhz), so indeed higher thus at 4.8pF distributed /selfcapacitance?
See diagram below, same as with the single coil measurement as shown in post #25
The key to understanding this behavior is remembering that at the resonance frequency:
1) An ideal series LC circuit acts as a short circuit.
2) An ideal parallel LC circuit acts as an open circuit.
But these are not ideal LC circuits, thus at resonance:
a) The current flowing in a series LCR circuit is at the MAXIMUM.
b) The current flowing in a parallel LCR circuit is at the MINIMUM.
Because of the distributed intrawinding capacitance, these pancake coils act as two imperfect parallel LCR circuits...and when they are connected in series and are far apart, they act as two parallel LC circuits connected in series
When they are close together, their mutual inductance and and interwinding capacitance comes into play.
It is surprising that the two frequency peaks are so far apart for coils that are virtually identical.
Anyway, the SA essentially measures voltage at its input, but since its input has the 50Ω impedance, this voltage is proportional to the current flowing in this input impedance, too (V= i * 50Ω).
When you are measuring one coil and we treat it as a parallel LC circuit, we can expect it to act as on open circuit at resonance. Obviously, an open circuit doesn't allow any current through. No current flowing through the 50Ω input impedance, means no voltage appearing at the SA's input, because 0A * 50Ω = 0V.
With an imperfect LCR circuit, the voltage at the SA's input, would not be 0V, but it would be at the
minimum, which represents the dip denoted as dip #2. (it is noisy because the amplitude is low at that point - as it should be in this measurement setup).
If you want the parallel LC resonance to manifest itself as a high amplitude peak (not noisy), then connect it as on the diagram below. (calibrate it to a flat line after all the coil's cables and interconnects are in place ...but
before the coil itself is connected).
Author
Topic: Smudge proposed NMR experiment replication. (Read 104626 times)
