Another experiment I'll try is a simply aluminum shield around the breadboard as well as tying the circuit ground to the breadboard chassis as a shield. With the workbench LED off it still is noisy but not a disaster. With my workbench LED light on, it picks up all kinds of EMI (at LF 5KHz) and is unreadable. BTW it's also interesting that this works as an antenna. It's great that adding the 2.2k worked but I don't yet understand why. I do have to go back and understand why the source impedance matters. Either way that graph was very very helpful. Is your graph a real graph or a simulation? Because where'd you happen to get a 7.2mH inductor?! Or do you have a variable tap / adjustable inductor on hand. I'll post more results tomorrow or Wednesday when I have time to get back to this. I added a 2.2k resistor, it works! So cool! The Q is much better looking as well, although I don't yet know if that's the effect of the added resistor or if I'm doing something else. And again, in series it worked perfectly. In general, at this order of magnitude of values, is it challenging to build breadboard prototypes? It's LF stuff, I wouldn't have imagined that to be the case. I can imagine it has a lot of parasitic inductance, however shouldn't that have affected the series LC circuit equally? I'll try again after the long weekend with a single axial lead capacitor. My capacitor is a decade capacitance box. 6mH doesn't match the resonance frequency, and I don't think the meter is reading this value correctly (too much capacitance for it to read the inductance properly), but it's much, much closer. My HH LCR meter reads just 6mH for the total circuit, vs 7.2mH for the inductor alone. I added 10" of loop to the capacitor (5" to each terminal), and I added 5" (total 10") to the probe ground clip. I thought maybe my capacitance measurement is actually ok, and that with the parallel circuit I'm adding parallel inductance, ie reducing the net overall inductance. By the theoretical lumped circuit model (well under SRF), this parasitic capacitance, even if significant, would be in parallel. By the numbers, the parasitic capacitance of the inductor should be 1.8pF SRF (1400kHz). And it would be in parallel anyway, so it should add to the net circuit capacitance. The probe tip capacitance should be insignificant at ~10pF. At 5500 the effective capacitance is the same 53% of the measured capacitance. (could be some kind of x/x+y thing though.) But visually the resonance is around 5500, not 5000 (the scope is picking up a noisy peak). By the scope auto measurement it shifts but not by a linear proportional amount. In the second trace, I added ~100nF of capacitance in parallel. The effective capacitance is 53% of the measured capacitance. The resonance frequency shifted quite a lot. In the first trace, I have the same L & C but in parallel. I hope this is encouraging to anyone else getting started on LC resonant circuits!ĭidn't work out so well in parallel. As a first go at it, I felt I wanted the borders to verify correctness. I also should use all 10 horizontal divisions instead of 8. Also I should offset the display to see only the top half of the wave. That way it's easy to use the scope auto cursors to find the resonant point instead of all the zooming and manual manipulation I had to do. To do it more thoroughly, I think I should configure it as a parallel circuit, that way the current is minimized and the voltage is maximized at resonance. Ultimately I will build this parallel but the resonance is the same so I just left the series configuration in place to do the "bode plot". That's not possible as a parallel circuit. I did it as a series circuit because prior to this, I was looking at the individual L and C as individual components in the LC circuit, to see their individual contribution to the overall circuit. Next I'll play with series and parallel resistance to see the effect on Q. The Q looks pretty low to me but I don't have a really good intuition about that yet. Maybe if I turn on the FFT channel I'll effectively get that. I wish there were a way to display the vertical scale log instead of linear. I'm amazed how close to theoretical the result is. Read a bunch of web pages, watched w2aew #54 #55 #56 (all 3 re: L and C resonance), watched Dave's #396 (use scope to create bode plot), ran the theoretical numbers and built a circuit on breadboard.
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