# Resonance

Discussion in 'Radio and Wireless' started by Granpa, May 6, 2021.

1. ### Granpa

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May 4, 2021
I wanna build a tank circuit That resonates at 7.86hz by using an inductor 50mH and capacitor 8200uF, I wanna tune the frequency between 5hz and 50hz or 100hz. What size variable capacitor should I use in parallel with the cap.? I'm a beginner so any input is welcome.

My goal is to monitor the Schumann wave and possibly amplify it.

2. ### Granpa

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May 4, 2021
BTW I'm not that much of a beginner. I did my first audio amp in the late 70's and have an AS in Computer Tech And Electronics from Maui Community College around 1997. My last job before retiring was Computer network support...

3. ### Harald KappModeratorModerator

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Nov 17, 2011
Firstly: your 8200 µF capacitor would be an electrolytic capacitor. These are usually tolerated -20% ... + 50 % Also they have a considerable series resistance (ESR). Completely (my opinion) unsuitable for a reasonable LC filter.

But let's assume you had a perfect 8200 µF capacitor...
That would be one big capacitor...
For 5 Hz you'd need ~20000 µF.
For 50 Hz you'd need ~200 µF.
Since you want to use the variable capacitor in parallel to the fixed one, you'd use a fixed 200 µF capacitor and you'd require a 0 µF ... 20000 µF variable capacitor. You will not find such a beast.
Your 8200 µF capacitor will be useless above the 7.8 Hz you calculated as any other capacitor in parallel will increase the total capacitance and thus lower the frequency. Increasing the frequency is noz possible by paralleling capacitors.

For creating a reasonably accurate big capacitance, you can use a known good small capacitor plus an opamp to form a capacitance multiplier.
Another approach would be to use a fixed, known good capacitor of a small value plus a much larger inductor. Of course, a bigger inductor will be unwieldy. But rescue comes agian in the form of opamps. You can simulate a large inductance using capacitors, resistors and opamps.
With these circuits you can adjust the frequency by adjusting resistors (e.g. by using a potentometer).

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5. ### Granpa

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May 4, 2021
Thanks for the replies folks! Gonna look into another way to tune 5hz to 100hz.

6. ### Granpa

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May 4, 2021
Okay, so I wanna use a cap with better tolerances, what's the best cap to use? And maybe an op amp circuit to produce a capacitance multiplier, I used to have a book called the encyclopedia of electronic circuits, are these circuits common public domain? I was thinking of getting a copper coil from a still or something to get large induction. What does a capacitance multiplier look like. And I can see the use of potentometers. I'm a minimalist and wanna keep this project as simple as possible. In the R&D lab we had large budgets, but I can only spend less than a hundred a month. Any help and patience is appreciated.

7. ### Harald KappModeratorModerator

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Nov 17, 2011
That is going to give you a rather large tolerance, too.
Quality capacitors are film or ceramic, but an accuracy better than 5 % is above your budget. You'll have to do some trimming in any case.
A potentiometer is simple.

What kind of antenna are you going to use to pick up these waves?
Are you aware that you will pick up a lot of noise around 16.7 Hz (railway power supply) and 60 Hz (mains power in the USA) as well as integer multiples thereof? You will need to filter out these frequencies otherwise the signals from these frequencies will be much higher than the signal you want to measure.
Have you considered an alternative approach: pick up the signal with a coil, add a low pass filter to eliminate frequencies > 100 Hz, then digitize and do all the filtering in software? See e.g. here. You may have to add a mixer stage in front of the soundcard's input to bring the input signal to within the card's bandwidth (< 20 Hz any reasonably prized soundcard will be hard pressed to record). Or use an ADC module with USB interface instead of the sound card. In any way, you'll be up to some serious programming

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9. ### Granpa

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May 4, 2021
Mr. Kapp, I wanna thank you for your input, you've showed me enough to keep me busy for a week. I'll show my grand kids and see what they think. Just kidding. when I get my workbench set up and learn how to use the schematic software I'll have more questions.

davenn likes this.

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11. ### Granpa

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May 4, 2021
Here's what I have so far.

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12. ### Harald KappModeratorModerator

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Nov 17, 2011
I think this will not work:
1. Your resonant circuit made of L1 and C1 is shorted by the 12 V power supply (V2 is a short circuit for AC signals).
2. R1 is not adjustable. To use a potentiometer as a rheostat you'll have to connect the wiper to one end of the potentiometer. What is the use of R1 anyhow?
3. You seem to be trying to use R2 to create a positive feedback from the output of the opamp to the resonant circuit. The value of R2 is probably to low for this purpose.

13. ### Granpa

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May 4, 2021
Cool, V1 is just to represent the Schumann Resonant Frequency of 7.8Hz. Does it still short if I take V1 out? I added R1 to tune the receiver, I'm not sure what value it should be. How can I tune the circuit? I copied that value from the Capacitance Multiplier at
What value should R2 be? I changed the Capacitance from 100nF to 82mF on the CM to get 7.8Hz.

14. ### Harald KappModeratorModerator

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Nov 17, 2011
Unfortunately yes.
You don't tune an LC resonator by changing the resistive element. Changing the resistance will vary teh Q factor, but not the resonace. The latter is characterized by L and C. You'll have to chnage one of these to tune the receiver.
Now I see what you're aiming at. This is what your circuit should look like (note: I haven't checked that it actually works, only made the corrections to fit it to the circuit pf the capacitance multiplier):

Your values for R1 and R2 vary considerably from those used in the link you gave. Why? Read the description on the linekd website to find the values you need. Tip: start with the values provided tehre.

15. ### Granpa

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May 4, 2021
I just started using the https://www.circuitlab.com/ software early this morning. Kinda excited so making mistakes. Ill send the updated version.

16. ### Granpa

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May 4, 2021
Yes Sir, you gave me the idea to use a Capacitance Multiplier. I have noticed in these posts that I need to improve my communication skills, I will work on that.The capacitance I need is 8.2mF I believe(I wanna check my math) and a cap that size has large tolerances so you said that may not work well. So you directed me to use a Capacitance Multiplier with a times 100 function in order to use a smaller more suitable capacitor. You provided me with a link to a circuit https://www.falstad.com/circuit/e-capmult.html and I want to replace the value C1 100nF with 8.2mF/100 which is 82uF. I think I ordered a suitable 50mH inductor.

My goal is to build a simple receiver for 7.8Hz and three harmonics up. (14Hz, 20Hz, 26Hz I think)
How wide is the frequency band of this receiver, does it encompass the three harmonics?
What is the best antenna to use?
What adjustments do I need to make to the Capacitance Multiplier work well with C1 at 82uF?

This is just a hobby for me and I am not qualified to design and build this receiver without help. If I ask too many stupid questions and am too much for you let me know. The place I used to work had engineers and scientists and data analysis experts to ask any questions I wanted to. Now I live in rural Texas northwest of Austin and don't know many techies around here. So thank you!

17. ### Granpa

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May 4, 2021
U2 is an unknown component. An Antenna Amplifier?

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18. ### Harald KappModeratorModerator

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Nov 17, 2011
I have no idea. Where did you get that schematic from? The description should be there.

19. ### WHONOES

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May 20, 2017
Why don't you use a simple tuneable band pass filter using an opamp?