Tank Circuit Problem: Solving 0 Hz Frequency

In summary, the conversation is about building a simple tank circuit and testing its frequency using a multimeter or oscilloscope. The circuit consists of a coil and capacitor connected in a loop. The individual is having trouble generating a measurable frequency and is seeking advice on how to properly set up the circuit and verify its frequency. They mention using a square wave and a current-sensing resistor, but are unsure of the component values they have on hand. They also discuss the possibility of using a winding from a small transformer as a larger inductor. They estimate the frequency of the circuit to be around 11.6 MHz but are only able to generate a 17.5 kHz signal. They also mention issues with their microcontroller not working correctly.
  • #1
mearvk
133
0
Hello all.

I'm trying to make a dead simple tank circuit and test its frequency using either my multimeter (which has the Hz measurement) or my oscilloscope.

Now I've never built one of these circuits before but it looks fairly straightforward, however I've had no luck so far in generating a measurable frequency in the LC circuitry.

My understanding is that I can use something like this:

http://bioweb.biology.utah.edu/goldenberg/radioPics/tank.png

When I try to hook this up I see 0 Hz on my multimeter.

Here is how I have my circuit: http://imgur.com/lvhgZfT

The green is the path for current to flow. The capacitor is light blue but visible. The coil/capacitor loop actually connects back via the breadboard to the place where the positive terminal connects to the coil.
 
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  • #2
The circuit will have enough parasitic resistance to decay the oscillation faster then the meter can measure the Freq - not to mention the meter has resistance that absorbs the energy as well.
What would need is a scope - you could probably get away with a PC Audio Channel scope ( http://www.zeitnitz.de/Christian/scope_en - I have not used this it was just the first in my google search) -

If you are just getting into circuits - something like this would be quite handy!
 
  • #3
You'll have to feed it AC to get anything repetitive and useful for the oscilloscope. It looks like you are connecting it to DC? DC won't give you anything to see.

Have you got a widely variable AC signal generator, adjustable up to a few MHz? Otherwise, a fixed frequency square wave will do.
 
  • #4
Well I hooked up a 3.82 Hz square wave to it (+5v to +0) and all I don't see anything special on the scope. It's coming out of a PIC MCU so I'm not sure exactly what the output voltage is but I'd guess 0 to 5v.

Just to be clear where should I be hooking my scope lead(s) to to verify that this thing is producing a higher/different frequency? I don't want to bother you guys if it's just that kind of issue but I've checked in a few places (within loop) and at either end of entire tank circuit with oscilloscope and nothing so far.

Thanks.
 
  • #5
Can you get a square wave around 200kHz? The ringing will take place for a few microseconds or more after each square wave transition, so this will be lost from view unless you use a fast sweep. Trigger the sweep from the square-wave so the display is repetitive.

You haven't given any indication of the component values in your LC circuit, so we have no way of knowing what ballpark figure the oscillating frequency will be. What value inductors and capacitors do you have at hand?

Perhaps include a current-sensing resistor (say 10 Ohms) in series with the inductor and use the oscilloscope to examine the voltage across this resistor as you drive the circuit with a square-wave.
 
  • #6
Yeah, I'll see what I can do about a faster frequency tomorrow. Should be easy enough.

Dunno exactly what the inductance is since I don't have an LCR meter but it's about 5-6 coils with a diameter equivalent to a AAA battery, made of copper wire that just barely fits into a breadboard hole (~20-22 awg); it is an air core inductor. The capacitor is in the 500 pF range. I certainly have smaller/larger caps if need be.

The FM band (88 to 108 MHz) Youtube vids all seem to work with an 5-6 coil air inductor and a pF capacitor so I'd put my circuit between 1 MHz and 50 MHz at this point, since we are working with a larger capacitor than is typical for such applications.

Thanks.
 
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  • #7
Is your oscilloscope able to display a 10MHz oscillation? Otherwise, you could use a winding from a small power or audio transformer (salvaged from a broken cheap radio) as a large inductor.
 
  • #8
I've got the OWON PDS5022T which can do 25 MHz and 100 MS/s. I'm a little unsure how to set this up best to catch the echo effect you noted, I'll play with it tomorrow and try and report back what I've witnessed.
 
  • #9
I found this inductance estimator:L = n²∗d²/(18d + 40h)

where:
L = inductance in microhenries
n = number of turns
d = mean diameter in inches
h = height of winding in inches

L = (6^2) * (0.5^2) / (0.5*18 + 40*0.375)
L = 36 * 0.25 / (9 + 15)
L = 0.375 uHLCfreq = 1 / 6.28 * sqr_root( 0.500 nanofarad * 0.375 microhenry)
LCfreq = 1 / 6.28 * sqr_root( 0.0000000005f * 0.000000375h )

For the denominator I get: 0.000000086, check the math though.

LCfreq = 1/0.000000086= ~ 11,628,929 Hz.

Edit: re-re-tweaked some values and re-checked the capacitance which was 0.500 nanofarad, or 500 picofarad and the inductance calc was off too.
 
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  • #10
Your 500pF capacitor is now 500nF?
 
  • #11
Good catch Nascent. Changed that value back.
 
  • #12
Meh. I got a 17.5 kHz signal going into the tank circuit but I'm still not seeing anything interesting on the scope. Also, my micrcontroller IDE has decided that working is too much of a big deal and so I haven't been able to get the digital oscillation speed up higher than 17.5 kHz because it is happier not working correctly.

I thought I saw some of the echo on the scope but I removed the capacitor and the signal stays about the same, so I doubt that's the LC circuit.
 
  • #13
mearvk said:
Meh. I got a 17.5 kHz signal going into the tank circuit but I'm still not seeing anything interesting on the scope. Also, my micrcontroller IDE has decided that working is too much of a big deal and so I haven't been able to get the digital oscillation speed up higher than 17.5 kHz because it is happier not working correctly.

I thought I saw some of the echo on the scope but I removed the capacitor and the signal stays about the same, so I doubt that's the LC circuit.

well if you are feeding in 17kHz and the tank is resonant somewhere up in the MHz ...obviously it won't be resonant at 17kHz and you won't see any oscillations

Dave
 
  • #14
I guess I don't really understand the correlation between input frequency and output frequency. What mathematical formula states this relationship?

My very basic understanding was that if one feeds in any AC signal that the circuit would at least create some higher version of that frequency as a product of capacitor/inductor current interchange.
 
  • #15
mearvk said:
I guess I don't really understand the correlation between input frequency and output frequency. What mathematical formula states this relationship?
There is none, when all you are looking for is ringing.

Your square-wave source will see those few turns of copper wire as a short circuit, and be complaining loudly. I think you should add a 100Ω series resistor (or whatever is appropriate) so that it sees a reasonable resistance.

If you are displaying on the screen a few cycles of your 17kHz square wave (it is a square wave, is it?), then you won't be seeing any of the tank circuit oscillations. You have to stretch out the sweep speed so that, if your oscilloscope screen were really huge, one cycle of the square wave would stretch across the whole room. Then, you would see some of the 10MHz ringing on the few cm of your screen. There is a one-thousand-fold difference between repetition of the square wave and the ringing (if any) of the tank circuit. To put this another way, while triggering off the square wave you need to really speed up the sweep so that the [apparently] vertical transition of the square wave actually looks more like a 45° sloping line across half of your screen.

It may take some experimenting to get a good display of the ringing. Are you using a current-sensing resistor as I suggested?

My next suggestion will be to use a hundred or so turns of wire, to make something more substantial. Then wrap a few turns of insulated wire around that to form a primary winding, and power that primary through the 100Ω resistor. The tank circuit will then not be directly connected to the square wave source.
 
  • #16
The most my scope will stretch I think is down to 5ns. I had it down that low and didn't see any difference between the signal that was produced with the capacitor in place and without the capacitor in place. There was a sort of decaying sine wave at the rising and falling edges of the square wave. As I said, this stayed constant as the cap was pulled.

I have not tried putting a resistor in the LC circuit itself. Also, I am not sure what a current-sensing resistor is. I don't see a sweep feature on my scope either. I have the ability to change the time period on the horizontal scale and the voltage on the vertical scale. There may be a sweep speed function but I haven't found it yet.

I'll try tomorrow with a series resistor in the LC circuit and try and boost the frequency up higher if my microcontrollers stop dying. I got a new programmer and it seems to rejoice in lobotomizing my precious chips. :-(

I do appreciate your help Nascent.

PS I should add the smallest time period my scope runs at is 5ns. I wasn't sure if we could correlate this with a maximum frequency that it could reliably see or not.
 
  • #17
mearvk said:
The most my scope will stretch I think is down to 5ns.
That is 5ns/cm? That's plenty.
I had it down that low and didn't see any difference between the signal that was produced with the capacitor in place and without the capacitor in place. There was a sort of decaying sine wave at the rising and falling edges of the square wave.
That's exactly what ringing of the LC circuit will look like, except it will change as you change L, C and/or R.
I have not tried putting a resistor in the LC circuit itself.
That's where the oscillations are, so if you aren't looking there, you can't hope to see any.
Also, I am not sure what a current-sensing resistor is.
It's an ordinary resistor, the purpose of which is to reveal the waveform of the current in that wire.
I don't see a sweep feature on my scope either.
That's what I mean by the speed of the horizontal scale.
 
  • #18
Ok, latest status. 100 ohm resistor added between coil and capacitor.

It's still not working correctly but I did get a chip to output a square wave around 175 kHz.

Here are some pics: http://imgur.com/73ha149,saDv28Y,UxHXtQz#0

Yellow is the LC circuit probe, red is the square wave oscillator.

The first image is what bugs me; it shows the oscillation going into the LC circuit without the positive voltage even being connected.

The second scope image (you can see the oscillations are slightly more powerful) is with the power connected.

The final image is the two boards I have (left is LC, right is just square wave generator).

I suspect the LC circuit isn't close to being correct at this point.

Edit: I should note the relatively large capacitor was used to see if it made any difference to the oscilloscope output when compared against the very small .500 nF cap described above.
 
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  • #19
The first image is basically what to expect. When the square-wave transitions there are oscillations in the LC circuit. Your square wave shows a sharper rise than its fall, so the tank oscillation should be more pronounced on that rise than the fall. (Remember, I said they won't be discernible until you stretch the waveform out horizontally because of their much higher frequency.)

The mains hum on your signals needs to be eliminated, and it's probably an earthing problem. What are you doing with the black Earth clips on your oscilloscope probes? They should be earthed to the signal generator earth, along with part of the tank.

Try this series LC arrangement with the oscilloscope connected across the capacitor. It won't show ringing, the resistor gives too much damping, but at least it will show whether you are able to get rid of the mains hum. (If the hum can be cleaned up, try shorting out that 100 Ohm resistor and see how the waveform changes.)

attachment.php?attachmentid=56318.png


I'm going to have give more thought to how the parallel arrangement can demonstrate its ringing.
 

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  • #20
I did want to post that I have made considerable progress on this. I will post some more data this weekend if I can.

Cheers.
 
  • #21
Alright, I had to go back to the original photo I took for reference because the circuit seems very finicky in terms of the ordering of the pieces. This could be poor observation on my part but I know it was frustrating.

You can see that I have a monster cap in there. What I have observed is that smaller caps , with a 70 kHz square wave input, amplify the frequency less than larger caps do. The variation in frequency scales, more or less, proportionally with the amplification. You can see from the pictures we have swings from 250 kHz up to around 450 kHz; it averages about 350 kHz though. Without a way of stabilizing the frequency swings the LC is pretty useless.

I added another LED, to act as a simple diode, because I noticed the input frequency was showing up amplified sometimes. I haven't noticed this weirdness since so perhaps it is doing its job but I'm not 100%.

Notice that the IC square wave generator frequency is not 100% stable either. I chalk this up as a current draw issue.

Notice the current clips are now grounded. The green wire runs to the ground on the generator side. The orange is also connected to ground.

Here are some pictures.

http://imgur.com/ucZnGry,6aknvPZ,ayUkHjy,rltwOzV,lNRpxcr#0
 
  • #22
Ok, so I found another issue. The scope's setting for voltage/div greatly affects the amplified frequency. Moved it down to 1v for both input/output and the frequency is much more stable, albeit about 3x slower.

Also, FFT pictures where you can see the square wave harmonic effect.

http://imgur.com/PClbJsZ,QydnlLs,8Of22rT,K7n4JzA,lyaBnEx,fNrbObc
 
  • #23
Electrolytic capacitors will not work in resonant circuits.
If a ceramic capacitor is used, it should be COG. Most other ceramic capacitors don't work in resonant circuits.(I have tried X7R capacitors and they won't work)
Mylar film type capacitors usually work in resonant circuits.
There should be many more turns on the inductor so that the circuit is not operating at a frequency where the stray capacitance and inductance affect the frequency.
Don't use a times 1 probe on the oscilloscope. Use a X10 probe on oscilloscope.

Good Luck
Carl
 
  • #24
Carl Pugh said:
If a ceramic capacitor is used, it should be COG. Most other ceramic capacitors don't work in resonant circuits. I have tried X7R capacitors and they won't work.
I assume the explanation for this is that capacitors with C0G dielectrics have much lower dissipation factors (DF) than capacitors with the X7R dielectrics. As an example, AVX's C0G types are listed with DF = 0.1% while their X7R's have DF = 3.5%. Since the DF is the reciprocal of the quality factor (Q), and that oscillating systems are associated with high Q's, the C0G types are seemingly better suited for the job.

www.avx.com/docs/masterpubs/mccc.pdf [Broken]
http://en.wikipedia.org/wiki/Q_factor
http://en.wikipedia.org/wiki/Dissipation_factor
 
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  • #25
mearvk said:
Notice that the IC square wave generator frequency is not 100% stable either.
What IC are you using to generate the square waves? Does it look like a square wave when it is not loaded by the LC circuit? (It doesn't look like a square wave in your pics.)

For the inductor, the few turns you are using store very little energy; to get a ringing display like you expected to see I think you will end up having to use a much larger inductor.

The scope's setting for voltage/div greatly affects the amplified frequency.
You will have to explain what we are to understand by your phrase "amplified frequency".
 
  • #26
Why not use a higher inductance and aim at a lower resonant frequency (is the actual frequency important for your exercise?)? Also, if you want to obtain a better Q factor for your resonator you could consider feeding it via a 'tap' on the inductor - say across the bottom few turns (as in an auto-transformer) or across a large value C (Ten times the resonator C) placed in series with L and C. Damping due to the sig generator will be a lot less of a problem with this arrangement.
 

What is a tank circuit?

A tank circuit is a type of electrical circuit that consists of a capacitor and an inductor connected together in a loop. It is used to create a resonant frequency and is commonly used in radio frequency circuits.

What is the problem with a 0 Hz frequency in a tank circuit?

A 0 Hz frequency in a tank circuit means that the circuit has no resonance and is not functioning properly. This can be caused by a variety of factors such as faulty components, incorrect circuit design, or external interference.

How do you solve a 0 Hz frequency in a tank circuit?

The first step in solving a 0 Hz frequency in a tank circuit is to check all components for any potential issues such as loose connections or damaged components. If everything appears to be in working order, then the circuit design should be double-checked for any errors. If the problem persists, it may be necessary to shield the circuit from external interference or replace components.

What are some common causes of a 0 Hz frequency in a tank circuit?

A 0 Hz frequency in a tank circuit can be caused by a variety of factors, including faulty components, incorrect circuit design, and external interference. Other common causes include poor connections, inadequate power supply, and incorrect tuning of the circuit.

Can a 0 Hz frequency in a tank circuit be harmful?

A 0 Hz frequency in a tank circuit is not necessarily harmful, but it can indicate that the circuit is not functioning properly. If left unresolved, it can lead to inefficiencies in the circuit and potentially cause damage to components. It is important to address and solve the issue to ensure the circuit is operating at its optimal level.

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