B Why does my LC circuit not oscillate its energy between Electric & Magnetic fields?

[sophiecentaur]

I remember a similar experiment that I tried, which failed. Anyone who attempts an experiment like this, and questions the result, deserves every support. All the indications are, that they have the critical thinking skills needed, to make a great engineer or physicist one day.

To get a good result for this sort of 'obvious' phenomenon, you need the experience to make the right sort of guesses about what frequency to use, what inductor to use and how to display the result. That calls for a fair bit of experience or enthusiasm wanes before getting a decent result. (And a large source / choice of component values.) Plus you need the appropriate measuring equipment. A DVM makes things very difficult.

I (also) remember a similar play I had - that time with coupled LC oscillators which passed energy between them and back again (analogue of the two pendulums experiment). The main thing was to find suitable audio frequency inductors and appropriate coupling. The (successful) result was two strings of beads on two scope traces, which started with an impulse (switch off the DC supply) and gradually decayed. It lasted just long enough to satisfy me and impress colleagues who would also have done the same thing, given the time and inclination.

 

[Baluncore]

I did not use the values specified in the diagram... only the arrangement of circuit elements.

That circuit, with your changed values, will oscillate at about 63 kHz.
What frequency do you want ?

 

[Averagesupernova]

Ok, how do I know if the circuit oscillates with LC oscillations or not?

Do not try to build an oscillator that runs at radio frequencies. Start with audio which a cheap voltmeter is able to measure.

 

[James2018]

So C=Ca*Cb/(Ca+Cb) =(15*220) /(15+220) =14.0425 nF
The resonant frequency is f=1/(2*pi*sqrt{L*C}) = 61.951 KHz so approximately 62 KHz.
Thank you

 

[Sagittarius A-Star]

So C=Ca*Cb/(Ca+Cb) =(15*220) /(15+220) =14.0425 nF
The resonant frequency is f=1/(2*pi*sqrt{L*C}) = 61.951 KHz so approximately 62 KHz.
Thank you

That result is correct.

I propose that you use the next time for better readability LaTeX for formulas (see link to a LaTeX Guide below the input window and have esecially a look at chapter "Delimiting your LaTeX code").

In the middle term of your first formula, the unit $nF$ is missing.

 

[sophiecentaur]

Do not try to build an oscillator that runs at radio frequencies. Start with audio which a cheap voltmeter is able to measure.

Also bear in mind the old adage "Amplifiers oscillate and oscillators amplify"
Modern transistors (since 1955) can have an embarrassing amount of gain at high frequencies. They will 'see' signals, fed back from long bits of circuit wiring and wires laying close to each other and decide to oscillate many octaves above what you wanted. Short leads and a ground plane are not always available on a breadboard.

 

[Baluncore]

"Amplifiers oscillate and oscillators amplify"

There are many oscillators named after hopeful amplifier inventors, those who failed to tame positive feedback. Those oscillators are never simple, as they always have a subtle feedback mechanism. I would list some here, in a "rogues gallery", but that would not be fair, as I would probably forget to mention a dozen or more.

There are very few amplifiers named after their inventors. Can you name three?

 

[Sagittarius A-Star]

Video about a Colpitts oscillator with practical hints:

 

[James2018]

The multimeter detects the resulting output as being a voltage that oscillates between + 120 milliVolts and -120 milliVolts, far lower than the input 9 Volts, although the current from the battery reaches 7 milliAmperes. And I did use the capacitance values from the diagram, four 0,1 microfarads capacitors and one 1 microfarad capacitor.

The only thing I didn't follow was the resistance values, as I didn't find resistors for sale with these exact values: 2.2KΩ, 4.7KΩ, 10KΩ and 560 Ω.

Also I did use a 1.35 microhenries inductor to obtain a resonant frequency of 612,6 KHz for the tank circuit, given the effective capacitance of 50 nF.

 

[Sagittarius A-Star]

The multimeter detects the resulting output as being a voltage that oscillates between + 120 milliVolts and -120 milliVolts, far lower than the input 9 Volts
...to obtain a resonant frequency of 612,6 KHz

What is the AC frequency response of your multimeter?

See
https://electronics.stackexchange.c...-a-dmm-decrease-as-the-frequency-is-increased

 

[James2018]

What is the AC frequency response of your multimeter?

See
https://electronics.stackexchange.c...-a-dmm-decrease-as-the-frequency-is-increased

It's not that, but I keep the circuit turned on 24 hours out of 24, the batteries run out. And when I replace with new batteries, the output voltage is 0.23 V AC or so. I know the multimeter does not record the values like an oscilloscope, but the fact that voltage oscillates and changes polarity is a sign that the output current is AC.

 

[Baluncore]

So, have you listened to the 612 KHz oscillator with your AM receiver?

I didn't find resistors for sale with these exact values: 2.2KΩ, 4.7KΩ, 10KΩ and 560 Ω.

Those are E12 standard resistor values.
https://en.wikipedia.org/wiki/E_series_of_preferred_numbers#E12

 

[marcusl]

There are many oscillators named after hopeful amplifier inventors, those who failed to tame positive feedback. Those oscillators are never simple, as they always have a subtle feedback mechanism. I would list some here, in a "rogues gallery", but that would not be fair, as I would probably forget to mention a dozen or more.

There are very few amplifiers named after their inventors. Can you name three?

Circuits were only named after their inventors in the early days of radio and electronics (Doherty amplifier of the 1930s, e.g., still used today for some high power RF transmitters). Names are rare because many inventors came up with their own fanciful descriptors (Lee de Forest called his triode amplifier valve an Audion tube, Edwin Armstrong called his first amplifier a regenerative amplifier, etc.). Names are rarely attached in modern times as this tradition continues. Charles Townes called his parametric microwave amplifier a maser, for example, and Robert Dicke called his radiometer amplifier a lock-in. And sometimes you can’t attach a single name. The invention of the DC SQUID amplifier was shared by a team of something like six researchers.

 

[James2018]

So, have you listened to the 612 KHz oscillator with your AM receiver?


Those are E12 standard resistor values.
https://en.wikipedia.org/wiki/E_series_of_preferred_numbers#E12

Yes
Without circuit at 612 KHz AM the spectrum is

With circuit turned on the spectrum is

I would say in the presence of the circuit the radio speaker has some frequency spikes at 565, 599, 1217, 1396 regarding the sound spectrum.

 

[Baluncore]

That audio spectrum is next to useless in assessing the RF spectrum.

When tuned to a nearby unmodulated RF oscillator, an AM radio should be almost silent.
Only the oscillator power supply ripple, at double the power frequency, may show in the audio spectrum, but you are using a battery supply, so the audio should be quite silent.

When no carrier signal is present, the AM radio will turn up its Automatic Gain Control, AGC, to amplify the weaker transmissions and atmospheric noise, that should then fill the audio with detail.

I suspect the noise you are seeing in the audio spectrum, is radio interference from the computer you are using to record the audio spectrum. That RFI is dominating the oscillator you have built.

Try it with the computer turned off. See if you can tune to a quiet spot, with the oscillator close to the AM radio antenna. To verify reception, turn the oscillator off and on again.

 

[James2018]

That audio spectrum is next to useless in assessing the RF spectrum.

When tuned to a nearby unmodulated RF oscillator, an AM radio should be almost silent.
Only the oscillator power supply ripple, at double the power frequency, may show in the audio spectrum, but you are using a battery supply, so the audio should be quite silent.

When no carrier signal is present, the AM radio will turn up its Automatic Gain Control, AGC, to amplify the weaker transmissions and atmospheric noise, that should then fill the audio with detail.

I suspect the noise you are seeing in the audio spectrum, is radio interference from the computer you are using to record the audio spectrum. That RFI is dominating the oscillator you have built.

Try it with the computer turned off. See if you can tune to a quiet spot, with the oscillator close to the AM radio antenna. To verify reception, turn the oscillator off and on again.

The multimeter certainly detects a voltage that oscillates between + 0.1 Volts and - 0.1 Volts at the antenna of my oscillator, although random values in this range at random times because cannot measure continously like an oscilloscope can,
while detecting + 2.5 Volts near the battery terminal. So I guess AC is present.
Perhaps the radio signal is too weak in Watts to be detected by the radio receiver. Perhaps static noise is stronger in Watts.

 

[Baluncore]

Perhaps the radio signal is too weak in Watts to be detected by the radio receiver. Perhaps static noise is stronger in Watts.

If the oscillator functioned, it would be heard, if the radio was tuned to that frequency.
Try turning off the computer, then find the oscillator with the AM radio.

 

[James2018]

If the oscillator functioned, it would be heard, if the radio was tuned to that frequency.
Try turning off the computer, then find the oscillator with the AM radio.

I took the computer and the radio receiver to a different room than the oscillator. The computer perturbs the radio receiver with a beep on top of the static noise. My circuit creates a more loud buzzing sound like a bee on the radio receiver. Perhaps my circuit's antenna does not resonate to 612 KHz, perhaps I should use a different antenna length, for only a tone to be heard on the radio receiver.

 

[Baluncore]

I took the computer and the radio receiver to a different room than the oscillator.

That was a real bad move.
Also, if you hear a buzz from a battery powered oscillator, something is very wrong.

A good AM receiver should have noise across the RF band, except where your AM radio is tuned to the oscillator, as the oscillator there will desensitise the AM receiver, giving you silent audio.

You must turn off the computer and perform the above test with the oscillator close to the AM radio antenna.

 

[James2018]

That was a real bad move.
Also, if you hear a buzz from a battery powered oscillator, something is very wrong.

A good AM receiver should have noise across the RF band, except where your AM radio is tuned to the oscillator, as the oscillator there will desensitise the AM receiver, giving you silent audio.

You must turn off the computer and perform the above test with the oscillator close to the AM radio antenna.

I found a similar experiment, where a Collpits oscillator is heard on radio at time 4:15 in the video. Perhaps my antenna is not impendance matched. But good news, my multimeter value oscillates even faster in values and polarity than before when I use two 3 V coin batteries on top of each other, giving an input voltage of +5.21 volts and an output AC voltage of between +1.35 and -1.35 Volts.

 

[James2018]

I have an idea though, what if I connect my headphones to the AC output of my oscillator? won't the AC be converted to audio?

 

[Baluncore]

Some of us have been working with radio frequency systems for more than sixty years. We do not need to look at examples on YouTube.

Have you yet heard that quietening when your AM radio is tuned to your oscillator?

I have an idea though, what if I connect my headphones to the AC output of my oscillator? won't the AC be converted to audio?

NO.

 

[James2018]

Some of us have been working with radio frequency systems for more than sixty years. We do not need to look at examples on YouTube.

Have you yet heard that quietening when your AM radio is tuned to your oscillator?


NO.

Ok, I have not heard that silence on radio but what if my signal is not strong enough to be picked up by the radio? Maybe I need an amplifier to boost the signal before emitting it. Or if the capacitors have tolerances of 10%? The code 104K on the capacitors means they have a 10% tolerance, their capacitance is not exactly 0.1 uF. So the resonant frequency won't be exactly 612.6 KHz.

" In addition, by varying the amplitude of the broadcast signal for AM radio, the power at which that signal is broadcast is also changed, since amplitude represents the strength of the signal. Thus, instead of picking up low amplitude signals, some radios may pick up no signal at all. FM radio, on the other hand, always remains at a constant amplitude, so the strength of the signal does not change." ( https://illumin.usc.edu/catch-a-wave-radio-waves-and-how-they-work/ )


Also,

"
The third reason AM radio does not sound as good is that AM radio tends to have a lot of interference. There are many natural sources of radio waves, most of which are AM waves, and your radio is unable to differentiate between natural AM waves and those being broadcast by radio stations. Perhaps you have noticed that AM radio tends to sound better at night than during the day. The reason for this is that the sun is a big source of natural AM radio waves.
"

 

[Baluncore]

So the resonant frequency won't be exactly 612.6 KHz.

612 kHz is at the low frequency end of the AM BC band. Maybe you should reduce the two capacitor values, to move the frequency up near 1 MHz. Tolerance will then not be a problem. If you cannot hear it then, you have a problem with your oscillator.
You will need to sweep across the band to find the oscillator frequency.

If all that fails, you will need an good oscilloscope, or lower the frequency down to a couple of kilohertz where you can hear it without the radio.

What sort of AM radio are you using?

The third reason AM radio does not sound as good is that AM radio tends to have a lot of interference.

You need that interference, so it will only be quiet when you are tuned to your oscillator. How good AM radio sounds compared to FM is irrelevant.

 

[James2018]

612 kHz is at the low frequency end of the AM BC band. Maybe you should reduce the two capacitor values, to move the frequency up near 1 MHz. Tolerance will then not be a problem. If you cannot hear it then, you have a problem with your oscillator.
You will need to sweep across the band to find the oscillator frequency.

If all that fails, you will need an good oscilloscope, or lower the frequency down to a couple of kilohertz where you can hear it without the radio.

What sort of AM radio are you using?


You need that interference, so it will only be quiet when you are tuned to your oscillator. How good AM radio sounds compared to FM is irrelevant.

There may be a problem, the multimeter detects an output voltage on the antenna that oscillates in strength and polarity (AC) even if I remove the inductor altogether and open the circuit where the inductor was.

 

[Baluncore]

Do not use a multimeter to measure RF.
Check your battery voltage.

What do you mean by "the antenna"? The oscillator inductor will radiate more than sufficient signal to the AM radio.

 

[James2018]

Do not use a multimeter to measure RF.
Check your battery voltage.

What do you mean by "the antenna"? The oscillator inductor will radiate more than sufficient signal to the AM radio.

The voltage measured with the multimeter connected in parallel at the wires exactly near the battery, is always fixed and +5.21 volts. The output wire, where the waves are represented as A,B,C,D is where the multimeter measures an output voltage changing in value and sign, in strength and polarity between +1 V and -1 V.

 

[Baluncore]

How does your simulation know that the negative of the battery is the ground reference?

If you are getting ± voltages on the inductor, then I guess you must be using a DC range on the meter to measure the AC.
You may not be aware that the inductor is floating, without DC bias, since every DC bias path is blocked by a capacitor. That might explain partly why your meter is confused.

Measure the DC voltage of the emitter and collector of Q1 to ground. That should identify if the transistor might be oscillating.

 

[James2018]

How does your simulation know that the negative of the battery is the ground reference?

If you are getting ± voltages on the inductor, then I guess you must be using a DC range on the meter to measure the AC.
You may not be aware that the inductor is floating, without DC bias, since every DC bias path is blocked by a capacitor. That might explain partly why your meter is confused.

Measure the DC voltage of the emitter and collector of Q1 to ground. That should identify if the transistor might be oscillating.

Yes, every path from the battery to the antenna is blocked by a capacitor connected in series. DC voltage cannot pass through a capacitor connected in series. Why I think that? Because DC charges the capacitor then the capacitor stays charged and acts like an open circuit.

Only AC voltage can pass through a capacitor because it regularly charges and discharges the capacitor. The base of the transistor is connected in series to the capacitor C1, the collector to the capacitor C4 and the emitter to C3.

Am I wrong? Can DC voltage pass through a capacitor connected in series?

 

[James2018]

Ok, here is the AC output interpreted by the sound card of my computer as "Line In" signal through a Jack connector. It is a buzzing sound, just like on the radio receiver. Like a bee.
Spectrum:

Waveform:

 

[Baluncore]

Yes, every path from the battery to the antenna is blocked by a capacitor connected in series. DC voltage cannot pass through a capacitor connected in series. Why I think that?

It does not matter why you think that, but you can throw out one of those two coupling capacitors, either the one on the collector or the base. The two tuning caps must stay, with the same ratio. I would throw out the collector coupling capacitor, C4.

Ok, here is the AC output interpreted by the sound card of my computer as "Line In" signal through a Jack connector. It is a buzzing sound, just like on the radio receiver. Like a bee.

You need to turn off the computer and stop plotting meaningless spectra. Those fascinating distractions, will only confuse you.

You must listen with your ear, for quietening of the AM audio, when tuned to the oscillator. That quieting should be replaced by noise when the oscillator is turned off.

I ask again; what sort of AM radio are you using?

 

[James2018]

It does not matter why you think that, but you can throw out one of those two coupling capacitors, either the one on the collector or the base. The two tuning caps must stay, with the same ratio. I would throw out the collector coupling capacitor, C4.


You need to turn off the computer and stop plotting meaningless spectra. Those fascinating distractions, will only confuse you.

You must listen with your ear, for quietening of the AM audio, when tuned to the oscillator. That quieting should be replaced by noise when the oscillator is turned off.

I ask again; what sort of AM radio are you using?

This sort of AM radio. But what IF I am using the wrong resistor values in the circuit, too low resistance, 56 ohms, 13 ohms, what if I need to use a lot higher? I thought a lot higher resistance would stop current from flowing at all.

 

[Tom.G]

But what IF I am using the wrong resistor values in the circuit, too low resistance, 56 ohms, 13 ohms, what if I need to use a lot higher?

So, are you saying the diagrams you have supplied do not match what you have built?!!

That is sort of like putting a bicycle tire on a car and wondering why it always looks flat.

 

[Baluncore]

While the oscillator will not work if the DC bias is too far out, resistor values are a red-herring. Your oscillator will produce sufficient signal for the AM radio, when you sit them next to each other. You just need to turn the computer off, and make an oscillator that operates in the MW band.

 

[Baluncore]

The component values you have used are a bit of a mystery.
It is important that you list the values you have actually used, before progressing.

The inductor value is critical. What is the value ?
The inductor you show in the photo, in post #47, just does not look sufficient for the bottom of the MW band.

 

[James2018]

The component values you have used are a bit of a mystery.
It is important that you list the values you have actually used, before progressing.

The inductor value is critical. What is the value ?
The inductor you show in the photo, in post #47, just does not look sufficient for the bottom of the MW band.

Finally I have received on the AM radio, a sound beep composed of three sound frequency spikes: 1000 Hz, 2000 Hz and 3000 Hz. 1000 Hz was the loudest, 24 decibels louder than the rest of the spectrum. I have moved the circuit away from other devices to hear the beep on the AM radio, then a minute later I recorded the beep. Although the AM radio frequency was more driven towards 700 KHz (somewhere in the middle between 600 and 700) rather than 612 KHz. I guess that would be the 10% tolerance of the capacitors.

 

[James2018]

The component values you have used are a bit of a mystery.
It is important that you list the values you have actually used, before progressing.

The inductor value is critical. What is the value ?
The inductor you show in the photo, in post #47, just does not look sufficient for the bottom of the MW band.

The value is 1.35 uH but I cannot show it because it is covered in a yellow insulator tape. It has 37 turns, 0.4 cm radius and 6,4 cm length. I calculated it here: https://www.allaboutcircuits.com/tools/coil-inductance-calculator/

The capacitors have the code 104K which means 0,1 uF with tolerance 10% except for C3 which has 1 uF. All resistors have 56 ohms with exception for resistor R3 which has 33 ohms.

 

[davenn]

Finally I have received on the AM radio, a sound beep composed of three sound frequency spikes: 1000 Hz, 2000 Hz and 3000 Hz. I have moved the circuit away from other devices to hear the beep, then a minute later I recorded the beep. Although the AM radio frequency was more driven towards 700 KHz rather than 612 KHz. I guess that would be the 10% tolerance of the capacitors.

Any beeps or buzzes etc you hear IS NOT your oscillator.... its coming
from something else On your work bench.... probably the computer.
Which Baluncore has also told you to turn off.
Baluncore has told you multiple times that you need to tune the radio
carefully to find the quiet spot = the oscillator. If you don't find that spot then your oscillator isn't working OR it is on some far away frequency.
You need an oscilloscope else you will not figure out what is happening with your circuit.
You also need to start listening carefully to what people are trying to teach you and answer their questions instead of responding with random comments

Cheers
Dave

 

[James2018]

Any beeps or buzzes etc you hear IS NOT your oscillator.... its coming
from something else On your work bench.... probably the computer.
Which Baluncore has also told you to turn off.
Baluncore has told you multiple times that you need to tune the radio
carefully to find the quiet spot = the oscillator. If you don't find that spot then your oscillator isn't working OR it is on some far away frequency.
You need an oscilloscope else you will not figure out what is happening with your circuit.
You also need to start listening carefully to what people are trying to teach you and answer their questions instead of responding with random comments

Cheers
Dave

Yes, but I cannot find the quiet spot anywhere between 530 KHz and 1700 KHz. I did follow the online scheme for the Collpits oscillator except I used lower resistances. If it did work for the one who invented the scheme, why wouldn't it work for me?

 

[davenn]

Yes, but I cannot find the quiet spot anywhere between 530 KHz and 1700 KHz.

Then, as I said, either the oscillator isn't working, or if it is working, its frequency is outside of that range.
You need an oscilloscope to determine what your circuit is doing

 

[Tom.G]

View attachment 351647
If that spectrum is realistic, I suspect that those low resistor values have made the circuit into an RC oscillator.

I recommend that you put the correct resistors (as shown in the simulation) in the circuit; you might actually get the expected results. (The batteries will last longer too.)

Just for grins, change the resistor values in the simulation to the values you actually used. It would be interesting to see what results you get.

Cheers,
Tom

 

[James2018]

View attachment 351647
If that spectrum is realistic, I suspect that those low resistor values have made the circuit into an RC oscillator.

I recommend that you put the correct resistors (as shown in the simulation) in the circuit; you might actually get the expected results. (The batteries will last longer too.)

Just for grins, change the resistor values in the simulation to the values you actually used. It would be interesting to see what results you get.

Cheers,
Tom

Yes, that is what I am going to do, I am going to make a simulation of the actual circuit and see what it does.

 

[James2018]

Oops, seems resonant frequency of my circuit is actually 40 Hertz. In this simulation I used every value for resistor, capacitance and inductance and input DC voltage like in real circuit.

Here is the animation to see AC is present in the tank circuit

 

[James2018]

Antenna oscilloscope - not a pure sine wave, but beats

 

[Baluncore]

The Colpitts circuit you are using has bias voltage dependent on the Re and Rc values.
Here is a different Colpitts circuit, one that I prefer, it uses fewer components and the bias is not dependent on the exact resistance values. Design is for 1 MHz. It has no antenna, the RF output is from the coil current of ±2.7 mA peak, while the circuit runs on less than 0.5 mA.

 

[James2018]

The Colpitts circuit you are using has bias voltage dependent on the Re and Rc values.
Here is a different Colpitts circuit, one that I prefer, it uses fewer components and the bias is not dependent on the exact resistance values. Design is for 1 MHz. It has no antenna, the RF output is from the coil current of ±2.7 mA peak, while the circuit runs on less than 1 mA.

View attachment 351662

And it doesn't matter what resistance values I use, at all? And what values are for capacitance for C1, C2, C3? I don't understand the codes.

 

[Baluncore]

C1, C2, C3? I don't understand the codes.

The convention on circuit diagrams is to move the SI prefix to where the decimal point was. That prevents problems with missing decimal points.
C1 = 0u1 = 0.1 uF = 100 nF.
C2 = 1n0 = 1.0 nF.
C3 = 220p = 220 pF = 0n22.

Keep R1 = R2, at about 33k. Anything from 22k to 47k is OK.
Make R3 about half of R1. 12k to 22k should be OK.

 

[James2018]

The convention on circuit diagrams is to move the SI prefix to where the decimal point was. That prevents problems with missing decimal points.
C1 = 0u1 = 0.1 uF = 100 nF.
C2 = 1n0 = 1.0 nF.
C3 = 220p = 220 pF = 0n22.

Keep R1 = R2, at about 33k. Anything from 22k to 47k is OK.
Make R3 about half of R1. 12k to 22k should be OK.

But my battery cannot deliver currents through resistors with values above 1k. It is not alkaline. I will try but I have to order online these high resistance values they are not found in the local electronics shop. But wait I can make them out of graphite and paper. https://pxt.azureedge.net/blob/fe8d...tutorial/make-a-resistor/clip-to-resistor.jpg

 

[davenn]

But my battery cannot deliver currents through resistors with values above 1k. It is not alkaline. I will try but I have to order online these high resistance values they are not found in the local electronics shop.

Rubbish .... what sort of battery ?

 

[Baluncore]

But my battery cannot deliver currents through resistors with values above 1k. It is not alkaline

Your battery is 5 volts, is it not?
Ohms law applies. It is low value resistors you must avoid.
Your battery may have trouble supplying the current of 45 mA needed for your 56 ohm base bias circuit.