How can RLC circuits and LCAM be used together to demodulate AM signals?

[IssacBinary]

Hey everyone,

Ive been learning RLC circuits recently and all is ok now in the understanding side of things. We have also been learning about AM transmition and LCAM. Also how LCAM uses just a diode, capacitor and resistor to demodulate as that circuit will trace the peaks and recreate the signal.

So my first question is, RLC circuits you can use to tune and select frequency's, now would you be tuning it to select a frequency to multiply with the AM signal to recreate the original signal?

Would the LCAM circuit and RLC circuit be used together?

Ive seen several sites on how to make your own AM radio using an RLC circuit, but its using crystal headphones (not to bad) but also a germanium diode (to get the power from the radio signals them self).

Is there a way where we can do away with the sensitive headphones and diode and just use a battery to supply power?

Is it possible to make a receiver with just a resistor inductor and capacitor and without any other parts (apart from battery and basic headphones and wire) using just resonant circuit principles?

What about building a receiver with the LCAM circuit?

Thanks

 

[Jiggy-Ninja]

The tuning circuit and demodulator are the bare minimum that you need to get an audible signal out. If you do it without the demodulator, you'll just be feeding straight RF into the headphones, and I don't know how well they'd handle them.

And if you're going to boost the power with a battery, you'll need amplifier circuitry too.

 

[IssacBinary]

Ah right so the RLC acts like a band pass to select the frequencies straight from the source with the sidebands, and then the diode brings them back down to the original frequency?
(but to do that you multiple the modulated signal by the carrier...so how does the diode the diode do that?)

Ok so what about the LCAM circuit? as it just traces the peaks to recreate the original signal at the correct sound frequencies .

edit: I am talking about series RLC circuit

 

[IssacBinary]

Ok so I just built an AM radio.

I use wire wrapped around a bottle and a diode and used a crystal earpiece.

At first nothing came through, but I live in a very bad area for radio so I got the lead from the aerial from the top of our house and touched the end onto the circuit and I could hear radio!.

At this time nothing was variable, it was just to see what happened but at times I could hear clearly as I was basically holding most of the connections.

But when replaying the aerial with a long wire nothing came through. But that's not to bad at the moment.

So a couple of questions.

How is this actually working?

Because from my understanding I needed a diode with a capacitor and resistor in parallel. This would trace the peak of the signal which would give you the sound. (Im guessing all AM is LCAM)

So what is happening with JUST an inductor and diode?

If I used a variable inductor, that would just do what?
As I though to tune you need an inductor AND capacitor, and then adjusting one would adjust the resonant frequency?
So using a series RLC circuit is just acting as a band pass before it goes into the diode to your ear?

 

[Jiggy-Ninja]

You're going to have to tell me what LCAM stands for, and what it is. I've never heard the term before, and Google searching just brings me right back to this thread.

As a guess, does the "LC" stand for "low carrier" or something?

A tuning circuit requires an inductor and capacitor. You can't get by with just an inductor. Then, you put that signal into a diode detector, which needs a diode, resistor, and another capacitor of its own.

I don't know enough about antennas to help you with your aerial issue.

 

[IssacBinary]

LCAM = Large Carrier Amplitude Modulation. Thats what we where told anyway.

Where before multiplying by a carrier frequency you first add a DC offset. The large carrier part comes in when you multiply the offseted signal creating a high amplitude frequency at the carrier...due to sum and difference of 0hz is going to give you the carrier.

Dont worry about the aerial problem, its just my area.

But I don't know how its working with just an inductor and a diode.

When would the series RLC come into it?

And what about the diode + capacitor and resistor in parallel.

 

[Jiggy-Ninja]

Ah, so just a basic AM signal. I believe that my book calls it Double Side Band Full Carrier, or something like that. There are several other kinds of AM besides LCAM, but as you've found, it looks like broadcasts use LCAM.

The RLC circuit is the tuner, which gets its input right from the antenna. The antenna is going to pick up all of the radio signals in the area. The tuner is needed to filter out all the ones you don't want, so that only one station gets through at a time.

You can't make a tuner with just an inductor, at least not by any method that I know of.

The diode detector comes after the tuner, and is used to demodulate the signal.

You're going to have to draw out what you're thinking, because I don't know exactly what you're talking about with the "just an inductor and diode" stuff.

 

[IssacBinary]

Ah, I thought names wouldn't be totally universal. We were told basic AM is just AM and that to receive it you have to match the original carrier and then multiply the signal by it to bring it back down to the original frequencies. But that isn't used because its expensive compared to LCAM as you need an oscillator to match it 100%...

Anyway,

So I've uploaded a drawing of my circuits.

Number 1:
This is what I made, and I was able to hear radio stations. At first it didn't work, but I touched my outdoor aerial lead onto the circuit and then it was working...but the circuit worked (the aerial problem is just because I am in a bad area, even handing a long wire out my window didnt work)

The earphone is a crystal earpiece, and the inductor is just some wire I wrapped around a bottle. It says to make notches in the windings along the way and you connect the cable to the notches to "tune" it, but I didn't bother with that yet, but it still worked, could hear a lot quite clearly.

Again it was very quickly done, nothing was soldered or taped, just twisted bits of wire together. If I moved the aerial connection slightly or connections slightly I could "tune" it (it would make it louder or clearer) , but I don't know if its just bad connections and it was just getting contact between wires that was causing that.?

...but basically I was picking up sound.

So firstly HOW, how does that work with just an inductor and diode?

From all that I've been taught and my understanding I thought I would have needed circuit number 2. As we were taught this traces the peaks which recreates the original signal. So how is it working without that?

And circuit 3 just a series RLC. Acts like a band pass...so I guess using a variable capacitor you adjust it to find the right channel to pick up right? Without it you would hear multiple stations?...

So I thought the minimum would be number 2, and if you wanted to, hook the aerial to number 3 and then that to number 2 to create a basic tuner.

But circuit 1 is making me stumped.

 

[IssacBinary]

Anyone have any ideas? Or could help me out?

 

[Averagesupernova]

An inductor is never JUST an inductor. There is interwinding capacitance that can form a tuned circuit with the inductor. You may also think about the fact that if you have few stations in your area (but strong) you really shouldn't need a tuned circuit.

 

[IssacBinary]

After some more research, I found that as long as you have a super strong station near by. i.e your standing next to it you can use JUST a diode and earphone.

But then how is that working? As I thought you needed circuit 2 to create the orignal sound signal, otherwise you just get loads of spikes?...Or does it work both ways but circuit 2 just smooths it out to create a clearer signal?

Also, after taking apart an old radio I have and also taking the retactable aerial off. It still worked.

So does that mean even without an actual aerial (so its just using the circuit as an aerial) if you include an amplifier you can get an audible signal?
(so the massive aerials when building your own radio is just so you have enough power from the radio waves to power your earphone without and amplifier?

Sorry for all the questions, I am just trying to piece together what's I've learn and fill in the gaps

 

[IssacBinary]

bump?

 

[jsgruszynski]

After some more research, I found that as long as you have a super strong station near by. i.e your standing next to it you can use JUST a diode and earphone.

As @Averagesupernova says, the "just an inductor" isn't really: it has parasitic capacitances so it's a weak LC filter. The "Q" or bandwidth of the LC is pretty bad so you can pick up more than one station if they are close enough.

But even with just a wire, you aren't tuning at all, but a crystal/diode radio isn't not super sensitive. Because of this, only the strongest nearby station will be detected by the diode. You may get others but they could be so soft you don't hear them below the stronger.

But then how is that working? As I thought you needed circuit 2 to create the orignal sound signal, otherwise you just get loads of spikes?...Or does it work both ways but circuit 2 just smooths it out to create a clearer signal?

The capacitor and resistor are just there to filter out the rectification-created harmonics and residual RF left over after diode rectification and to get rid of side-effects caused by a super strong signal interacting with the diode and itself or another signal: aka intermodulation and harmonic distortion forming "mixer spurs" - the low frequency component created could be in the audio band. Mostly these second-order effect so strictly you can get by without the filter most of the time based on my experience.

Also, after taking apart an old radio I have and also taking the retactable aerial off. It still worked.

Amplifiers. The RF section has gain which picks up stations even without a decent antenna. It's a good selling feature for a radio.

So does that mean even without an actual aerial (so its just using the circuit as an aerial) if you include an amplifier you can get an audible signal?
(so the massive aerials when building your own radio is just so you have enough power from the radio waves to power your earphone without and amplifier?

Sorry for all the questions, I am just trying to piece together what's I've learn and fill in the gaps

Basically yes. With a near-enough and strong-enough station no antenna is needed or rather no external antenna: the wires connecting to the antenna connection work as an antenna well enough for a strong signal. It can be (but isn't always) enough to require an amplifier.

For example, AM broadcast is pretty strong and expected to be local. But with distance or if you are trying to pick up AM in the shortwave band instead, you'll need amplification and a big antenna together.

 

[IssacBinary]

AH I see,

So with "just" an inductor the aerial acts as the capacitor?

But as its harder to use a variable inductor, (giant sliding rod or having notches) another capacitor is added and that is a variable one to be the tuning part as its easier and smaller (just turn the plates)

So I am guessing if you take the aerial away, (And the signal was strong enough to be detected by the circuit itself) the inductor on its own acts as a capacitor due to the coils near each other simulating plates? And thus it would still resonate?

So then,

If I built a circuit with an inductor and variable capacitor with NO aerial, but I had an amplifier would it be enough to work? (even though using a crystal earpiece doesn't detect anything?)

Lastly...

The crystal earpieces you buy in the shops, are they any better than the 1920s high impedance headphones? Better as in sensitivity (im guess the headphones are clearer?)

 

[jsgruszynski]

AH I see,

So with "just" an inductor the aerial acts as the capacitor?

BTW the http://en.wikipedia.org/wiki/Crystal_radio" is pretty good.

An inductor does have parasitic capacitance but it may or may not result in a resonant frequency where you want them (thus the radio is untuned) or the resonance is so broad it doesn't actually tune to a specific station but instead more than one.

But as its harder to use a variable inductor, (giant sliding rod or having notches) another capacitor is added and that is a variable one to be the tuning part as its easier and smaller (just turn the plates)

A parallel LC looks like an open circuit to the resonant frequency but looks like a short (antenna to ground) to any other frequency. This is the "normal" way of tuning the RF in a crystal radio.

If you look at the above Wiki page, the first diagram has only a coil in parallel with detector and phones. The parasitic capacitances are in parallel with incremental inductances per winding turn so the entire coils is approximately a parallel LC.

So I am guessing if you take the aerial away, (And the signal was strong enough to be detected by the circuit itself) the inductor on its own acts as a capacitor due to the coils near each other simulating plates? And thus it would still resonate?

Resonance isn't necessary to make a radio receive signals. It simply allows you to separate multiple stations from one another. You can make a radio without any LC (just an antenna to a diode) and what you'll get is all the power stations jumbled together.

This ability to separate signals is "selectivity".

So then,

If I built a circuit with an inductor and variable capacitor with NO aerial, but I had an amplifier would it be enough to work? (even though using a crystal earpiece doesn't detect anything?)

It can if there is still enough RF radiation captured. For an antenna, especially a "long wire", this is proportional to the length of the wire down. Basically a longer antenna "collects" more current so the signal is stronger. A very short wire may capture enough.

Without some captured energy (some antenna action) however, no amount of amplification can make up for no or too small a signal. And on top of that there is a noise floor that limits how low a signal the amplifier can amplify.

Best to think of it as an antenna still being there but maybe not an obvious one. Just the connector for the antenna and the wiring up to the coil may be enough, or the coil itself can sometimes work as an antenna (a magnetic antenna rather than an electric antenna - you can have either because radio is about electromagnetic waves - there is both an electric field and a magnetic field in the wave).

Lastly...

The crystal earpieces you buy in the shops, are they any better than the 1920s high impedance headphones? Better as in sensitivity (im guess the headphones are clearer?)

The "Earphones" section of the above wiki page talks about this.

 

[IssacBinary]

Arh its all very confusing at times. But I think I am getting there.

I understand that you you can receive singles with just a wire and diode, but it would be basically every station all overlaid.

But then to get ANY kind of tuning, you would need some form of capacitor and inductor?...Even if its "just" an inductor..and the capacitance is from the aerial?

BUT in this case,
http://www.radiointel.com/review-amloop.htm

The looped aerial acts as the inductor part also, so only a variable capacitor is needed.

I understand the parallel LC circuit for tuning now I believe. As when the resonant frequency is being received its causing the LC branch to resonant causing high resistance ... the way I see it, is that because its resonating, there is current already there so no current needs to go down that path. And when its not resonating its empty so the current splits...

So how would you, if at all possible could you use a series LC to tune?...Im guessing at resonance the impedance is the lowest so maximum current flows through. So you would have the aerial LC and headphones all in series?

As for the circuit 2 in my picture, I don't quite understand the intermodulation and harmonic distortion forming "mixer spurs" that occurs without using a capacitor and resistor...but basically using the capacitor and resistor just improves the sound quality?

Or without it are other frequencies being created some how from the intermodulation or is it just less clear because of the spikes and not a smooth tracing of the tops...

What I am not quite understanding is that even though the peaks recreate the original signal, using just a diode you would still be able to hear sound because its still spikes of the high carrier frequency which frequency doesn't change and its also a non audio frequency. So I was surprised to be able to hear something without the capacitor tracing the peaks to create the audio frequencies...does that make sense?I guess I might be a bit of a pain but I really appreciate the help from everyone, without this site id just be for ever stuck. As the more I read the more questions I get and there wouldn't be anyone to turn to. :)