All About Circuits

Modulation?

M

Thread Starter

Mathematics!

Joined Jul 21, 2008
1,036
Ok , so I made an ossilator circuit. (it basically outputs a carrier wave around 57Mhz )
I want to know how I can modulate the output 57Mhz carrier wave of this circuit using AM.

So basically what do I add on to the output to do AM modulation to my carrier wave?

Attached is the LTSpice file
Also is their any LTSpice component for mic's

Also I am curious why do we need to use a carrier wave couldn't we just amplify the 3.4khz voice signal and send that out the antenna.
Just have a reciever listening for that wave.... Or another words just have the voice signal go thru the air and not the carrier with the voice on it like always???
Curious?


Hope everybody had a merry xmas
Thanks again for your help
 

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M

Thread Starter

Mathematics!

Joined Jul 21, 2008
1,036
here is the best I can do right now.
This is a screen shot of it

Normally you can do it by putting the mic at the base of the transistor and this will control the voltage which modulates AM and also modulates it FM as well.

But I am trying to learn how to build transmitters in stages...
i.e first create an ossilator circuit ,then create the modulation circuit , then the amplifier / antenna circuit ....

Thanks for your help
I have tested this out and it is correctly ossilating now I just have to modulate onto this output
 

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M

Thread Starter

Mathematics!

Joined Jul 21, 2008
1,036
How can I modify this ossilator circuit so it modulates AM.

I saw your link but I really want to understand ever component...
Right now I have a Colpitts ossilator circuit

is their anyway I can add to the output to vary the amplitude
Or if I have to add something to the ossilator circuit at the base of the transistor....

I am trying to learn from scratch not copy somebody's circuit completely
 
Wendy

Wendy

Joined Mar 24, 2008
23,421
You could use a balanced mixer if the power levels are low. Another traditional way is to use a variable device such as a transistor as one leg of an attenuator.
 
PRS

PRS

Joined Aug 24, 2008
989
Mathematics! said:
Ok , so I made an ossilator circuit. (it basically outputs a carrier wave around 57Mhz )
I want to know how I can modulate the output 57Mhz carrier wave of this circuit using AM.

So basically what do I add on to the output to do AM modulation to my carrier wave?

Attached is the LTSpice file
Also is their any LTSpice component for mic's

Also I am curious why do we need to use a carrier wave couldn't we just amplify the 3.4khz voice signal and send that out the antenna.
Just have a reciever listening for that wave.... Or another words just have the voice signal go thru the air and not the carrier with the voice on it like always???
Curious?


Hope everybody had a merry xmas
Thanks again for your help
Click to expand...
You need to mix the rf (radio frequency) signal with an audio signal. There are a variety of ways to do this.

As for sending out a 3.4kHz signal all by itself that is only possible with CW (morse code) and lots of power, and really big antennas. And that costs alot of money. Also, if everyone did that how in the world could we have selectivity such that we could discriminate between one signal and the other. Believe me, it is all worked out. If you get the book I recommend it will explain it.
 
electrojim

electrojim

Joined Jun 3, 2009
12
What's your end-use application? An AM transmitter generally does not modulate the oscillator or intermediate amplifier stages; rather, it uses "high level" modulation of the final amplifier stage for greatest efficiency, which means that you need an audio amplifier capable of about the same power output as you want to put into your antenna. Tell us what you want to do and we can recommend a course of action. If you want to play DJ on the AM broadcast band, take great care; the FCC is nailing people right and left nowadays, even so-called "legal" Part 15 (100mW) broadcasters with type-accepted equipment. You don't need a great deal of power to cover a square mile, for example. Attention to your antenna and grounding system will prove most useful.
 
M

Thread Starter

Mathematics!

Joined Jul 21, 2008
1,036
You need to mix the rf (radio frequency) signal with an audio signal. There are a variety of ways to do this.
Click to expand...
Take a look at this. It seems this would work to mix the signals.

Notice that the output from the soundcard is merge with the output of the ossilator the end output would be the modulated waveform.

Am I missing anything or will this work. I believe it should work for AM.
All I did is mix the 2 output sources. One being my computers speaker port the other being the ossilator output

What's your end-use application? An AM transmitter generally does not modulate the oscillator or intermediate amplifier stages; rather, it uses "high level" modulation of the final amplifier stage for greatest efficiency, which means that you need an audio amplifier capable of about the same power output as you want to put into your antenna. Tell us what you want to do and we can recommend a course of action. If you want to play DJ on the AM broadcast band, take great care; the FCC is nailing people right and left nowadays, even so-called "legal" Part 15 (100mW) broadcasters with type-accepted equipment. You don't need a great deal of power to cover a square mile, for example. Attention to your antenna and grounding system will prove most useful.
Click to expand...
Well I was think of modulating before the amplifing stage but I general don't care which way I do it.
But curious to know why modulating after amplification is beter then before?
 

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Wendy

Wendy

Joined Mar 24, 2008
23,421
I could be wrong, but I don't think that will work. The term mix in electronics has two distinct meanings. One is a simple adding of frequencies, which is what you have. The other is to heterodyne the two together, which is what balanced mixer does.

Heterodyning produces the sum and the difference of the two frequencies, the very definition of AM.

*****************

Looking at it again, you don't want a balanced mixer.

http://en.wikipedia.org/wiki/Frequency_mixer

The reason is it suppresses the two input frequencies, and only generates the sum and the difference. This is excellent for many applications, but you need to keep your carrier intact in the signal.

I'll think about it, but simply adding two frequencies without heterodyne will not work.
 
M

Thread Starter

Mathematics!

Joined Jul 21, 2008
1,036
I could be wrong, but I don't think that will work. The term mix in electronics has two distinct meanings. One is a simple adding of frequencies, which is what you have. The other is to heterodyne the two together, which is what balanced mixer does.

Heterodyning produces the sum and the difference of the two frequencies, the very definition of AM.
Click to expand...
wait isn't my way simply adding the carrier sine wave with the audio wave of the soundcard. How is this different then what heterodyneing does?

Is their some simple product mixer I can create to combined the carrier with the audio soundcard wave.
I was looking at this

http://www.radio-electronics.com/in...alanced_mixer/diode_single_balanced_mixer.php

I realize I was just adding the 2 waves together when I needed to be producting them together.
So their must be an easy way to make a mixer to do product mixing .... any help would be great
 
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Wendy

Wendy

Joined Mar 24, 2008
23,421
The sum and the difference of the two frequencies are created. It is what AM literally produces, and it is also the definition of heterodyne.

http://en.wikipedia.org/wiki/Amplitude_modulation

With heterodyne, two completely new frequencies are created.

If you amplitude modulate a 1Mhz signal with a 1Khz signal, two sidebands are created, 990Khz and 1.001Mhz, the difference and the sum. Interestingly if you graph out either side band with 1Mhz, you can see the classic AM modulation pattern.

AM is created by putting both frequencies through a nonlinear device. It can also be created by varying the gain of an RF amp with the audio signal.
 
M

Thread Starter

Mathematics!

Joined Jul 21, 2008
1,036
Ok , so all I need is to create a mixer that products the 2 waves together instead of adding them together like I originally had.

Is their any simple product mixer I can build to take in the carrier and audio from the soundcard and produce the modulated signal?

Basically I am asking how you create a simple product mixer?
 
Wendy

Wendy

Joined Mar 24, 2008
23,421
I was thinking about that. There are several approaches, some better than others.

I don't have any plans handy. A simple diode will mix two frequencies together, but it will be ugly and not very satisfying.

Another way may be to go with a transistor biased in a linear mode, as its CE resistance goes down it will attenuate the signal more then if it doesn't. I don't know if this will work, but here goes.



The top schematic is a variable attenuator. The bottom is a transistorized version. I don't know if this will work, but it is the best I could come up with on a moments notice.

It is also possible if you introduce your audio signal into the base of your original oscillator (attenuated and capacitive coupled) you might get the same effect.
 

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electrojim

electrojim

Joined Jun 3, 2009
12
Sorry, but "mixing" will not do the job. You will have both the RF and the audio signals at the same point at the same time, but the RF will not be modulated by the audio. The 'balanced modulator' will, as pointed out, automatically suppress both the audio and the RF, giving you sidebands without a carrier, which is not the AM you want. (You can, however, offset the balanced modulator with a DC bias and get good AM. This is not generally done with transmitters, but common in modulated signal generators.)

If you want true amplitude modulation (that can be heard on an AM radio), then the audio signal must change the amplitude of the carrier signal in accordance with the program source. In your original diagram, if you connect a transformer secondary in series with the power supply voltage source, and apply the audio to the transformer's primary, you will get amplitude modulation, more or less. You can replace the transformer with a transistor in a commom-collector "emitter-follower" configuration and get pretty much the same results by applying the audio to the base of the transistor. However, simple VFO (variable-frequency oscillator) circuits such as yours do not amplitude-modulate well, as you will get a lot of IPM (incidental phase modulation) that will severely distort the signal. This is why the oscillator is always followed by an amplifier stage that the amplifier is modulated in broadcast transmitters.

Hey, tell me exactly what you want to do and I'll shoot you off a schematic. Bill has the right idea in the previous post, but it's difficult to use a transistor as a modulator as shown without a good deal of distortion.
 
M

Thread Starter

Mathematics!

Joined Jul 21, 2008
1,036
Sorry, but "mixing" will not do the job. You will have both the RF and the audio signals at the same point at the same time, but the RF will not be modulated by the audio. The 'balanced modulator' will, as pointed out, automatically suppress both the audio and the RF, giving you sidebands without a carrier, which is not the AM you want. (You can, however, offset the balanced modulator with a DC bias and get good AM. This is not generally done with transmitters, but common in modulated signal generators.)

If you want true amplitude modulation (that can be heard on an AM radio), then the audio signal must change the amplitude of the carrier signal in accordance with the program source. In your original diagram, if you connect a transformer secondary in series with the power supply voltage source, and apply the audio to the transformer's primary, you will get amplitude modulation, more or less. You can replace the transformer with a transistor in a commom-collector "emitter-follower" configuration and get pretty much the same results by applying the audio to the base of the transistor. However, simple VFO (variable-frequency oscillator) circuits such as yours do not amplitude-modulate well, as you will get a lot of IPM (incidental phase modulation) that will severely distort the signal. This is why the oscillator is always followed by an amplifier stage that the amplifier is modulated in broadcast transmitters.

Hey, tell me exactly what you want to do and I'll shoot you off a schematic. Bill has the right idea in the previous post, but it's difficult to use a transistor as a modulator as shown without a good deal of distortion.
Click to expand...
I have built an AM transmitter before with a crystal ossilator and a transformer in series on the primary side and the secondary side I plugged into the audio port of my computer. It's quality was ok and your transformer way, would just be substituting my ossilator circuit for the crystal ossilator. Either way I am looking for a more proffessional quality.

What happens if I put a diode after the output_from_soundcard ?
Then each time I output from the soundcard it will raise the total voltage
from the ossilator to be ossilator voltage + soundcards voltage.

Or maybe put a transistor base at the output_from_soundcard and have an external voltage supply ?

Attached are my thoughts.
I am still looking into the to diagrams may have some question on them latter.

Either way I don't see anyway yet to do pure AM without varying the frequency in some way???

Basically I only know how to do it by varying the base of the transistor or using the transformer method.
How do the proffesional transmitters do it?

Thanks
Note I have know problem in doing the amplifier stage first before the modulation stage I just don't know how to proffesional AM modulate a carrier wave ....
 

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M

Thread Starter

Mathematics!

Joined Jul 21, 2008
1,036
A resonant antenna at 3.7 KHz will be about 140,000 feet long. Be my guest.
Click to expand...
Well other then the antenna being optimal I am curious how the proffesional AM modulation is done. Plus assume I can make any professional antenna I want . I am more concerned with everything other then the antenna right now....

Should I have used a different ossilator circuit other then colpitts....if so what is the best ossilator circuit I can use Colpitts oscillators, crystal oscillator, Meiszner Oscillator, Hartley oscillator ,...etc ? what one...
I don't really understand what the difference of using different ossilators would be interms of quality of transmission.... I would think this depends on the modulation , amplification , and antenna.

Seems once you have the carrier wave created by any ossilator circuit you choose and it is correctly ossilating indefinitly with out to much flucation then the quality or distance of transmission is determined by the power amplifier , modulation , and antenna.... But correct me if I am wrong....

So the many question is what the proffesional way or proper way of modulation a carrier wave with AM. (i.e the best way or most common way of doing it is )

I thought it was mixing but I don't know how or what to do...
 
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