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Mathematics!
12-26-2009, 07:33 PM
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

bertus
12-26-2009, 07:36 PM
Hello,

Can you also post a "normal" schematic, as I do not have the software?

Greetings,
Bertus

Mathematics!
12-26-2009, 07:42 PM
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

bertus
12-26-2009, 07:47 PM
Hello,

An AM transmitter ofthen uses a buffer behind the oscillator.
The powersupply of this buffer is modulated with the "voice".

See the schematic in this link for an example:
http://electroschematics.com/2522/am-transmitter-circuit/

Greetings,
Bertus

Mathematics!
12-26-2009, 08:09 PM
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

Bill_Marsden
12-26-2009, 08:59 PM
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
12-26-2009, 09:15 PM
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

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
12-27-2009, 12:35 AM
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.

Mathematics!
12-27-2009, 01:37 AM
You need to mix the rf (radio frequency) signal with an audio signal. There are a variety of ways to do this.

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.


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?

Bill_Marsden
12-27-2009, 01:47 AM
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.

Mathematics!
12-27-2009, 01:56 AM
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.


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/info/circuits/diode_single_balanced_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

Bill_Marsden
12-27-2009, 02:07 AM
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.

Mathematics!
12-27-2009, 02:12 AM
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?

Bill_Marsden
12-27-2009, 02:15 AM
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.

http://forum.allaboutcircuits.com/attachment.php?attachmentid=14723&stc=1&d=1261884320

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.

electrojim
12-27-2009, 02:28 AM
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.

Bill_Marsden
12-27-2009, 02:30 AM
I would be interested. Mine is a pure kludge.

Mathematics!
12-27-2009, 03:30 AM
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.


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 ....

KL7AJ
12-27-2009, 03:34 AM
A resonant antenna at 3.7 KHz will be about 140,000 feet long. Be my guest.


eric

Mathematics!
12-27-2009, 03:50 AM
A resonant antenna at 3.7 KHz will be about 140,000 feet long. Be my guest.


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...

bertus
12-27-2009, 05:28 AM
Hello,

Here are some more schematics:
http://www.techlib.com/electronics/amxmit.htm

The link comes from this EDUCYPEDIA page:
http://www.educypedia.be/electronics/circuitsrf.htm

Greetings,
Bertus

Mathematics!
12-27-2009, 04:53 PM
Thanks but I am not trying to build someone elses. I can read a schematic and solder the components in to build theirs but I want to know how to build one from scratch....

From my ossilator circuit what do I have to create to take in the carrier wave and audio wave and output the modulated carrier....

Maybe the only way is the transformer method to get pure AM modulation?

I am still curious why product mixers won't work....
And curious how you design a circuit to product 2 input signals together?

I will try the attenuation diagrams but I am betting they will not give me pure AM modulation...

Thanks for your help I am looking for a quick circuit that I can add to the output of the ossilator circuit to modulate the carrier with pure AM using the signal from my audio port?

Bill_Marsden
12-27-2009, 07:09 PM
A transformer by itself won't do it, it is a linear component (usually). It can be part of a amplifier in such a way to vary the amps gain. Ultimately this is the cleanest way to modulate a signal.

Try what I suggested, put a capacitor and resistor in front of the base of your oscillator, and injected the audio there. The reason I think this will work is the variable voltage on the emitter will cause minor gain shift (I think).

The classic way I made AM transmitters during the CB craze of the 70's was to use a carbon mike, the type found in old telephones. Those are rare and hard to find nowdays, but a variable resistor (which is what they were) is perfect to vary the gain of the circuit they were in, so modulation was easy.

PRS
12-27-2009, 10:11 PM
After looking your ideas over I'm a bit confused, but I think you want to take output from your computer sound card, modulate an rf signal and project it as radio. Am I right so far?

If so, your oscillator is probably okay if you used SPICE (and probably a design from somewhere else), but the 1k resistor seems unnecessary and your idea of mixing is simply not right.

If you want the audio wave from the sound card to modulate an rf carrier, you have to use a mixer. This can be done very simply. More if you want it.

PRS
12-27-2009, 10:18 PM
Another point. There are two kinds of mixing: One is nonlinear and the other is linear. Linear mixing is used when you simply want to add two signals. An Op Amp adder will do that. This is usually used to mix audio signals. Another way to do this is simply join two similar resistors and input two signals at the ends of the resistors not connected to their junction. You can add as many resistors as you like and also have as many inputs, but there will be some attenuation at the output (the common junction of the resistors.

Nonlinear mixing -- which is what you need --occurs when you pass the two signals to be mixed through a nonlinear device such as a diode or a transistor. More on this if you want. I can show you a simple circuit that will probably get it done.

Bill_Marsden
12-27-2009, 10:34 PM
See post #10

Yes, this is what he wants to do.

I would be interested. I'm thinking unbalance a balanced mixer at the moment.

Mathematics!
12-28-2009, 01:16 AM
I realize that their are 2 types of mixers .... the ones that add 2 signals by just connecting the 2 signals with resistors (these add the signals ) (these where the ones I original had in my schematic but was wrong and should have been looking at the other kind of mixer)

The other is product mixers (which is the ones that I need to do )...They product the 2 signals together....
Since AM is m(t)* sin(2pi*f * t + theta) where m(t) is your audio trig function...etc This means you must product mix the carrier
sin(2pi*f*t + theta ) with the audio m(t).... enough with the math theory...

I understand why I have to "product mix the 2 signals together" to modulate AM wise.

But I am unsure how if the only way is using transistors and diodes then how? (nonlinear devices)


A transformer by itself won't do it, it is a linear component (usually). It can be part of a amplifier in such a way to vary the amps gain. Ultimately this is the cleanest way to modulate a signal.

Try what I suggested, put a capacitor and resistor in front of the base of your oscillator, and injected the audio there. The reason I think this will work is the variable voltage on the emitter will cause minor gain shift (I think).

The classic way I made AM transmitters during the CB craze of the 70's was to use a carbon mike, the type found in old telephones. Those are rare and hard to find nowdays, but a variable resistor (which is what they were) is perfect to vary the gain of the circuit they were in, so modulation was easy.

I know the transformer by itself won't do AM but what I made before was an AM transmiter with a transformer and a crystal ossilator in parrell on the primary side and the other side was plugged into my audio port of my computer the audio would change the magnetic resistance of the inductor which would change the amplitude of the crystal ossilator ... work fine for me. What I just got from your post is this method of using transformers is the cleanest to do AM.... (seems to simple to be true)

As for put a capacitor and resistor in front of the base of your oscillator, and injected the audio there ( could you give me a picture of what I should do for this?)

And do they still sell carbon mikes? because that would be an easy solution to modulate AM all I would have to do is put the mic at the output of the ossilator.

Nonlinear mixing -- which is what you need --occurs when you pass the two signals to be mixed through a nonlinear device such as a diode or a transistor. More on this if you want. I can show you a simple circuit that will probably get it done.


After looking your ideas over I'm a bit confused, but I think you want to take output from your computer sound card, modulate an rf signal and project it as radio. Am I right so far?

If so, your oscillator is probably okay if you used SPICE (and probably a design from somewhere else), but the 1k resistor seems unnecessary and your idea of mixing is simply not right.

If you want the audio wave from the sound card to modulate an rf carrier, you have to use a mixer. This can be done very simply. More if you want it.

I realize that I don't need a few things like the 1kohm resistor. And I need to some how create a product mixer that takes the carrier and audio from the soundcard and products the 2 together.
Seems by the previous posts the only way to make product mixers is by using nonlinear devices (like transistors, diodes ) but how that is the question?

Thanks for any help on this

Bill_Marsden
12-28-2009, 02:47 PM
Unfortunately I don't think they still make carbon mikes. You might be able to find an old one, they used to be in every telephone in the world. They are simple, but their freq response is atrocious.

PRS
12-28-2009, 04:41 PM
Mathematics, you could use an IC such as a 1596. Google it and find the application notes. I'd show you how to use diodes or a transistor, but I forgot how to put pictures on my posts.

A passive mixer consists of a ring of 4 diodes and two transformers and the output has to have a filter to get rid of all the unwanted harmonics. This is a complicated method.

You might try using a single common emitter amplifier. Inject the rf into the base and the audio into the emitter. The output is the collector and you have to follow it with a filter to get rid of harmonics.

bertus
12-28-2009, 04:50 PM
Hello,

You could also have a look at the LM13700 IC.
In the datasheet there is an AM modulator written.

The AD633 is also a ringmodulator like the MC1596.
Here is an audio example:
http://www.musicfromouterspace.com/analogsynth/RingModulator/RingModulator.html

Greetings,
Bertus

Mathematics!
12-30-2009, 02:06 AM
Mathematics, you could use an IC such as a 1596. Google it and find the application notes. I'd show you how to use diodes or a transistor, but I forgot how to put pictures on my posts.

A passive mixer consists of a ring of 4 diodes and two transformers and the output has to have a filter to get rid of all the unwanted harmonics. This is a complicated method.

You might try using a single common emitter amplifier. Inject the rf into the base and the audio into the emitter. The output is the collector and you have to follow it with a filter to get rid of harmonics.


First , I want to make the modulator circuit from scratch NO IC chips...

This limits me to the passive mixer or common emitter amplier of your post in quotes...

Curious if you have a schematic you could post for the passive mixer and common emitter amplifer..... for the AM modulation

Thanks

Bill_Marsden
12-30-2009, 04:29 AM
I've already posted a design I've never built, you'd get to be the first! :D Of course, it might not work either.

Did you ever look up the balanced mixer schematic I linked to? A simple diode setup would do the same thing.

bertus
12-30-2009, 08:14 AM
Hello,

Did you have a look at the balanced mixer using transistors, I gave you in a former link:

http://www.techlib.com/electronics/graphics/amsch.gif

Greetings,
Bertus

Mathematics!
12-30-2009, 05:04 PM
I've already posted a design I've never built, you'd get to be the first! :D Of course, it might not work either.

Did you ever look up the balanced mixer schematic I linked to? A simple diode setup would do the same thing.

Do you mean this attenutation/transistor diagram
http://forum.allaboutcircuits.com/attachment.php?attachmentid=14723&d=1261884320

If so I am going to try this when I get home in a few days it should work in theory
Except what resistor values should I use and what capacitor values / transistor should I use...?
Also is VCC the 9volt battery of the ossilator circuit or is it another external battery if external what is it's value...

Bill_Marsden
12-30-2009, 05:06 PM
Yep. Bertus design look more promising. You could replace the crystal oscillator with yours, and use the rest. Basically it is a differential amplifier.

Mathematics!
12-30-2009, 07:38 PM
Well , I tried your attenuation transistor circuit... wasn't to bad it works but it didn't give me to much range though.... (at least I don't think but I am still working some of the bugs out on this one filtering harmonics ,...etc etc )

As Bertus's design above
Question , I am unsure what components to take out/leave in so I can swap his crystal ossilator with my ossilator circuit ?

Here is what I am thinking but I know it is somewhat wrong
attached is the picture .... circled is what I am taking out in place of my ossilator????

Mathematics!
12-30-2009, 07:40 PM
forgot to attach

bertus
12-30-2009, 07:46 PM
Hello,

You can put your oscillator at the place of the circled parts.
You will need the coupling capacitor to block the DC from the oscillator.

Greetings,
Bertus

PRS
12-30-2009, 08:06 PM
First , I want to make the modulator circuit from scratch NO IC chips...

This limits me to the passive mixer or common emitter amplier of your post in quotes...

Curious if you have a schematic you could post for the passive mixer and common emitter amplifer..... for the AM modulation

Thanks

I hope this attachment works....

... upon edit, I see that it did. The transformer is just a toroid with a 2:1 stepdown ratio. Pick a small capacitor and put an even smaller variable cap in parallel with it. Then calculate the L from: f=1/(2*pi*sqrt(L*C))

f is your radio frequency.

Mathematics!
12-30-2009, 10:58 PM
Thanks for all that I am curious about one thing when I was studying transistors on the allaboutcircuits book....

It is in the attached picture....

The transistor can amplify the total sinewave at the base if their is a suitible foward basised voltage to keep the transistor from going into cut off or saturation ..... (in the picture the base sine wave is amplified and inverted)

What I want to know is what would happen if the 15volt dc emitter-collector supply voltage was an AC sinewave voltage? ( then would the base wave by am modulated/amplified instead of just amplified )

Would the output from combineding the base sinewave and the ac sinewave voltage be equivalent to Amplitude modulation of the base sinewave.... Would it be in theory the product of these to waves ????

If the common-emitter inverting is going to be a problem then how about the same question but using a common-collector ???

Or will this also somehow effect the frequency/period of the output wave....

Curious because in the AAC book they never talk about if the voltage connect to the emitter-collector is sinewave....
And weather being sinewave will effect the frequency/period of the output wave?

Thanks for any help
And when I get a chance I will build the above schematic you just gave me.

Mathematics!
01-02-2010, 07:50 PM
Does anybody know the answer to my above post ?
Curious to know...
Since the AAC internet book only does examples when the high voltage/current battery is DC and not ossilating...

Thanks if it is equivalent to AM modulation then a one transistor could be used as a product mixer...

Thanks

bertus
01-02-2010, 08:54 PM
Hello,

In the "old" days with tube amplifiers, they used an audio transformer in the anode of the tube to modulate.
The power supply voltage and the modulation voltage will superponate.

http://upload.wikimedia.org/wikipedia/commons/b/bb/Ammodstage.png

This picture come from the wiki:
http://en.wikipedia.org/wiki/Radio_transmitter_design

Greetings,
Bertus

Mathematics!
01-03-2010, 08:15 AM
Thanks for all that I am curious about one thing when I was studying transistors on the allaboutcircuits book....

It is in the attached picture....

The transistor can amplify the total sinewave at the base if their is a suitible foward basised voltage to keep the transistor from going into cut off or saturation ..... (in the picture the base sine wave is amplified and inverted)

What I want to know is what would happen if the 15volt dc emitter-collector supply voltage was an AC sinewave voltage? ( then would the base wave by am modulated/amplified instead of just amplified )

Would the output from combineding the base sinewave and the ac sinewave voltage be equivalent to Amplitude modulation of the base sinewave.... Would it be in theory the product of these to waves ????

If the common-emitter inverting is going to be a problem then how about the same question but using a common-collector ???

Or will this also somehow effect the frequency/period of the output wave....

Curious because in the AAC book they never talk about if the voltage connect to the emitter-collector is sinewave....
And weather being sinewave will effect the frequency/period of the output wave?

Thanks for any help
And when I get a chance I will build the above schematic you just gave me.
Attached Thumbnailshttp://forum.allaboutcircuits.com/attachment.php?attachmentid=14838&stc=1&thumb=1&d=1262217495 (http://forum.allaboutcircuits.com/attachment.php?attachmentid=14838&d=1262217495)


Still curious about this question?

bertus
01-03-2010, 09:00 AM
Hello,

When the powersource of the amplifier is changed to an AC source the power also can go negative.
As transistors do not work correctly at negative power, you need a positive DC offset to the AC source that is a little bit greater than the negative value of the AC voltage.
In that way you can modulate AM with an transistor.
The base of the transistor can have the carrier signal.
Also here goes the offset as needed.

In the tube example I gave you this is also done.

Greetings,
Bertus

Bill_Marsden
01-03-2010, 09:12 AM
You might be able to put a transformer in the collector circuit to modulate. Be aware that it goes both ways though, the RF will also go back through the transformer. This could be a bad thing for your sound card, so some buffering might be in order.

Mathematics!
01-03-2010, 08:07 PM
Hello,

When the powersource of the amplifier is changed to an AC source the power also can go negative.
As transistors do not work correctly at negative power, you need a positive DC offset to the AC source that is a little bit greater than the negative value of the AC voltage.
In that way you can modulate AM with an transistor.
The base of the transistor can have the carrier signal.
Also here goes the offset as needed.

In the tube example I gave you this is also done.


Ok so let me get this straight.
If the DC source was AC and I was able to forward shift the voltage so the base never goes negative or below cut off then this would be AM modulation?
So one transistor can be used for a product mixer/ Amplitude modulation?

If this is all true then AM transmitters amount to nothing more then creating an ossilator circuit , then using a transistor to modulate AM , then creating filters/ amplifiers and sending it out an anntena.
Ofcourse you could do this in an infinity amount of ways an some way/components are less buggy/better quality ... can be used in ranges of different tempatures ,...etc
(the basic princples are create an ossilation , AM modulate it with a transistor/mic or IC or whatever ..., amplify/filter...,Send it to an antenna... your done!)

Thanks

Bill_Marsden
01-04-2010, 05:35 AM
You don't even need a transistor. Did you look how a balanced modulator works? 2 transformers, 4 matched diodes.

Mathematics!
01-05-2010, 04:36 AM
You don't even need a transistor. Did you look how a balanced modulator works? 2 transformers, 4 matched diodes.

If this is what you are refering to then yes see attachment.

Question in this circuit their is LO , IF , and RF is the LO the carrier/ossilating circuit or is it the IF (intermediate frequency )...

I am just unsure with this picture where the audio signal should go into and where the carrier wave should be?

And I am still looking for confirmation on the correctness of a transistor with base with audio signal and carrier being the large ac voltage between collector and emitter ===> output at the collector is going to be AM modulation... I think it is true "provide that you basis the transistor so it never goes below cutoff but in the AAC book they never varied the main voltage only the base voltage/current so I am wondering if varying both modulates as AM still? Below is the orignal post look back a few post to get a better picture of what I am asking....


Quote:

Hello,

When the powersource of the amplifier is changed to an AC source the power also can go negative.
As transistors do not work correctly at negative power, you need a positive DC offset to the AC source that is a little bit greater than the negative value of the AC voltage.
In that way you can modulate AM with an transistor.
The base of the transistor can have the carrier signal.
Also here goes the offset as needed.

In the tube example I gave you this is also done.
Ok so let me get this straight.
If the DC source was AC and I was able to forward shift the voltage so the base never goes negative or below cut off then this would be AM modulation?
So one transistor can be used for a product mixer/ Amplitude modulation?

If this is all true then AM transmitters amount to nothing more then creating an ossilator circuit , then using a transistor to modulate AM , then creating filters/ amplifiers and sending it out an anntena.
Ofcourse you could do this in an infinity amount of ways an some way/components are less buggy/better quality ... can be used in ranges of different tempatures ,...etc
(the basic princples are create an ossilation , AM modulate it with a transistor/mic or IC or whatever ..., amplify/filter...,Send it to an antenna... your done!)

Thanks

bertus
01-05-2010, 05:28 AM
Hello,

The attached picture is indeed a ringmodulator.
The LO is the signal from the oscillator.
The IF is the modulation signal.
The RF is the modulated output.

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

Greetings,
Bertus

Mathematics!
01-05-2010, 07:19 AM
Is this different in anyway to a balanced modulator that Bill_Marsden was refering to?

And can anybody answer my transistor question in previous post.
I just want to be positive I got this straight.

My next question is if AM modulation is done with product mixers either built by transistors or diodes/transformer pairs....

Then how do you modulate a carrrier with FM modulation? Another words how do you use a transistor , or diodes/transformers to vary the frequency while keeping the amplitude the same.... Would this be called a frequency mixer (or what type of mixer is this for FM modulation called if any) And how do you go about making one....

Thanks

Paulo540
01-05-2010, 07:13 PM
And can anybody answer my transistor question in previous post.
I just want to be positive I got this straight.

Thanks

You would not be biasing the transistor if you used AC. Bias is a stable DC current.
For proper reproduction of a complete AC signal (also known as class A), you need to bias the transistor with DC current between the cutoff and saturation points of the transistor. Check out diagrams of transitor characteristic curves. An NPN will need a need positive current to bias and a PNP would need the opposite.

If you were trying to make a sine wave from a jump rope, would it work if you were lying on the ground? You could get a postive (upward) movement, but it wouldn't go negative, right? You have to raise the midpoint of the wave so that it can give a downward wave without touching the ground.



http://www.allaboutcircuits.com/vol_3/chpt_4/4.html talks about biasing much better than I could hope to.


There are other arrangements (class B C D etc etc) but none of them involve injecting power supply AC current into the base.

Mathematics!
01-05-2010, 07:37 PM
There are other arrangements (class B C D etc etc) but none of them involve injecting power supply AC current into the base.

well I meant if you used biasing and had the ac at the base of the transistor shifted up by the dc battery...etc assuming this was done then if the supply source between the emitter /collector was ac would the output at the collector be AM modulation?

I have read the transistor chapter and know all about class A B AB ,...etc but nothing in this chapter covers if the main source between collector and emitter was AC at the same time as the base has shifted up ac to remain forward biased.

Thanks for any help here.

As for my other questions about FM modulation still waiting on this one ...?

bertus
01-05-2010, 08:08 PM
Hello,

I found an interesting page for you:
http://www.users.cloud9.net/~stark/commbook.htm

See chapter 8 for FM

Greetings,
Bertus

Mathematics!
01-05-2010, 09:36 PM
Thanks for the link I will look into FM modulation more and post another thread specific to FM at a later date...

But I still have my transistor question to be confirmed above? Yes/or No?
I believe this would be amplitude modulation if basied correctly...(i.e I beleive one transistor could be used as a product mixer)

bertus
01-06-2010, 08:03 PM
Hello,

Take a look at page 2 of chapter 7 for AM modulation with a transistor.

Greetings,
Bertus

Mathematics!
01-07-2010, 09:16 PM
Ok , but now I am curious for this ring modulator (product mixer )
http://en.wikipedia.org/wiki/Ring_modulation

Would switch diodes be ok to use for making the diode ring?
And what 2 transformers or at least how many turns of a coil of where should I use?

And should the taps for carrier be made in the center of the coils or does it not matter that much to be exactly centered?

Thanks

Also wondering is their any other way to modulate AM without using a ring modulator , or a transistor circuit.... (leaving out for the moment useing vaccum tubes in place of the transistor )?

I would think only product mixers are the only way to AM modulate ?
Curious to know if I am wrong and if their is a better/best way to modulate AM other then using product mixers.... My thoughts is probably not...

bertus
01-08-2010, 05:19 AM
Hello,

Here I have a link on making a ringmodulator.

http://www.cgs.synth.net/modules/cgsrr.html

For RF use other transformers.

Greetings,
Bertus

Mathematics!
01-08-2010, 11:03 PM
For the link you gave me I don't have the abilty to get those germanium diodes could I use in their place switch diodes 1N914 - 1N4148???

And for the transformers would any audio transformer do?
I have only radioshack 273-1380 audio output transfomer or radioshack isolatation transformer part 273-1374?

By looking at the ring modulator circuit it seems the only thing that would be significant is that the components don't drop significant voltage i.e diodes don't drop to much forward voltage and audio transformer don't resist to much. Eitherway If this occurs I should beable to amplify it after it is modulate with an LM386 or something...

But I am wondering if anybody see's a reason why the parts have to be specific.... before I try it and if it doesn't work try amplifying it????

Thanks for your reply
I think I am save to use almost any thing as long as resistance isn't to much and the transformer can respond to the frequency. Now that I think about it frequency response could be an issue.

Looking at the spec's I see nothing that stands out differently then the audio transformers of radioshack that I have from theirs
http://www.mouser.com/catalog/specsheets/XC-600134.pdf

Bill_Marsden
01-08-2010, 11:31 PM
You could also use Shottky diodes, which drop even less voltage than germanium. Generally toriod transformers are used for RF, the kind you wind yourself.

Mathematics!
01-09-2010, 12:07 AM
ok , last thing will the switch diodes be ok I see picture of them used here
http://www.electronics-tutorials.com/basics/diodes.htm

Half way down the page....
seems I am all set then just need to know how many turns for the coils of wire?

Bill_Marsden
01-09-2010, 12:13 AM
Actually that isn't as critical as you'd think. It is the ratio that is more important. The more turns the tighter the coupling, up to a point. Too few and it can still work, but efficiencies go down.

Mathematics!
01-09-2010, 06:46 AM
so for the two transformer i.e the 4 coils of wire how many turns should use should the ratio be the same for both coils,...I am just unsure about how many turns ,...etc I am using 22gage wire?

Thanks