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Help me in my AM radio transmitter project ..

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salvazero

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Hello ,
I'm working on a simple short range AM radio transmitter , and I faced some problems which I hope to find solutions here :)

1st Problem : I designed 1MHz wein bridge oscillator and it's Pspice model works like a charm . so what is the problem ?? :evil: ... the problem is that I was forced to use the fast AD 745 Op-Amp instead of LM741 :???: and I hope to find a way of making an 1MHz oscillator works with LM741 is it possible ?
Hint : I don't want to use colpitts , Hartley or 555 oscillators !

2nd Problem : I need to multiply the voice signal by the 1MHz carrier ,So I need a multiplier , what is the best way of making a simple multiplier ? , I was thinking of using the logarithmic and anti logarithmic applications of the op-amp but I found it hard and complex process ! :cry:

3rd Problem : I want to put a High Pass filter to ensure that the dc components is blocked ! , should I apply the HPF on the voice signal or the modulated signal or both of them ? :!:


thx ,best regards :-D :-D
 
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goldsmith

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Hello ,
I'm working on a simple short range AM radio transmitter , and I faced some problems which I hope to find solutions here

1st Problem : I designed 1MHz wein bridge oscillator and it's Pspice model works like a charm . so what is the problem ?? ... the problem is that I was forced to use the fast AD 745 Op-Amp instead of LM741 and I hope to find a way of making an 1MHz oscillator works with LM741 is it possible ?
Hint : I don't want to use colpitts , Hartley or 555 oscillators !

2nd Problem : I need to multiply the voice signal by the 1MHz carrier ,So I need a multiplier , what is the best way of making a simple multiplier ? , I was thinking of using the logarithmic and anti logarithmic applications of the op-amp but I found it hard and complex process !

3rd Problem : I want to put a High Pass filter to ensure that the dc components is blocked ! , should I apply the HPF on the voice signal or the modulated signal or both of them ?


thx ,best regards

Hi Salvazero
A wrong manner ! why you want go through this way ? for a simple AM transmitter you can easily use a transistor oscillator . and then a simple amplifier .
Best Wishes
Goldsmith
 

salvazero

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Thank You for ur fast reply :D , Can u please clarify more ?
 

goldsmith

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Thank You for ur fast reply :D , Can u please clarify more ?
Certainly .
But before trying to answer your question , in a right path you should mention your exact required power .
Anywhere , for low wattage transmitters , a simple colpitts oscillator in CB arrangement or each , other type of oscillator would be ok . and then an AF amplifier and then easily a power stage if you need higher powers .
Good luck
Goldsmith
 

Eshal

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Hello guys..

As salvazero is asking for low power AM transmitter so I think he want to transmit his voice. If it is so then frequency should be 0.5MHz to 1.5MHz and power may be 1mW enough. If you want to amplify your power then you can use class B push pull. Isn't it Goldsmith?
 

goldsmith

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As salvazero is asking for low power AM transmitter so I think he want to transmit his voice. If it is so then frequency should be 0.5MHz to 1.5MHz and power may be 1mW enough. If you want to amplify your power then you can use class B push pull. Isn't it Goldsmith?

Hi Eshal
1mW ???!!! do you expect that such low power signal be able to go through the improper path ???!!
Class B ? Why class B ? Why amplifying like this ? power modulating systems are based in a power modulator . not amplifying a low power AM wave .
Best Wishes
Goldsmith
 

betwixt

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Goldsmith, Eshal is doing something similar in a different thread but only needs enough power to go across a room. 1mW is plenty for that.

Any LINEAR amplifier can be used for AM, it can be class A, AB, B, C etc. As long as the input and output are directly proportional it doesn't matter what class it is although there are advantages in using certain types in certain situations.

Brian.
 

goldsmith

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Goldsmith, Eshal is doing something similar in a different thread
Hi dear Brian
Unfortunately yes ! ;-)
1 mw for a room ? i've never created such low power because it is like a toy . and i don't want to make toy . hence i think when we can easily design a transmitter for higher powers with the same price why we don't go through this path ?
Any LINEAR amplifier can be used for AM,
( however a non linear amplifier can be used too ! for higher powers ! with power modulation )
it can be class A, AB, B, C
Is class C amplifier, a linear amplifier ??! who told this ? that's a nonlinear stage . and of course isn't a good idea to amplify AM waves ! it is just good to be used as a power modulator stage . isn't it ?
I really hate to see something like this most of the times . for example a transmitter with very low power ! most of the times i prevent to answer questions such as this !
Best Regards
Goldsmith
 

betwixt

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1mW is plenty of power to cross a room. I regularly used 10mW for communication over about 15kM a few years ago and there are records of under 100uW per KM being reached with no troubles.
It worries me that some of the designs on the Internet are very bad. I have seen circuits designd to produce fast rise and fall time square waves being used with an antenna connected to them! I think the worst I've seen was a 7805 regulator with a tuned circuit in it's ground pin to make it unstable and an antenna at it's output. 8-O

True, you can use a non-linear amplifier but you have to apply correction to the envelope to restore linearity again so overall it's a linear amplifier.

You can use class C for AM. In class C the amplifier conducts less than the full RF drive waveform which can make it more efficient. The RF waveform will be distorted and has to be filtered later. The modulation envelope can still be linear though.

Brian.
 

goldsmith

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Hi Brian

I regularly used 10mW for communication over about 15kM
Of course that's true but not for an AM wave . right ? yeah i've seen some FSK transmitters in some spacecrafts that with power of 4 watts , transmitting a signal from some hundred thousand KMs to the earth .
It depends on two parameters :
1-what kind of modulation we've used
2- what is our receiver ? we can easily design a pretty expensive receiver to receiver pretty weak signals but when we can increase the transmitted power , instead of creating a big antenna and some precise circuits with expensive .

It worries me that some of the designs on the Internet are very bad. I have seen circuits designd to produce fast rise and fall time square waves being used with an antenna connected to them! I think the worst I've seen was a 7805 regulator with a tuned circuit in it's ground pin to make it unstable and an antenna at it's output.
Yes i agree . i've seen some awful designs , in the net and i think most of them , weren't designers . they were , hobby toy makers instead of circuit designers .

You can use class C for AM. In class C the amplifier conducts less than the full RF drive waveform which can make it more efficient. The RF waveform will be distorted and has to be filtered later. The modulation envelope can still be linear though.
Brian , Just consider that you've an AM wave with modulation depth of 75 percent and you want to amplify it with a class C amplifier . let me tell the scenario that what will happen . it will conduct just in a little piece of cycle . hence your message signal will be destroyed , easily .
But there is an easy solution available , here . that we just amplify a carrier signal with a class C amplifier ( without modulation ) . and then we use a summed signal as it's VCC ( a DC wave which summed with a sine wave ) ( our message signal ) we can easily design such a circuit for low powers or very high powers .
Isn't it ?

Best Regards
Goldsmith
 

goldsmith

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Hi!
I am back again friends. Welcome me. Lol heheheheh....
goldsmith here is one AM transmitter,
https://obrazki.elektroda.pl/7342738200_1350706868.jpg
What you want to say about this one? It has class C amplifier in the final power section.

Hi Eshal
Before trying to tell you about your circuit , i've two suggestions for you .
1- learn how to read a post carefully
2- try to learn circuit analysis and microelectronics before trying to design a circuit .

Why i've suggested these parameters to you ? because as it appears you didn't understand how a class C amplifier does work .
And you didn't read my former posts carefully .
What i've told before ? i've mentioned a class C amplifier can't amplify an AM signal . and i've told it can be used as a power AM modulator . So if you try to understand what is that circuit , you'll see that base is drove with an RF signal ( without any modulation ) and the collector voltage is a DC signal that has been summed with a sine wave ( message signal or as a matter of fact audio signal )
So it means a power modulation . it is exactly what i've mentioned before . it can amplify a carrier signal and then because of variations in supply voltage it will deliver a pure AM wave .
Do you understand ? i'm sure not ! because you don't have backgrounds of telecommunications .

Best Wishes
Goldsmith
 

BradtheRad

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yeah i've seen some FSK transmitters in some spacecrafts that with power of 4 watts , transmitting a signal from some hundred thousand KMs to the earth .

This is remarkable.

I once read that a well-made antenna is crucial to efficient radio transmitting. In the sense that a good antenna can reach the world even if transmitter power is under the legal watt rating.

Steps toward achieving efficiency: Optimize the size and shape of the antenna for the frequency band. Make the antenna impedance matched to the transmitter output.
 

goldsmith

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This is remarkable.

I once read that a well-made antenna is crucial to efficient radio transmitting. In the sense that a good antenna can reach the world even if transmitter power is under the legal watt rating.

Steps toward achieving efficiency: Optimize the size and shape of the antenna for the frequency band. Make the antenna impedance matched to the transmitter output.
Hi dear Bradtherad
Yes I agree with you about antenna for example we can use an antenna with high value of focus ratio then if the power is low , the distance will be higher .

Best Regards
Goldsmith
 

Eshal

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OK :-(

BTW: We are going off topic of what salvazero posted. Be on track.
 

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