Low level AM Transmitter Circuit and little description.

[goldsmith]

Well i think you are an oracle !
No, but I can read. You wrote here that you are a university student and that you were born in 1992.

because it is around 9 years that i'm studying electronics ! under guidance of old professors and old engineers and of course some peoples younger than that !
Maybe so but less than one year ago you started this thread because you did not know how to design a one transistor CE amplifier, and you started this thread because you could not understand a simple circuit with just one opamp and four resistors.

Godfreyl , an advise , if you don't improve your writing and don't be polite , i can talk with moderators regarding your behaviors . i told you if you have any problem with me because of each reason you can tell me . but this behavior is like baby's behavior in school .
Yes i'm an student of university . but here in this forum there are many peoples with ages lower than me and with same or higher abilities . don't you know them ? is age an important issue ? ( i know some pretty young boys and girls in U.S.A ! they are professors of university ! with pretty low age ! )
And about those threads , i think you didn't understand my aim by those threads . first one was regarding large signal , and another one was regarding an op amp , they was because of a doubt that created by a friend of mine and i had to be sure . but the only thing that i can't understand is why you are not polite ? didn't you learn ?

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Hi Eshal

@ goldsmith

How are you?

I need your help.

I'm fine thank you , how are you too ?
Sure , but which section is unclear , now , tell me please , i can explain . ;-)
Best wishes
for you !
Goldsmith

 

[Eshal]

I decided to make this transmitter, here is link
**broken link removed**

Now it is your job to teach me this circuit in such a way that major and important portions of this transmitter is covered in 2 to 3 weeks.

 

[goldsmith]

Hi Eshal
Humm it is a simple transmitter based on colpitts oscillator ( if you increase frequency of oscillator it will be an FM transmitter but now it is AM ! )yes i can help you in this too . where is the problem my friend ?

 

[Eshal]

I like that you call me your friend. But remember I am girl . hehehehehe...

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Can you please tell me what are major portion of this circuit. I think they are, one is modulator, second is oscillator and third one is RF. Am I right?

 

[goldsmith]

Sorry ! i thought you are a boy too ! pardon me .
Lest analyze this circuit together :
First stage with Q1 is a simple CE amplifier to amplify weak signals . R7 and C6 are used because that microphone is capacitive microphone and it needs supply . Q2 is a simple colpitts oscillator in CB arrangement .
when base voltage increasing , oscillator will go through more conduction and thus out put will be decreased ( this oscillator will appear as a CE stage instead of input from base (audio signal ) ) so , if your input be decreased transistor will be at lower conduction and thus out put will be increased . and because your oscillator has RF signal thus out put will be an AM wave . a pure AM wave .
Is that clear ?

 

[Eshal]

Sorry ! i thought you are a boy too ! pardon me .

Its OK. You are not first who regarded me as a boy. When I joined beauty tips forum then some members there regarded me as a boy too.
BTW: don't you know ESHAL is girl name?

microphone is capacitive microphone and it needs supply .

Can you explain this? What do you mean by capacitive microphone and why it needs a supply, doesn't other microphone needs supply?

because your oscillator has RF signal thus out put will be an AM wave . a pure AM wave .

When oscillator has RF signal then this implies the output will be pure AM wave. Is it?

 

[goldsmith]

BTW: don't you know ESHAL is girl name?

However our countries are neighbors together but i think it is local name of your country because i didn't hear it . but now i know it !

Can you explain this? What do you mean by capacitive microphone and why it needs a supply, doesn't other microphone needs supply?

What a capacitive microphone is : it is a capacitor that when your voice is coming through it , it's capacity will change . so , we should detect this effect , hence if we use a supply , thus voltage across the microphone will be variable when you are talking on it ! most of the microphones don't need supply because most of them are as a self generator ! ( coil and magnet or crystal ) .

When oscillator has RF signal then this implies the output will be pure AM wave. Is it?

Of course ! consider a simple oscillator , it has RF wave , but when you are applying another wave into it , it's amplitude will be variable , because gain depends on voltage too .

 

[zsolt1]

hi,
The stage with Q1 is a low signal amplifier in common emiter conection, Q 2 is in a oscilator circuit, between the oscilator and the antenna there is a "strange RF filter" like circuit. The circuit looks 80% FM than AM to me ... maybe someone will explain to us how much the oscillations are affected in amplitude by the audio signal. It is true that when modulateing in AM or FM , there is not a pure AM or a pure FM modulation. For exemple in the circuit i drawn to you, the modulation is mostly AM , but there is a little frequency modulation too due to the "misterious oscilators" sensibility and frequency deviation with it's power suply . In the same way for a circuit where the frequency is modulated, the amplitude may be affected to in a little proportion.
Since the circuit is simple you could quickly try it out and tell us about the results

 

[goldsmith]

circuit looks 80% FM than AM to me ... maybe someone will explain to us how much the oscillations are affected in amplitude by the audi

Hi Zsolt1 !
At high frequencies this circuit will be changed to FM because of effect of capacitors ! internally capacitors ! their capacity will be changed instead of amplitude , but this change is low ! so at high frequencies it is important . but this circuit is both AM and FM at high frequencies ! it means the out put is M at high frequencies but it's amplitude will have changes too !
Recall :
Consider conduction of transistor is depends on it's base voltage . thus if audio signal can change it ( base voltage because of it's current ) thus amplitude of RF will have change . it can be derived mathematically too .
Regards
Goldsmith

 

[Eshal]

I am back.

consider a simple oscillator , it has RF wave

Can you tell me how do you know the oscillator given in the circuit has RF wave?

 

[zsolt1]

i se the balance between AM & FM you are pointing at , in case of this circuit, but what is with the filter and the ground conection (or shaft) ?

PS: https://en.wikipedia.org/wiki/Radio_frequency

 

[Eshal]

At high frequencies this circuit will be changed to FM because of effect of capacitors ! internally capacitors ! their capacity will be changed instead of amplitude , but this change is low ! so at high frequencies it is important . but this circuit is both AM and FM at high frequencies ! it means the out put is M at high frequencies but it's amplitude will have changes too !

At high frequencies, you said. This means if I speak loud to the microphone then this circuit will behave like a FM transmitter and if I speak low to the microphone then this circuit will behave like AM transmitter. Is it?
If so, then isn't there any way that we could avoid FM transmission?

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but what is with the filter and the ground conection (or shaft) ?

Shaft is a coaxial wire. Its cold water pipe case is grounded.

 

[zsolt1]

hehe .. the GND conection around the antena bothers my fantasy because i think it will atenuate rf signals

 

[Eshal]

Bro can you tell me what is the output frequency of this transmitter? If my teacher asked, I would tell him.

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Will it really attenuate RF signals?

 

[goldsmith]

i se the balance between AM & FM you are pointing at , in case of this circuit, but what is with the filter and the ground conection (or shaft) ?

Ground connection ? is your mean that coaxial filter ? it can be because of two reasons : 1 - perhaps user wants to use dipole antenna ( family of lambda/2) thus user will need ground path . 2- because of matching of antenna with filter of that circuit . however i don't think for this pretty low power that filter ( with cable ) be required .


Hi again Eshal

Can you tell me how do you know the oscillator given in the circuit has RF wave?

You can calculate ! range of feed back capacitor and tuned circuit !

At high frequencies, you said. This means if I speak loud to the microphone then this circuit will behave like a FM transmitter and if I speak low to the microphone then this circuit will behave like AM transmitter. Is it?
If so, then isn't there any way that we could avoid FM transmission?

No don't confuse ! in all of the times this circuit is an FM and AM too but at high frequencies it is FM with higher modulation percent . so it doesn't matter for your aim . at high frequencies variation in internal capacitors ( however a bit ) will be pretty important because your tuned circuit will be with low values such as some nH inductors and some PF capacitors .

 

[Eshal]

You can calculate ! range of feed back capacitor and tuned circuit !

Will you teach me how to calculate? :???:

will be pretty important because your tuned circuit will be with low values such as some nH inductors and some PF capacitors .

So if we increase there values then its FM modulation percent may be decreased. Can it be? Can we do this?

 

[goldsmith]

Will you teach me how to calculate?

Sure , best lead of oscillation is when fed capacitor is around C tank/2 co Ctank should be twice of that ( internally capacitor of transistor ( CE ) should be considered too ) so it should be 200 pf . so fout=1/2pisqrt(LC) so it should be around 790KHZ ! however you can change it's frequency but it's power will be decreased if you don't change fed capacitor .

So if we increase there values then its FM modulation percent may be decreased. Can it be? Can we do this?

You can decrease frequency but up to 535 KHZ ( MW range ! ) . so i told you it is not important for your aim . you can use higher frequency transistor too ( thus its capacitors will have lower values ! ) .

 

[Eshal]

Yes, I calculated frequency, it is 795.77KHz
So this means oscillator has RF wave ?

you can use higher frequency transistor too ( thus its capacitors will have lower values ! ) .

I am not understanding, why this transmitter is categorized as AM transmitter if it is FM too. And what should we do in order to reduce FM effect?

 

[zsolt1]

why this transmitter is categorized as AM transmitter if it is FM too

because the modulation is not pure AM or FM.

And what should we do in order to reduce FM effect?

nothing , it is alredy reduced to minimum by the circuit itself ....

I still suggest to test it, so you will know for shore if it works or not :wink:

 

[goldsmith]

Yes, I calculated frequency, it is 795.77KHz
So this means oscillator has RF wave ?

Yes . congratulations !

I am not understanding, why this transmitter is categorized as AM transmitter if it is FM too. And what should we do in order to reduce FM effect?

Again : that FM has not effect in your aim ! because FM detection is different with AM detection thus , your signal ( detected will be a good voice ) .