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
Certainly .Thank You for ur fast reply, Can u please clarify more ?
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 dear BrianGoldsmith, Eshal is doing something similar in a different thread
( however a non linear amplifier can be used too ! for higher powers ! with power modulation )Any LINEAR amplifier can be used for AM,
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 ?it can be class A, AB, B, C
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 .I regularly used 10mW for communication over about 15kM
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 .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.
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 .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.
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.
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 .
Hi dear BradtheradThis 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.
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