Hey Community
As a part of a project, I am building an Amplitude Modulator and synchronous demodulator on breadboard. For that, I first simulate a circuit on LTSpice. The circuit and modulated output is shown in the image below. The issue is that the complete sinusoidal signal is not getting modulated. when you zoom the modulated signal, you see that two waves are located with a distance between them. How can I correctly modulate the signal? The reference images and the LTSpice model are attached alongside. Thanks in advance for the help.

 

hi 477,
Welcome to AAC.
This point is not connected.??
The circuit needs a redesign.
E

 

Ahhhh...........silly mistake no 1.
I actually took the reference from this YT video -

. Yes the point is connected. The RF input is the carrier signal.
But now after connecting, the result is even more worse. I have attached the pics. Why is there a spike in the modulated signal. Which value is letting this happen. Actually, I need to demonstrate this for an audio signal. (Taking sine wave is just for test). Can you please suggest me the changes I should make to this circuit in order to get a perfectly modulated signal atleast for sine wave. Thanks in advance.

 

Hi,
If you build the circuit as shown by Fesz, it works fine, you have changed a number of parameters.
E

 

Actually, I don't have all values of capacitances and resistances available for hardware model. So, I had to change parameters according to the values I had. I made my circuit through hit-and-trial method i.e checking by increasing and decreasing values since I don't know the exact calculation behind the values. Can you please tell me how to calculate which values to take and how to change the parameters? I know this is a very tedious task to write so much just to explain me but I request you because I am unable to find how to do so on the internet.
A very sincere thanks in advance

 

Hi 477,
The first step is to decide the specification of the circuit and its purpose?

E

 

Its just a project of Hardware demonstration of Amplitude Modulation for audio wave(maybe a song) as an input signal and take its output. What do you want to know by specification?

 

Hi,
If you build the circuit as shown by Fesz, it works fine, you have changed a number of parameters.
E
View attachment 321058

That is not an AM waveform, it is basically one frequency added to another, not multiplied.
Below is how it should look.

 

That is not an AM waveform, it is basically one frequency added to another, not multiplied.
Below is how it should look.

View attachment 321068

Can you help me?

 

Hello,

Put an capacitor from the emittor of the transistor to ground.
See this page for more info:
https://sound-au.com/articles/am-modulation.htm
Look for figure 3.
Also a tuned circuit is at the output.

Bertus

 

Site not working. Connecting capacitor like you told isn't working.

 

Hello,

If you mean C4, that is not the correct place.
Read the page I gave you.
From that page:


Bertus

 

From that page:

LTspice sim without the optional tuned circuit on the output:
A little distorted, (Fourier analysis shows a little less than 4%) but it is indeed AM modulation.

 

Thanks for it. But the circuit you provided does not work well for audio file. Also, can u tell me how did you determine the resistor and capacitor values used in the circuit. the audio file (test2.wav (zipped)) is attached along.

 

Hi gup,
The amplitude of your wav audio signal is much too low, it has to be amplified by a factor of ~7000 in order to modulate the carrier signal correctly.
E
Note, I have edited your wav file name.


Update: I have just noticed that the Modulated carrier signal is the inverse of the
audio modulating signal! See 2nd Image.
If you look at approx 2.1 mSec in the plot you can see the maximum carrier and audio are inverted.
@crutschow will advise.

 

I don't have hardware amplifier. Also, when I checked the peak-to-peak voltage of audio signal through laptop is able to reach max of 0.2mV. What should be the changes I can do in my circuit design to properly modulate the audio signal? Also, the frequency of that sound is around 11kHz. Please suggest the changes for the same as well.
Thanks in advance