Time domin of AM

Hello there,

The output result of my conventional AM modulator at 200kHz of carrier and 5kHz of modulating signal is bellow:

the second picture is just the above one but maginfied to see more details of the output signal. What is your idea about those distortions at the second pic? are they natural??
The modulator is made by 4066 which the said chipis a quad "SWITCH" CMOS chip.

It looks like the CD4066 has a square-wave as the carrier frequency or is severely clipping the carrier frequency. Maybe your modulator is shorting (overloading) its output to create AM.

A 4066 is a switch; not a "modulator".

How do you expect to get normal AM modulation using a 4066 switch?

And the schematic reference says SSB. If the circuit is supposed to do single-side-band then that's different than standard AM modulation.

Post your schematic.

This square wave carrier AM spectrum must be bizarre.

A 200kHz carrier with a 5kHz SSB signal will look like a 200kHz and 205kHz or 195kHz (depending on whether it's the lower or upper side-band) voltage source connected in series. I'll post a plot if you like.

As already suggested, this isn't modulated, it's 'chopped' - and no way is it anything like SSB. A 4066 CMOS switch is entirely the wrong device to try and use.

Hero999 said:

A 200kHz carrier with a 5kHz SSB signal will look like a 200kHz and 205kHz or 195kHz (depending on whether it's the lower or upper side-band) voltage source connected in series. I'll post a plot if you like.

Click to expand...

I mean, instead of a dirac at 200 kHz, you are going to have a Sinc function envelope centered at 200 kHz as the carrier, due the square wave carrier.

A 150 kHz to 250 kHz window:


Both square-wave and cosine-wave carrier AM looks the same.


When we use a larger window, from 190 kHz to 1805 kHz:
Cosine:


Square:


Observe that the square-wave carrier AM spreads across the spectrum.

The carrier should always be sinusoidal, a squarewave isn't the way to do it.

Hayato is correct, however I would argue that he is slightly overmodulated because he is showing a spread of side frequencies, not just the carrier at 200khz and 195 and 205khz side frequencies.

If a 200khz square wave is modulated at 5khz, there will be a modulated carrier at 200khz, and, carriers at 600, 1000, 1400khz etc, every odd harmonic of 200khz.
(Remember that in the frequency domain, a square wave is composed of all the odd harmonics.)
A low pass filter which reduced the harmonics to a low level, would convert the modulated square wave into a modulated sine wave.

JimB

@Hero999
That's why those cheap AM transmitters schematics based on the 555, and logic chips are all aberrations. =P


Jimb. The wave is not overmodulated nor undersampled, take a look at the carrier frequency about 66% higher than the sidebands. (At the first pic).

As I used FFT to calculate the spectrums, I have to sample (I sampled at 10 MHz -> 25x Nyquist Freq. ), use a number of points for FFT (16384), and use a window (in this case was a rectangular window). All of this allegory causes the spectrum to be shown that way.

Think about it, a rectangular window in time is going to be a Sinc-shaped window in frequency domain, plus the sample pulses and FFT resolution will give us a lot of Sinc-shape forms at the frequency domain. (And if you see those Scopes that measure the spectrum based on FFT display spectrum like those I posted (noisy)).

But, if you observe, the ratio between a sideband (Magnitude = 0.6) and the highest noise peak at 208 kHz is about -10 dB.


If I use another window, the spectrum becomes clearer, but I have some losses, take a look, this is the same spectrum using a Tukey window, with a rolloff = 0.55. (0 = rectangular, 1 = Hanning).
You are able to see the carrier, the sidebands (with degraded amplitude), with much less noise:

Audiuguru, Ron and me, have tried it and it worked just fine as a good and stable AM modulator.

The main problem of the circuit I designed last weak was due to an fult in its input op-amps, which caused the Waveform to be distorted.

I'm not saying it doesn't work. Look at the spectrum I posted, they are pretty similar. But, if you use a non-sinoidal carrier, your spectrum becomes "dirt" and spreaded. For a 200kHz square-wave carrier, you are going to get signal on 600 kHz, 1 MHz, 1.4 MHz... (Generating interferences on those channels).

Any nonsinoidal waveform craetes harmonics, A aquare waveform craetes odd harmonics. So do you want that REAL modulators do not craete harmonics while they are working with Sine waves as carrier?!!

I suspect that you are using an ordinary ultrasonic piezo transducer that resonates at its rated frequency so it will not produce harmonics and will not produce your modulation. It also will not produce enough power for your application.

Electronman said:

Any nonsinoidal waveform craetes harmonics, A aquare waveform craetes odd harmonics. So do you want that REAL modulators do not craete harmonics while they are working with Sine waves as carrier?!!

Click to expand...

That's why good comercial transmitters work with sinewave carrier.

audioguru said:

I suspect that you are using an ordinary ultrasonic piezo transducer that resonates at its rated frequency so it will not produce harmonics and will not produce your modulation. It also will not produce enough power for your application.

Click to expand...

Agreed.

audioguru said:

I suspect that you are using an ordinary ultrasonic piezo transducer that resonates at its rated frequency so it will not produce harmonics and will not produce your modulation. It also will not produce enough power for your application.

Click to expand...

Hi audioguru,
What do you mean by that??!

Electronman said:

Hi audioguru,
What do you mean by that??!

Click to expand...

1) You are amplitude-modulating the piezo transducer. If you modulate with voices then you need a bandwidth for the audio of from 300Hz to 3400Hz for telephone quality. But the AM sidebands will have a bandwidth of 6800Hz.
If your piezo transducers resonate (have a sharp peak) at 40kHz then their bandwidth is too narrow to produce the AM sidebands and the result will be unsatisfactory.

2) The companies that make these "sounds from anywhere" devices use custom-made very high power and wide bandwidth arrays (many transducers together) of piezo transducers. Their many transducers in an array produces a focussed narrow beam of ultrasonic energy.
You don't have them.

audioguru said:

But the AM sidebands will have a bandwidth of 6800Hz.

Click to expand...

No, as he's using SSB, the required bandwidth will be 3.4kHz.

Oh yes, it is SSB.
I wonder if he found a 200kHz piezo transducer with a bandwidth as wide as 3.4kHz?