Test and measure AM and FM modulators

[audioguru]

If your scope is sync'd to the modulation's frequency, then you should see that each cycle of the carrier has a different amplitude caused by AM modulation of the carrier.
The envelope is the voltage tips of the modulated carrier. The "bubble" is an imaginary line that connects the carrier's tips so that the modulation is seen easier.

You won't see sidebands nor any waveform that is outside the carrier frequency unless a real-time- swept-frequency-display is used on the scope, just like the pics in articles in Google.

If the carrier is modulated and filtered as a single sideband or as a suppressed-carrier signal then the scope's display will look different, just like the pics in articles in Google.

 

[walters]

THanks for the information

carrier filtered as a single sideband

How do i filter out sidebands? the lower and upper sides bands do i put a EQ after the modulator section/stage?

whats the difference between a suppressed-carrier signal VS a regular carrier signal?

What is doing the AM modulation the actual modulation part is it a transformer? diodes? coupling? transistors how and what does the modulation please?

 

[mstechca]

What is doing the AM modulation the actual modulation part is it a transformer? diodes? coupling?

The entire circuit.

A transformer can't do modulation alone because a transformer is basically two groups of wound-up wires separated by something. Maybe a paper plate.

A diode can't do modulation alone because it passes current in one direction.

how and what does the modulation please?

The best way to examine the circuit is to "expand" and then "compress" it.
For example, if you see a transistor, picture it as a back-to-back diode.
If you see two resistors in parallel, use math to obtain one resistance value.

You might want to learn more about transistors.

 

[audioguru]

How do i filter out sidebands? the lower and upper sides bands do i put a EQ after the modulator section/stage?

The sidebands are at a radio frequency. An audio EQ filter doesn't work at radio frequencies. Look in the article you posted that shows an RF bandpass filter to select a sideband.

whats the difference between a suppressed-carrier signal VS a regular carrier signal?

The articles that you posted explain the difference very well.

What is doing the AM modulation the actual modulation part is it a transformer? diodes? coupling? transistors how and what does the modulation please?

An AM modulator is usually a circuit that changes the supply voltage to the carrier oscillator's output transistor, or any other circuit that changes the amplitude of the carrier.

 

[walters]

Thanks guys alot for the information

"expand" and then "compress the carrier supply voltages

Yes thats what the "bubble" or the "outside waveform looks like on the outside of the carrier frequency it looks like its expanding and compressing the supply voltages to make it envelope

The envelope voltages= the supply voltages being compressed and expanded?

The carrier is static its FIXED frequency the envelope voltages expand and compress and on the outside of the carrier frequency inside the "bubble"


""changes the amplitude of the carrier"'

Whats the different between amplitude VS amplitude modulation?

Amplitude would be a voltage divider or volume control

Amplitude modulation is compressing and expanding the amplitude
envelope voltages

 

[audioguru]

An amplitude modulator is a volume control for the carrier that operates at the frequencies of audio. The audio is the modulation. The video in TV broadcasts is also AM.

 

[walters]

amplitude modulator is a volume control for the carrier that operates at the frequencies of audio

Operates the Frequency of audio?

The audio stereo track is sitting on the outside of the carrier. The audio stereo track makes the envelope voltages that compress and expand the supply voltages which make the bubble?

 

[audioguru]

Amplitude modulation changes the amplitude of the carrier. At full modulation when the audio is loud, the power of the carrier changes from 0% to 200% at the frequency rate of the audio. Without any modulation, the carrier power is 100%.

Where in the world today does an AM station broadcast in stereo?
AM is for distorted muffled voices, not for stereo music.

 

[walters]

At full modulation when the audio is loud, the power of the carrier changes from 0% to 200% at the frequency rate of the audio.

Won't this cause a Tremolo amplitude volume effect?

If the audio dynamics changes up and down then the modulation of the amplitude of the carrier is going to changes from 0% to 200%


Without any modulation, the carrier power is 100%.

Whats the point in changing the carriers amplitude then?

Why would i want to change the carriers amplitude?

If no AM modulation then no audio dynamics from loud to soft?

 

[walters]

So the modulator stage takes a modulation frequency input and a carrier frequency input

The modulation frequency input "frequency rate" changes the amplitude of the carrier signal

Example

So if the frequency rate is going up and down , up ,up and down , down , up

The amplitude of the carries is going to go up and down, up , up and down, down , up


This is like a LFO frequency interfaced with a Volume pot


But a modulator is still different if u look at the pictures/graphs it compresses and expands the "envelope voltages"

This Envelope voltage compressing and expanding is what im confussed about

 

[audioguru]

What don't you understand? The height of the envelope is the amplitude of the carrier.
You understand that in an AM signal, the audio is increasing and decreasing the amplitude of the carrier, and you see the envelope of the modulated carrier increasing and decreasing at the rate of the audio. They are the same.

Maybe you are confused by the frequency rate of the audio:
1) Play a 1kHz sinewave tone. Its voltage increases each ms.
2) AM modulate an RF carrier with the 1kHz tone.
3) Then the carrier's amplitude increases each ms.
4) And the envelope of the carrier increases each ms.
5) Turn up the volume of the tone. The amplitude of the carrier and its envelope increase and decrease more.
6) Turn down the volume of the tone. The amplitude of the carrier and its envelope increase and decrease less.
The 1kHz tone is the modulation which is the rate that the carrier's amplitude is increasing and decreasing. The 1kHz tone can be any audio frequency.

 

[mstechca]

nice description audioguru :wink:

Walters, A waveform looks like a bunch of waves. To chart it, stick a horizontal line in it to divide the waves evenly.

anything above the line represents positive voltage. anything below represents negative voltage.

the voltage changes when amplitude changes.

Sorry, I can't draw you an example today. If I drew it, it would be total c**p.

 

[walters]

thanks alot Audioguru for your information

the frequency rate of the audio "controls" and "Changes" the envelope shape and modulation voltages?
The moduation mono audio track is "Sweeping" the amplitude up and down ask the audio music is being played

The Modulation signal is the Mono audio track?

The Carrier signal is the the radio station frequency data?

We don't hear the Carrier frequency signal because then the carrier and modulation signals come in the reciever of the radio the demodulation circuit the "diode" rectifys the signal

The Demodulation is a half wave rectifier circuit?

 

[audioguru]

Yes, yes, yes. I'm glad you understand. :lol:

 

[walters]

Whats the different of a OTA op amp VS a modulator stage is seems like the same thing. With a OTA op amp you can modulate the volume by sending a low frequency waveform in the control pin on the OTA and have the carrier frequency going to the input of the OTA

 

[audioguru]

An OTA (operational transconductance amplifier) has its gain voltage-controlled at its control pin. Therefore a signal at its control pin amplitude-modulates a signal through it.
I haven't seen a OTA IC that works at radio frequencies, most are used for audio effects.

 

[walters]

Thanks for the information

so a OTA does amplitude-modulates a signal through the OTA so its like the same thing

But when i put my probe on the output of a OTA i don't see a carrier and the amplitude modulation like what the radio modulator chip/stage does or look like with that bubble shapes

 

[audioguru]

But when i put my probe on the output of a OTA i don't see a carrier and the amplitude modulation like what the radio modulator chip/stage does or look like with that bubble shapes

Which OTA IC? What is its supply voltage? Attach your circuit's schematic.
What is the frequency and amplitude of the carrier?
What is the frequency and amplitude of the modulation?

 

[walters]

Which OTA IC? The 3080A operational transconductance amplifier

What is its supply voltage? +9 volts

What is the frequency and amplitude of the carrier? its audio signal guitar
or my 1K function generator

What is the frequency and amplitude of the modulation? its a low frequency 2 volts peak and 20hz


Its sweep the volume up and down but its doesn't 'modulate" like amplitude modulation does

 

[audioguru]

A CA3080A has a modulation input for a positive currrent. You fed it an AC voltage that might have destroyed it.

You don't show a schematic of your circuit so we can't see if its inputs are biased correctly.