Test and measure AM and FM modulators

[audioguru]

Walters, you are totally confused about modulation.

 

[walters]

Amplitude Modulation of the carrier.

The amplitude modulation only changes the carries amplitude not the frequency or timbre?

The radio volume control changes the amplitude modulation?

The radio station tuner changes the carrier frequency?

 

[audioguru]

Amplitude Modulation of the carrier.

The amplitude modulation only changes the carrier's amplitude not the frequency or timbre?

Yes, only the amplitude is changed.

The radio volume control changes the amplitude modulation?

No. An AM radio has demodulated audio at its volume control. The amount of audio that feeds the modulator at the radio station changes the amount of amplitude modulation.

The radio station tuner changes the carrier frequency?

Radio stations operate on a fixed RF frequency.

 

[walters]

The amount of audio that feeds the modulator at the radio station changes the amount of amplitude modulation.

This is what give dynamics to the radio song is from the amplitude modulation?

Why even use amplitude modulation why not just use a multiplexer to change the stereo audio tracks of the radio song to a carrier data line and use just the carrier frequency signal what does the amplitude modulation do?

I thought radio's used a mutliplier chip to change the stereo tracks to a carrier data line

So the stereo audio tracks go to a AM modulator chip instead of a multiplexer chip?

 

[walters]

Why do radio have a mixer/oscillator section?

What kind of mixer is this inside a radio?

what does the oscillator do with the mixer please?

 

[theinfamousbob]

walters - I'm going to point you to "Basic Communications Electronics" - it should help answer all your questions. Radio Shack might still stock it, but I couldn't find it in their online store.

https://www.amazon.com/gp/product/0...3512-8484856?_encoding=UTF8&v=glance&n=283155

good luck.

-Infamous

 

[walters]

thanks for the book

 

[theinfamousbob]

No problem. It covers all the basics of radio - here's a quick rundown of the chapters:

1)Basic Communications Systems
2)Analog System Functions
3)A Refresher (basics of electronics i.e. transistors, ohm's law, etc.)
4)Amplifiers and Oscillators
5)Modulation
6)Mixing and Heterodyning
7)Transmitters
8)Receiving - Including Detection
9)Transmission Links
10)Analog Integrated Circuits
11)Digital Signal Processing

Great book. Need to read it myself again.

 

[audioguru]

By the way, AM radio sound quality is garbage. Use FM stereo, it is much better because it is much more modern.

www.howstuffworks.com has a good explanation about how radios work.

 

[walters]

But it doesn't explain about the mixer and oscillator in the radio what does the mixer and oscillator do what kind of mixer is this it seems to be a special kind of mixer maybe a modulator?

 

[audioguru]

Correct, Howstuffworks is too basic.
The web is full of information about how radios worh. Look at:
https://en.wikipedia.org/wiki/Superheterodyne_receiver

 

[walters]

AM radios, the left plus right information must be transmitted by conventional amplitude modulation. It's how the left minus right stereo information is handled that differs between the various systems.

The left minus right stereo information is transmitted by low level phase modulation of the carrier


envelope detector recovers the AM while

What is envelope dectector chips?

a PM detector demodulates the left minus right stereo information


left minus right phase modulation by adding the proper phase shifts to the audio it is possible to generate an independent sideband signal with right on the upper sideband and left on the lower.

The left plus right signal is phase shifted -45 degrees while the left minus right is shifted +45 degrees (a total of 90 degrees).

The AM and PM sidebands?

phasing sidebands?

audio matrix?

This system transmits stereo by using 2 phases of the RF carrier 90 degrees apart. Each phase of the carrier is fed to a balanced modulator. The balanced modulator that is in phase with the original RF signal receives left plus right mono audio. The balanced modulator that is 90 degrees out of phase receives the left minus right stereo information. The balanced modulator outputs are summed together with the original in phase carrier and then passed through a limiter so only the phase information is retained.

quadrature AM signal?

phase-shifted left minus right information?

 

[walters]

**broken link removed**

https://zone.ni.com/devzone/nidzgloss.nsf/webmain/7F3E1AAEA57FB09B8625686A007943BA



Whats a carrier "null" trimmer?

 

[walters]

They like to ""phase shift the carrier frequency"'' Before the modualor chip and then use the RF modulator/mixer when using 2 modulator chips

What does phase shifting the carries frequency do?

 

[audioguru]

AM radios, the left plus right information must be transmitted by conventional amplitude modulation. It's how the left minus right stereo information is handled that differs between the various systems.

AM stereo? AM sounds terrible. Most of the high frequencies of audio are missing and the distortion and interference are very high.

What is envelope dectector chips?

It isn't a chip. It is a diode.

left minus right phase modulation by adding the proper phase shifts to the audio it is possible to generate an independent sideband signal with right on the upper sideband and left on the lower.

Left channel on one sideband and right channel on the other sideband? Both in the narrow bandwidth of an AM broadcast station? It would sound even worse than a mono AM radio.

audio matrix?

This system transmits stereo by using 2 phases of the RF carrier 90 degrees apart. Each phase of the carrier is fed to a balanced modulator. The balanced modulator that is in phase with the original RF signal receives left plus right mono audio. The balanced modulator that is 90 degrees out of phase receives the left minus right stereo information. The balanced modulator outputs are summed together with the original in phase carrier and then passed through a limiter so only the phase information is retained.

quadrature AM signal?

Yes. AM radio stations played with stereo many years ago. There were a few incompatible competing systems. Do any AM stations broadcast stereo today?

 

[walters]

Thanks for the information and correcting me


I think the mixer/oscillator in a Radio is actaully a AM modulator chip but they call it a mixer because it does f1-f2 and f1+f2 so its seem like the same thing a mixer=modulator i guess

Modulators:
- Diode used as modulator (there are many variations of this)
- voltage controlled amplifier circuit (use video signal as the control voltage)
- voltage or current controlled attenuator (transistor, fet or diode based)
- dual gate FET (one gate gets RF signal, other video signal, output is a combination of them)
- special mixer IC circuit (analogue multiplier)


Does the data sheets for AM modulators show a schematic of whats inside a AM modulator? im trying to find a schematic of the inside of a AM modulator because how does it produce the AM modulation waveform?

I see the carrier frequency and i understand that but i don't understand about the AM modulation waveform how it creates thats sort of modulation how can diodes or a VCO makes this type of modulation it loook like it has a "Bubble" around the carrier frequency the modulation frequency changes the "Bubble shape" around the carrier frequency how does this modulator chip create that "Bubble shape" around the carrier frequency?

 

[audioguru]

AM modulators aren't a chip. They are usually just a circuit that modulates the supply voltage of the RF oscillator.

The waveform envelope doesn't have "a bubble" It is drawn to show the differing heights of the amplitude-modulated carrier.

 

[mstechca]

I think the mixer/oscillator in a Radio is actaully a AM modulator chip...

I don't think there is such thing as an AM modulator chip. If there was, then young kids with some electronic knowledge could make transmitters and potentially screw up the entire neighbourhood :lol:

a mixer=modulator i guess

Not really.

A mixer takes two analog signals and combines them to produce one output signal.

A modulator (especially when dealing with AM and FM) takes an analog signal, and outputs the same signal at a different frequency.

Modulators:
- Diode used as modulator (there are many variations of this)

Two diodes back-to-back are common. Better known as a transistor modulator.

- voltage controlled amplifier circuit (use video signal as the control voltage)

Many modulators are voltage controlled oscillators.

- voltage or current controlled attenuator (transistor, fet or diode based)

I think the only use for an attenuator is to make the signal more clear. Other than that, it will hinder the range of the receiver, or hinder the transmitters range.

Resistors make perfect attenuators :wink:

- special mixer IC circuit (analogue multiplier)

You could make a simple mixer. Take two resistors and connect one end of each one together. The tied (connected) ends make an output, and the untied ends make the inputs to each signal.

Does the data sheets for AM modulators show a schematic of whats inside a AM modulator?

You will be lucky if you find any, and if you do, you will find a datasheet on an IC that helps form the modulator. If you want a schematic, search google for "transistor transmitter".

how does it produce the AM modulation waveform?

AM = amplitude modulation. Amplitude = volume. so basically, the volume (amplitude) changes when the input to the modulator changes.
With FM, the frequency of the waveform changes when the input changes. I find it better altering the frequency rather than the amplitude, because the amplitude has a higher effect on gain. This explains why AM is terrible.

I see the carrier frequency...

Can I have your eyes? :shock: :lol:
We can't see the carrier frequency directly. LOL

i don't understand about the AM modulation waveform how it creates thats sort of modulation

I explained it above.

...VCO makes this type of modulation...

VCO = Voltage controlled oscillator.
As it states, the oscillator changes with respect to the input voltage. They can help you achieve FM modulation because oscillators produce frequency, and in a VCO, changing the voltage can change the frequency.

how does this modulator chip create that "Bubble shape" around the carrier frequency?

The best way to learn is to not depend on multi-functional chips. In fact, start with transistors.

Try this circuit. It is an FM transmitter.

**broken link removed**

Also, talkingelectronics.com has alot of info about FM transmitters.

 

[audioguru]

The transmitter circuit posted by MStechca transmits both AM and FM, but on the frequency used by FM radio stations.

 

[walters]

Thanks guys for the help

I guess a modulator circuit or network is a coupling network capacitive or transformer coupling with 2 transistors to do the sum and difference modulation

The "Bubble" is the Envelope voltages and its called the carrier modulation

Without the carrier input frequency how do i get a hollow envelope voltages?

The Envelope voltages surround the carrier frequency

Are the Envelope voltages the side bands? or is the carrier frequencys the side band

I see the carrier frequency on the oscilloscope but what i don't understand is how do i just get the Envelope voltages the waveform on the "outside" of the carrier frequency

What is that waveform on the "outside" of the carrier frequency called?

The waveform on the "outside" of the carrier frequency has envelope voltages how are they made?