AM modulation and demodulation circuit

Thread Starter

MrsssSu

Joined Sep 28, 2021
250
1663419313269.png
1663419532639.png
1663420742457.png
Dear all, above shows a modulator (transmitter) and a demodulator (receiver) and the circuit works well. However, this circuit only works if the transmitter and receiver are attached by wire. What modifications should be made to transmit the wave signal wirelessly from the transmitter to receiver? My attempt is I seperated the 2 circuit by building a resonant LC that is the same, hopefully it will resonate together? Is my approach fine?
LT Spice file is attached below for convenience.

Thank you and have a nice day!!
 

Attachments

Last edited:

ronsimpson

Joined Oct 7, 2019
2,385
.k l1 l2 .98
This command ties L1 to L2 by .98, this is an example from a switching power supply.
I have not used very small numbers like .01 which seems more like two coils separated some distance.
RonSimpson
 

Thread Starter

MrsssSu

Joined Sep 28, 2021
250
.k l1 l2 .98
This command ties L1 to L2 by .98, this is an example from a switching power supply.
I have not used very small numbers like .01 which seems more like two coils separated some distance.
RonSimpson
Hmm is my approach correct to transmit it wirelessly? ? :)
 

DickCappels

Joined Aug 21, 2008
9,297
Let's say you want to transmit at 1.0 MHz. I do not know what the laws related to radio transmission in your country. In the Untied States a transmitter is allowed 0.1 watts of power into the final radio frequency amplifier, which can connect a single element up to 3 meters long.

You would need to connect antennas to your two resonant circuits. As @ronsimpson pointed out, you can't count on magnetic coupling but the antennas will provide electrical coupling plus a little magnetic coupling.

You maximum distance between the antennas will not be very great because the transmitter power will be low and your receiver will not be efficient, but what you will have is a fundamental transmitter and receiver.

Though it is very crude, I designed a very simple (On-Off-Keying, emission type A1A) a basic RF transmitter and a "companion" receiver. You might find that you can incorporate some of the concepts in your design.

The transmitter:
1663426352192.png

The receiver:
1663426381166.png

The tricky part, and the part other had trouble with is tuning the resonant tank circuits to very close to the same frequencies.

Only use very short antennas for this. I think mine were about 10 or 15 cm. You don't want to annoying your neighbors or interfering with radio communications since you are not licensed. Send me a private message for the URL to web pages that describe how the circuits work. By the way the ATtin12 chips are microcotrollers use encode and decode signal sent via ON-Off-Keying (OOK).
 

Thread Starter

MrsssSu

Joined Sep 28, 2021
250
Let's say you want to transmit at 1.0 MHz. I do not know what the laws related to radio transmission in your country. In the Untied States a transmitter is allowed 0.1 watts of power into the final radio frequency amplifier, which can connect a single element up to 3 meters long.

You would need to connect antennas to your two resonant circuits. As @ronsimpson pointed out, you can't count on magnetic coupling but the antennas will provide electrical coupling plus a little magnetic coupling.

You maximum distance between the antennas will not be very great because the transmitter power will be low and your receiver will not be efficient, but what you will have is a fundamental transmitter and receiver.

Though it is very crude, I designed a very simple (On-Off-Keying, emission type A1A) a basic RF transmitter and a "companion" receiver. You might find that you can incorporate some of the concepts in your design.

The transmitter:
View attachment 276420

The receiver:
View attachment 276421

The tricky part, and the part other had trouble with is tuning the resonant tank circuits to very close to the same frequencies.

Only use very short antennas for this. I think mine were about 10 or 15 cm. You don't want to annoying your neighbors or interfering with radio communications since you are not licensed. Send me a private message for the URL to web pages that describe how the circuits work. By the way the ATtin12 chips are microcotrollers use encode and decode signal sent via ON-Off-Keying (OOK).
Hi, sir, thanks for your reply and diagrams. Is my LC resonant tank correct and both are equal to each other, and if I were to attach an antenna on the resonant tank circuit on my diagram, will it work hopefully? :) Your diagrams are a bit complex for me as beginner haha
 

ronsimpson

Joined Oct 7, 2019
2,385
Top picture is what your transmitter is making. 1mhz + 5khz No amplitude modulation
Bottom picture is what you need. The 5khz is changing the amplitude of the 1mhz.
1663428537160.png
 

Thread Starter

MrsssSu

Joined Sep 28, 2021
250
Let's say you want to transmit at 1.0 MHz. I do not know what the laws related to radio transmission in your country. In the Untied States a transmitter is allowed 0.1 watts of power into the final radio frequency amplifier, which can connect a single element up to 3 meters long.

You would need to connect antennas to your two resonant circuits. As @ronsimpson pointed out, you can't count on magnetic coupling but the antennas will provide electrical coupling plus a little magnetic coupling.

You maximum distance between the antennas will not be very great because the transmitter power will be low and your receiver will not be efficient, but what you will have is a fundamental transmitter and receiver.

Though it is very crude, I designed a very simple (On-Off-Keying, emission type A1A) a basic RF transmitter and a "companion" receiver. You might find that you can incorporate some of the concepts in your design.

The transmitter:
View attachment 276420

The receiver:
View attachment 276421

The tricky part, and the part other had trouble with is tuning the resonant tank circuits to very close to the same frequencies.

Only use very short antennas for this. I think mine were about 10 or 15 cm. You don't want to annoying your neighbors or interfering with radio communications since you are not licensed. Send me a private message for the URL to web pages that describe how the circuits work. By the way the ATtin12 chips are microcotrollers use encode and decode signal sent via ON-Off-Keying (OOK).
1663428897568.jpeg
Should the 10cm antenna be placed at these red position at both transmitter and receiver for it to work? :)
 

DickCappels

Joined Aug 21, 2008
9,297
If you want to continue to experiment with your design, yes, connect the antennas as shown. You might find that you get better results without L1 and C1. You are going to need a very good op amp for this to work at 1 MHz.

10 cm is too short for 1 MHz, try 1 meter first. (The technically astute realize that a loading coil would help a lot.)

Also, you are going to lose a lot of power with R3 it might be a good idea to put a capacitor (.001 uf to 0.1 uf) in series with R3 and reduce R3 to a few hundred ohms. Find the value by experimenting.

Do you need those two 2 volt offsets from V1 and V3? The opamp should be ok without them since you are using ±15V power supplies.

In order to get much modulation of your signal you should modify your circuit to get the two signals multiplied instead of added.
Edit: Maybe we should explain that amplitude modulating the carrier is necessary to...cause changes in amplitude of the signal so that the amplitude detector (AM detector) can output a signal that responds to the amplitude to the changes.

Your chances of an enjoyable outcome increase if you use a proven circuit like the one that @ronsimpson published in post #10. That solves the problem of multiplying instead of merely adding the carrier and modulation. @ronsimpson , does your transmitter run near the bottom of the U.S. AM band? If so, MrsssSu can use an AM radio as the receiver.

The question is one of whether MrsssSu wants to make something that works right a way or to develop their own design.
 
Last edited:

sparky 1

Joined Nov 3, 2018
731
Yes, the general idea is correct. You can gain more insight by looking at different methods.
The tendency of a single op amp or single stage often the signal is weak because the harmonics diverge over the spectrum that
so that the parallel resonant receiving circuit has trouble developing a strong enough resonance and the harmonics are just lost.

Getting both carrier and tone signals to mix in just the right proportions contributes to symmetry. At the same time there are factors reduce or have a dampening effect. Any small impedance mismatch makes this project somewhat tedious. When the carrier is stable the tone can be adjusted.
The focus on the circuit is getting finer adjustment that produces a better mix.

By using a balanced mixer like NE612, by design gives good results that produce a good, modulated signal. The project you have can
be adjusted but it is nice when the inputs are fixed being careful to mix proportions and to follow that with a second stage.

In a simple example of a transistor configured as a mixer for modulating a carrier with audio.
The bias resistors are used as gain control. All of the components and their role in modulation can be compared to the op amp circuit.
 
Last edited:

Thread Starter

MrsssSu

Joined Sep 28, 2021
250
If you want to continue to experiment with your design, yes, connect the antennas as shown. You might find that you get better results without L1 and C1. You are going to need a very good op amp for this to work at 1 MHz.

10 cm is too short for 1 MHz, try 1 meter first. (The technically astute realize that a loading coil would help a lot.)

Also, you are going to lose a lot of power with R3 it might be a good idea to put a capacitor (.001 uf to 0.1 uf) in series with R3 and reduce R3 to a few hundred ohms. Find the value by experimenting.

Do you need those two 2 volt offsets from V1 and V3? The opamp should be ok without them since you are using ±15V power supplies.

In order to get much modulation of your signal you should modify your circuit to get the two signals multiplied instead of added.
Edit: Maybe we should explain that amplitude modulating the carrier is necessary to...cause changes in amplitude of the signal so that the amplitude detector (AM detector) can output a signal that responds to the amplitude to the changes.

Your chances of an enjoyable outcome increase if you use a proven circuit like the one that @ronsimpson published in post #10. That solves the problem of multiplying instead of merely adding the carrier and modulation. @ronsimpson , does your transmitter run near the bottom of the U.S. AM band? If so, MrsssSu can use an AM radio as the receiver.

The question is one of whether MrsssSu wants to make something that works right a way or to develop their own design.
Dear sir, I have the best op-amp which is LF 356N which is very common nowadays. I read the datasheet , it says it has a 5MegaHertz bandwidth, so is that enough for to handle 1 MegaHertz? What op-amp do you recommend for this purpose to handle 1 MegaHertz with best performance, are there any common ones in the market ?

Thank you:)
 

DickCappels

Joined Aug 21, 2008
9,297
Well...
A 5 MHz gain x bandwidth product means you will not get much gain at 1 MHz, and the circuit you have will have very little amplitude modulation and the opamp doesn't do much if anything, so you would probably be just as well off if you omit the opamp and connect your short antenna to the cathode of D1 and then a 1k to ground.

1663494091917.png

I strongly suggest that you take the advice offered by @ronsimpson and by @sparky 1 in posts #10 and #12 respectively so that you have a good amount of modulation. The single diode modulator is tricky to set up and even harder to obtain what you might call "good results". Consider that two knowledgeable members of this site have shown you significantly better amplitude modulator methods than a simple diode mixer is likely to deliver, even when the offsets and amplitudes are optimized.
 
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