Mixer

[ryanleung]

hello~~~

I would like to multiply the carrier and  the audio signal.  And I want to design the mixer to multiplier these two signal.  What can I do?? can I use the summing amplifier???

 

[audioguru2]

If the carrier is already modulated, a frequency multiplier or mixer circuit will increase or decrease the carrier frequency, not its modulating audio frequency.

A frequency multiplier, RF mixer or modulator isn't a summing amplifier.

I think you want to use the audio to modulate the carrier. Use an AM, FM or phase modulation circuit, not a mixer nor a multiplier.

What carrier frequency?
Which type of modulation?

 

[ryanleung]

I want to use the amplitude modulation and the carrier frequency is 40kHz

In my project, I would like to tansmit the carrier and the audio signal by infrared.  At the receiver side, I would like to use the envelope detector to take the original audio signal.

 

[audioguru2]

An AM modulator simply varies the level of the carrier. Since your IR doesn't care about harmonics, varying the supply voltage of a  transistor in the IR oscillator with the audio signal would give AM modulation.

Compact fluorescent lights also produce AM modulated 40kHz IR. Your receiver will pick them up. Also TV remote controls and other stuff use AM at 40kHz.
Your receiver's envelope detector should have highpass filter to feed it to avoid hum from incandescent and ordinary fluorescent lights. ;D

 

[ryanleung]

And can you suggest some circuit that can generate the 40kHz? Wien-bridge oscillator?

 

[audioguru2]

There are many different kinds of oscillators to choose from. A 555 IC is usually used.
I can't recommend an AM modulator circuit since I hate the poor sound quality of AM.
I would use FM like most IR communicators.