Modulator IC for sub 50khz

[Skillet49]

I'm working on a EE senior design team that is designing a laser key system.

At the moment we are simply modulating the baseband with a predetermined bit sequence and transmitting this through our laser to our receiver.

I was talking with a professor today and he suggested that we include a second level of modulation so that our signal was not affected by the DC source that is the sun and other light sources.

I believe that this can be implemented fairly easily but I do not know what to search for when I am on DigiKey or Mauser. Basically, I need an IC chip that will take a digital input signal and modulate it with a carrier frequency of between 20khz and 50khz. The reason for the range is that the rise and fall times on our photodiode are such that we want to limit any distortion on the receiver.

I was wondering if anyone had any ideas on what type of chip to look for, and unfortunately time is of the essence, our project is due in less than two weeks.

I know that our design will work without this step, but adding this to our project makes it a much more legitimate system.

Thanks for your help

 

[PottedMeat]

So you just want to shift the baseband up ~35KHz?

Could you just do a bit sequence where logic 0 = 20KHz and logic 1 = 50KHz, buffer the photodiode output to two bandpass filters at 20 and 50KHz.

Modulation: maybe a 555 timer, you can have a MCU switch the correct timing capacitor (a MOSFET or something inline ) into the circuit when it wants a 50KHz output or a 20KHz output.

 

[Skillet49]

So you are suggesting an FSK type system then? I think it would be simplier to just modulate with just one frequency (convolute the digital baseband signal with the 20khz carrier and output to the laser)

 

[Colt45]

Have a small xtal osc like this, using the ubiquitous CD4069 and a watch crystal link.

Then run it and your digital signal through an AND gate [CD4081]. The output will be 32kHz when your digital signal is high, and nothing when it is low.




What is the speed of the data going to be though? The mod freq should be substantially faster.
Obviously with 32kHz you can't transmit at 115.2kbaud, 300/600/1200 baud is more like it.

output should look something like this. Yellow being data, and blue being the modulated output. (except the yellow will lead the blue, not lag, as in this shot)

 

[Skillet49]

Because we are only transmitting (at the moment) three 8 bit hex numbers + two start and stop bits, the data transfer rate is not that important.

Obviously we want the signal to be received no matter how long you shine the laser on the photodiode, but even using 1200 baud would be acceptable.

Though 300 Baud might be a bit slow.

 

[Modelworks]

Easiest way is to use a IR remote IC .
http://www.rentron.com/remote_control/TX-IR.htm

[http://www.rentron.com/Files/TX-IR.pdf

Looks like it is good to 2400 baud.

 

[soccerballtux]

Originally posted by: Modelworks
Easiest way is to use a IR remote IC .
http://www.rentron.com/remote_control/TX-IR.htm

[http://www.rentron.com/Files/TX-IR.pdf

Looks like it is good to 2400 baud.

IR doesn't go nearly as far as laser.

 

[Born2bwire]

Originally posted by: soccerballtux

Originally posted by: Modelworks
Easiest way is to use a IR remote IC .
http://www.rentron.com/remote_control/TX-IR.htm

[http://www.rentron.com/Files/TX-IR.pdf

Looks like it is good to 2400 baud.

IR doesn't go nearly as far as laser.

And you can't blind your neighbors or drive your pets crazy with it.

 

[bobsmith1492]

Originally posted by: soccerballtux

Originally posted by: Modelworks
Easiest way is to use a IR remote IC .
http://www.rentron.com/remote_control/TX-IR.htm

[http://www.rentron.com/Files/TX-IR.pdf

Looks like it is good to 2400 baud.

IR doesn't go nearly as far as laser.

You can easily drive a transistor as an amplifier or switch to drive a laser diode. I did that with a laser pointer and a basic AM light modulation circuit to transmit audio over a laser beam. It was good out to about 100 yards.

 

[Modelworks]

Originally posted by: soccerballtux

Originally posted by: Modelworks
Easiest way is to use a IR remote IC .
http://www.rentron.com/remote_control/TX-IR.htm

[http://www.rentron.com/Files/TX-IR.pdf

Looks like it is good to 2400 baud.

IR doesn't go nearly as far as laser.

And what does that have to do with using the IC for a modulator ?