Re: 40kHz Infrared Serial Modulation?

redbrickhat wrote: 
I'm trying to transmit serial messages at 1200 bps to an IR receiver
such as a GP1U52X.
The data*** says that the 40kHz signal must be further modulated by a
100 to 1000 Hz signal, I think because after several cycles are
received the AGC in the receiver reduces gain.


it attempts to block a continuous carrier-only signal



I was wondering if this second modulation is necessary when sending
serial information (since the 1200 bps signal itself is like a
modulation), and what would be the ideal frequency for such a signal. I
suppose one problem would be if the UART were to send several "1" bits
in a row since the GP1U52X might only detect the first ones before the
AGC reduces gain.


look at the codes the receiver supports, then compare with say the vishay part mentioned below.


I was also wondering if the duty cycle of the 40kHz signal has to be
50%. I want to try a much lower duty cycle to pump more current into
the IR LED.


exhume monsieur Fourier, he'll tell you what will happen. there will be a bandpass filter after the AGC, centered on the carrier. as you reduce the duty cycle below 50%, the amplitude of the fundamental decreases, so your range reduces. reductio ad absurdum - at 0% duty cycle it definitely wont work.

Any feedback will be appreciated.

Thank you.


I'm not familiar with that particular detector but most IR receiver chips have circuitry to block CW (continuous wave) modulation. I recently designed a vishay TSOP32138.

their constraints are:

- burst length should be 6 cycles per burst or longer - >= 158us
- after each burst which is between 6 cycles and 70 cycles, a gap time of at least 10 cycles (263us) is necessary
- For each burst which is longer than 1.8ms, a corresponding gap time is necessary at some time in the data stream. This gap time should have at least the same length as the burst.

HTH

cheers
Terry

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