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Transmit composite video signal

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roboticvn

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I found this circuit on the Internet. I want to make it but i don't have information about hardware. Can you tell me what devices to make this circuit? Thank you.
1687-1499-2013-250-3.jpg
this is composite video signal
1687-1499-2013-250-2.jpg
and the linear ramp carrier signal at a frequency of 15 MHz
 

crutschow

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You can use an IC video amp, which are commonly available.
The filter can likely be a simple RC low-pass filter.
The comparator needs to be a high-speed IC type with a propagation delay of no more than a few ns. That's likely the most difficult part to find.
The level shifter and LED driver depends upon the LED characteristics and the LED current.
 

roboticvn

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You can use an IC video amp, which are commonly available.
The filter can likely be a simple RC low-pass filter.
The comparator needs to be a high-speed IC type with a propagation delay of no more than a few ns. That's likely the most difficult part to find.
The level shifter and LED driver depends upon the LED characteristics and the LED current.
NTSC bandwidth is just about 6MHz, so I want to a IC satisfying. Can you tell me exactly IC's name? And can you show me why use lowpass filter? How does it work? Thank you^^
 

roboticvn

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Help me! I can't find IC for 15MHz PWM and Vin=2V @@
 

betwixt

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Why do you want a PWM IC at 15MHz?

The comparator generates variable width pulses but as pointed out you need high speed, high current devices to drive the LEDs.

Please expain what exactly you are trying to do, you will have great difficulty using this circuit to transmit composite video,at least with resonable quality and you also need a receiver circuit that can convert the signal back to composite before you can view it. It probably won't work with visible LEDs unless you use low brighness types and shield the light path from other light sources.

Brian
 

roboticvn

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Why do you want a PWM IC at 15MHz?

The comparator generates variable width pulses but as pointed out you need high speed, high current devices to drive the LEDs.

Please expain what exactly you are trying to do, you will have great difficulty using this circuit to transmit composite video,at least with resonable quality and you also need a receiver circuit that can convert the signal back to composite before you can view it. It probably won't work with visible LEDs unless you use low brighness types and shield the light path from other light sources.

Brian
yeah, I just transmit signal in short distances. The receiver and transmitter located directly opposite each other. NTSC signal has a bandwidth of approximately 6MHz so I use 15MHz triangle wave for PWM. Is it OK? I need a IC for this job with Vin=2V. I hope you will help me. Thank you very much
 

Borber

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You can try to amplitude modulate laser beam directly with analogue video signal instead of converting it to PWM.
 

betwixt

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15MHz is far too slow to get a good response over a 6MHz bandwidth. Your problem isn't generating the triangle wave (which is what you want, not a ramp), it is pulsing LEDs on and off at 15MHz rate. Ordinary LEDs have quite a large capacitance and high brightness types use fluorescence to generate the light which has an 'afterglow'. You would also need a fast sensor to pick it up, probably a PIN diode and high frequency amplifier. Basically, you are tackling a simple problem in a complicated way.

If you make the LEDs pass half the DC current you intend to use, then apply the video through a large capacitor (>1000uf) directly across the LED it will achieve the same result in two components! With care, because the biasing is critical, you can even send video long distances with laser diodes this way. The record is several tens of Km.

Brian.
 

schmitt trigger

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Having recently worked on a fast -though not as fast- photodiode amplifier, I can warn you that this will be the most challenging portion of the whole circuit.
 

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