Video Over Fiber
MCP1700-500E2E/TT U6
3
5V
2 C12
1
1u
MCP1700-500E2E/TT
J2 +6V
1
GND
2
C13
C14
100n
10u 10V
5V
2
1
C10
C7
10u 10V
10n
C5
C9
10n
10n
L1
C4
10n
U4
4
10n
6
BCV62C 2
5 4
2
3
U2
BFT92
4
3
2
75R
&
R2
74LVC1G07
5
100n
R4
5 3
U3 2
MAX4090
1
3 4
5
U5 2
Q2 1
BFR540
ADCMP600
R5
R6
1k1
1
C3
Q1
U1
10n
1uH
VIDEO J3-B
10p
10n
L2
C1 1
C11
3
39R
1uH
R1 1k
C2
C8
2k2
U7
3
74LVC1G98
J1
2
OPTICAL FIBER
J3-A AUDIO
R3 75R
C6
HFBR1414Z 3
22p 130316 - 14
would be of the order of 11 MHz. The next problem is: find a very fast PWM circuit. The theory is quite simple. First you generate a triangular voltage. Then you put your video on one input of a fast comparator and your triangle voltage on the other input. The output of the comparator then supplies your PWM signal. That’s it!
For practical reasons we are limiting our supply voltage to 5 volts. It is also more convenient to move to a higher frequency as it relaxes the requirements on the filter after the PWM signal is received—plus it will also improve the quality of the signal. Let’s go to 20 MHz giving a period of 50 nanoseconds (ns).
Transmitter
For the switch we could use a MOSFET or a bipolar transistor. Both have their disadvantages: a MOSFET has too much capacitance and a transistor takes time to recover from saturation. A good solution is to employ open collector technology. The preferred logic is LVC being very fast, 5-volt tolerant and capable of high output currents (32 mA). A configurable logic LVC Schmitt trigger gate is used for generating the switching signal. Using a 5-volt supply voltage the sawtooth can reach 3 volts before the Schmitt trigger switches. In the Video Transmitter an R-C combination (R1C1) gives a pulsewidth of 10 ns which is enough to completely discharge a 22 pF capacitor (C8).
The schematic of the experimental video-over-fiber transmitter appears in Figure 1. Some of its design principles will be discussed below. First, back to the triangle voltage from the previous section. Of course it will have to be very linear, besides having to extend on both sides outside the video signal. A special case of the triangle voltage is the sawtooth voltage as used in analog oscilloscopes. A sawtooth is much easier to make than a pure triangular wave shape. The simplest form of the sawtooth generator consists of a capacitor, a constant current source and a switch—see Figure 2.
Figure 1. Schematic of the video transmitter for fiber optic links.
+5V
Figure 2. Let’s generate a sawtooth waveform.
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