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[Studies in Computational Intelligence 481] Artur Babiarz, Robert Bieda, Karol Jędrasiak, Aleksander Nawrat (auth.), Aleksander Nawrat, Zygmunt Kuś (eds.) - Vision Based Systemsfor UAV Applications (2013, Sprin

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128<br />

S. Fraś et al.<br />

4.2 Multiplex<strong>in</strong>g Unit<br />

One of the most important parts of OVAD project was design and implementation<br />

of multiplex<strong>in</strong>g unit (fig.<br />

5). From the very first concept this unit was meant to<br />

choose two from six <strong>in</strong>com<strong>in</strong>g PAL signals generated by cameras and to forward<br />

those two selected signals to transmitters. Channels, that should be connected to<br />

transmission l<strong>in</strong>e output, are determ<strong>in</strong>ed by two analog multiplexers (Texas In-<br />

struments CD4051B [3]), which are controlled by STMicroelectronics<br />

STM32F103C8T6 [4]. Microcontroller, depend<strong>in</strong>g on received <strong>in</strong>formation<br />

through CAN, sets or resets adequate p<strong>in</strong>s connected to control p<strong>in</strong>s (A, B, C) of<br />

multiplexers. By use of Texas Instruments SN65HVD230 [5] (known as VP230),<br />

CAN signal is converted to signals, that are accepted by microcontroller.<br />

Fig. 5. Multiplex<strong>in</strong>g unit<br />

Properly implemented unit should prevent signal from be<strong>in</strong>g reflected back. So-<br />

lution of that problem is electrical term<strong>in</strong>ation of signal by use of 75 [Ω] resistors<br />

(term<strong>in</strong>ators). Value of resistance is determ<strong>in</strong>ed by type of video output used by<br />

cameras (composite 75 [Ω] 1Vp-p). However, <strong>in</strong> a way <strong>in</strong> which they are used,<br />

term<strong>in</strong>ators are creat<strong>in</strong>g voltage divider. If not corrected, output signal would have<br />

been divided by two. This<br />

negative effect is be<strong>in</strong>g corrected by operational ampli-<br />

fier Texas Instruments LMV651 [6], with local feedback. Connected as <strong>in</strong> fig. 7,<br />

section C, amplifier proves ga<strong>in</strong> which equals two. Every s<strong>in</strong>gle one of six <strong>in</strong>put<br />

signal is hav<strong>in</strong>g its offset<br />

recovered, which is elim<strong>in</strong>ated when signal leaves mul-<br />

frequencies of signal will cross through capacitor – the less capacity is, the higher<br />

frequencies would be treat as offset and elim<strong>in</strong>ated by capacitor. Because of that it<br />

tiplex<strong>in</strong>g unit by 10 [μF] capacitor. Value of capacity ensures that every needed<br />

is very important to ma<strong>in</strong>ta<strong>in</strong> both large capacity and small size of capacitors.<br />

Whole circuit of multiplex<strong>in</strong>g unit uses 3.3 [V] voltage supply. This voltage is<br />

nom<strong>in</strong>al for microcontroller and every s<strong>in</strong>gle element was selected with considera-<br />

tion of that supply voltage, which is obta<strong>in</strong>ed us<strong>in</strong>g Texas Instruments LE33CD<br />

[7] positive voltage regulator.

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