TV Modulátor

Toto zapojení používá obvod MC1373. Mezi 1. a 2. pin ze poipojuje cívka a kondenzátor, které spole paralelni rezonují na frekvenci televizních kanálu. Hodnota kondenzátoru je 56pF a cívky 0.1uH, což dává frekvenci 4. kanálu (67,25 MHz). Pro 3. kanál (61,25 MHz) je potoeba zminit soueástky na C=75pF a L=0,1uH.  Pro podrobnijší informace je na konci anglický originál.

This circuit uses an MC1373 to form a TV video modulator.  The coil and capacitor connected between pins 1 and 2 should be selected to have a parallel resonance at the carrier frequency of the desired TV channel.  The values of 56pf and 0.1uH were chosen for channel 4 carrier frequency of 67.25 MHz.  For channel 3 operation, the resonant frequency should be 61.25 MHz (C=75pF, L=0.1uF).  Resistors R2 and R3 are chosen to provide an adequate amplitude of switching voltage, whereas R4 is used to lower the maximum DC level of switching voltage below Vcc, thus preventing saturation within the IC.  Composite luminance and sync should be DC coupled to luminance input, pin 4.  This signal must be within the luma input dynamic range to insure linearity.  Since an increase in DC voltage applied to pin 4 results in an increase in RF output, the input signal should have a positive going sync to generate an NTSC compatible signal.  As long as the input signal is positive, overmodulation is prevented by the IC.  Chrominance information should be AC coupled to chrominance input, pin 5.  This pin is internally connencted to a resistor divider consisting of a series 300 ohm and a shunt 500 ohm resistor.  The input impedance is thus 800 ohms, and a coupling capacitor should be appropriately used.  For a fully commercial application, a vestigial sideband filter will be required.

Zdroj : Motorola.