An alternate LED driver which uses the 74F3040 line driver was incorporated in this particular application example. The 74F3040.has a higher current rating, but not the variable threshold capabilities at the NE522. The LED diode is operated in the saturated on-off mode for best signal to noise. This receiver circuit consists of wideband differential amplifier NE592, VCO NE564 and LED driver NE522-tbe high-speed comparator. The video signal is ac coupled into the modulator preamplifier and followed by a sync tip clamp to provide de restoration on the composite video signal and to prevent variation of modulation deviation with varying picture content.
A video signal level of 250 to 300 m V peak is required to maintain optimnm picture modulation. Frequency compensation (preemphasis) is inserted in the form of a passive rc lead network at tbe input to the NE592 differential amplifier. The main FM modulator consists of an NE564 used only as a linear wideband VCO, but the other sections of the device are not used. Differential de coupling to tbe VCO terminals is attained via tbe loop filter terminals, pins 4 and 5.