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Maxim > Design Support > Technical Documents > Application Notes > Video Circuits > APP 3745
Keywords: audio subcarrier, notch filter, group-delay equalization, RF video modulator, NTSC,
APPLICATION NOTE 3745
Low-Cost Quad Op Amp Drives RF Modulator
Mar 13, 2006
Abstract: A video circuit combines an audio-subcarrier notch filter, group-delay equalization, and an
amplitude-adjustment capability for driving an RF video modulator in NTSC applications.
The video circuit of Figure 1 combines an audio-subcarrier notch filter and group-delay equalization as
required by the ITU-470 standard. The circuit also includes amplitude-adjustment capability for driving an
RF video modulator in NTSC applications. (PAL operation requires a minor adjustment of the filter and
allpass values.)
Figure 1. Bandstop filter and delay equalizer for NTSC applications.
For best performance, the input should be driven from a low-impedance source such as an op amp or
active filter. Two second-order allpass stages (U1a, R1, C1, L1) and (U1c, R13, C3, L3) and a first-order
allpass stage (U1d, C4, R14) form a fifth-order group-delay equalizer that compensates for group delay
introduced by the notch filter (U1b, R6, C2, L2). See Figure 2. Potentiometer R7 adjusts the output
amplitude as required by the modulator in use.
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