Electronic Design

Low-Cost Quad Op Amp Drives RF Modulator

The video circuit illustrated in Figure 1 combines an audio-subcarrier notch filter and group-delay equalization as required by the ITU-470 standard. It also includes an amplitude-adjustment capability for driving an RF video modulator in NTSC applications. (PAL operation requires a minor adjustment of the filter and all-pass values.)

For best performance, the input should be driven from a low-impedance source, such as an op amp or active filter. Two second-order all-pass stages (U1a, R1, C1, L1 and U1c, R13, C3, L3) and a first-order all-pass 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). Potentiometer R7 adjusts the output amplitude as required by the modulator in use.

The notch frequency is set by L2 and C2, and the notch depth and bandwidth are set by R4, R5, and R6. Typical notch depth is greater than ­16 dB (Fig. 2). Flatness above 2.5 MHz is ±0.5 dB.

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