Low-Cost Quad Op Amp Drives RF Modulator

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要約

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.

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.

Figure 2. Gain and group-delay response for the notch filter and group-delay compensation of Figure 1.

Figure 2. Gain and group-delay response for the notch filter and group-delay compensation of Figure 1.

The notch frequency is set by L2 and C2; the notch depth and bandwidth are set by R4, R5, and R6. The typical notch depth is greater than −16dB. Flatness above 2.5MHz is ±0.5dB.

A similar article appeared as a Design Brief in the July 21, 2005 issue of Electronic Design.