Lower cost, improved performance HDTV active reconstruction filters

In order to maintain the quality of HDTV signals and progressive scan DVD video signal quality, according to the requirements of EIA770-3, a set of three-channel (RGB/YPbPr), 5-pole reconstruction filters are required. The filter configuration is 30MHz bandwidth, providing greater than 40dB Frequency selection characteristics.

These filters traditionally use LC passive filters. Considering the steep frequency selection characteristics of the filter, group delay compensation is required. In addition, they also need to provide a certain gain to drive the back-terminated load and provide isolation between the filter, DAC and load. The cost of LC filters in these applications is very high due to difficulty in implementation, custom components and tuning required, and the snubber circuit further increases the cost pressure.

The active reconstruction filter introduced in this article is a low-cost alternative that uses RC components to provide a bandwidth of 30MHz and greater than 40dB suppression at the 74.25MHz frequency. It also has good group delay compensation and buffering The converter is implemented by a four-channel MAX4383 op amp. Although PCB parasitic parameters need to be considered when selecting components, compared to the 5% to 10% inductance tolerance, the improved capacitance tolerance of 1% to 2% can eliminate the need for adjustments in the production process. The circuit presented in this article uses dual power supplies, but the MAX4383 can also be powered by a single power supply by providing appropriate bias.

The filter itself is a Rauch architecture, providing 5-pole, multi-feedback, improved Butterworth filter characteristics, using a first-order +6dB gain stage and two-stage second-order unity gain amplifiers. The last stage of the circuit is a first-order group delay compensator that drives a back-terminated 75Ω load with unity gain. There are three sensitive points in the circuit: R14 is used to control the real part of the pole, C1 and C4 control the high-frequency pole, and C5 sets the GD compensation.

R16 and connectors are provided at the input of the circuit to replace the DAC load for circuit testing. The DAC load resistor is typically 35Ω to 40Ω, with R14 and R16 installed here. In some MPEG decoders, this resistance may be as high as 150Ω to 160Ω. In order to meet the requirements of different DAC loads, the R14 and C6 products need to be adjusted accordingly. Compared with the parasitic capacitance of the PCB at fs frequency (equivalent to a two-layer FR-4 PCB, using a 1/2oz copper-clad, 3/16-inch-thick ground plane around the op amp pins), the capacitance of C1 and C4 The value is smaller. C5 is used to adjust GD compensation. When wiring the circuit, it is best to first adjust the C1 and C4 values ​​to set the bandwidth before adjusting C5 to compensate GD, so that a fully configured PCB can be obtained.