The MAX3580 is a direct conversion tuner designed for Digital Video Terrestrial Broadcasting (DVB-T). This article explores the implementation of a duplex filter at the input stage of the MAX3580 to enhance the second-order input intercept point (IIP2). The integrated RF input switch enables the MAX3580 to select one of two duplex filters, which effectively isolates the desired frequency band while suppressing interference from the other. This feature is particularly beneficial in DVB-T applications, where the standard specifies two distinct frequency bands—VHF and UHF. Since these bands operate independently, the system can selectively activate the appropriate filter to suppress unwanted signals, thereby reducing second-order distortion caused by interference.
**Working Principle**
The MAX3580 operates across a wide input frequency range, covering 170MHz to 230MHz (VHF-III) and 470MHz to 878MHz (UHF). It integrates an RF input switch and multi-band tracking filters, offering a low-power solution that eliminates the need for a secondary conversion stage. By using a zero-IF architecture, the device directly outputs baseband I/Q signals without requiring SAW filters, simplifying the design and improving performance.
**Duplex Filter**
In DVB-T systems, where both VHF and UHF bands are used, a duplex filter can be employed alongside the MAX3580’s input switch to improve IIP2 and suppress strong out-of-band signals. As shown in Figure 1, the integrated input switch allows the selection between two filters. The VHF filter output connects to RFIN2, and the UHF filter output connects to RFIN. This setup ensures that only the desired signal is passed through, with the other band being significantly attenuated.
Figure 2 illustrates the recommended duplex filter schematic, while Figure 3 shows the measured frequency response of the filters on the MAX3580 evaluation board. The results demonstrate effective rejection of out-of-band signals, with the UHF band being suppressed by approximately 15dB when the receiver is tuned to VHF, and the VHF band being attenuated by at least 32dB when the system is set to UHF.
Figure 4 highlights the isolation between the two bands when the duplex filter is used. This high level of isolation contributes to improved IIP2 performance, as it minimizes the impact of intermodulation distortion caused by signals in the adjacent band.
The use of the duplex filter not only enhances the IIP2 but also maintains a minimal degradation in the overall noise figure. According to Table 2, the IIP2 improves by more than 30dB for selected input frequencies and LO settings, with a noise figure reduction of less than 0.8dB. Furthermore, even when both input signals are outside the operating band, the combined attenuation of both filters leads to significant IIP2 improvement. For example, at an input frequency of 707.5MHz and an LO frequency of 170MHz, the IIP2 can still increase by up to 25dB. This makes the duplex filter an essential component in achieving robust and reliable DVB-T reception.
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