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 of the MAX3580 to enhance the second-order input intercept point (IIP2). The integrated RF input switch enables the MAX3580 to select one of the duplex filters, allowing signals in the desired frequency band to pass while effectively suppressing interference from other bands. This feature makes the duplex filter particularly well-suited for DVB-T applications, as the standard specifies two separate frequency bands—VHF and UHF. Since these bands are independent, only one filter needs to be activated when suppressing signals from one band. By reducing interference, the second-order distortion is significantly improved, as the main source of such distortion comes from out-of-band signals.
**Working Principle**
The MAX3580 is a fully integrated direct conversion tuner that operates across two key frequency ranges: 170MHz to 230MHz (VHF-III) and 470MHz to 878MHz (UHF). It includes an RF input switch and multi-band tracking filters, offering a low-power solution that eliminates the need for secondary conversion stages. This zero-IF architecture removes the requirement for SAW filters and directly outputs baseband I/Q signals, simplifying the overall design and improving performance.
**Duplex Filter**
In DVB-T systems, where both VHF and UHF bands are used, a duplex filter can be combined with the MAX3580’s input switches to improve IIP2 and suppress strong out-of-band signals. The integrated input switch is illustrated in Figure 1. Figures 2 and Table 1 provide details on the recommended duplex filters. Both inputs of the duplex filter are connected to the antenna, with the VHF filter output connected to RFIN2 and the UHF filter output connected to RFIN.
Figure 1 shows the functional block diagram of the MAX3580. Figure 2 presents the schematic of the duplex filter, and Figure 3 displays the bill of materials for the filter.
When the receiver is tuned to the VHF band, the input switch directs the signal through a low-pass filter, allowing VHF signals to pass while blocking UHF interference. Conversely, when tuned to the UHF band, the high-pass filter allows UHF signals to pass while suppressing VHF signals. Figure 3 illustrates the measured frequency response of the two filters using the MAX3580 evaluation board (rev 1).
Figure 3 also shows the suppression and insertion loss characteristics of the duplex filter. When the MAX3580 is paired with the duplex filter, the isolation between the VHF and UHF bands is clearly visible in Figure 4. When tuned to the VHF band, the UHF signal is attenuated by approximately 15dB, and when tuned to the UHF band, the VHF signal is suppressed by at least 32dB.
This proprietary approach enhances the IIP2 metric without significantly affecting the noise figure. The improvement in the VHF band IIP2 is due to the attenuation of the UHF band, which causes AB distortion in the VHF band. Similarly, the UHF band IIP2 improves because the VHF band generates A+B and AB distortion in the UHF band.
As shown in Table 2, the duplex filtering scheme improves the IIP2 by over 30dB for selected input frequencies and LO settings, with a noise figure degradation of less than 0.8dB. As seen in Figure 4, the IIP2 is further enhanced through the isolation of out-of-band signals. When both input signals are outside the target band, the total attenuation contributes to the IIP2 improvement. For example, at an input frequency of 707.5MHz and 878MHz with a local oscillator frequency of 170MHz, the IIP2 can still increase by 25dB.
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