Front-end duplex filter for MAX3580

Operating Principle

The MAX3580 fully integrated direct-conversion tuner covers the input frequency range of 170MHz to 230MHz (VHF-III) and 470MHz to 878MHz (UHF). The MAX3580 integrates an RF input switch and multi-band tracking filter, providing a low-power tuner solution without considering the cost and power consumption issues in the secondary frequency conversion solution. This zero-IF architecture eliminates all surface acoustic filters and directly outputs baseband I/Q signals.

Figure 1. MAX3580 functional block diagram

Duplex Filter

Since two independent frequency bands (VHF and UHF) are used in DVB-T applications, a duplex filter can be used to work with the MAX3580 input switch to improve the second-order input intercept point (IIP2). performance and suppress strong out-of-band signals. The integrated input switch is shown in Figure 1, and Figure 2 and Table 1 are the recommended duplex filters. Both inputs of the duplex filter are connected to the antenna. The output of the VHF filter is connected to the RFIN2 pin of the MAX3580, and the output of the UHF filter is connected to the RFIN pin of the MAX3580.
When the receiver is tuned to the VHF band, the input switch is set to receive signals through a low-pass filter that passes signals in the VHF band while suppressing signals in the UHF band. Likewise, when the receiver is tuned to the UHF band, the input switch is set to receive the signal through a high-pass filter that passes signals in the UHF band while rejecting signals in the VHF band. Figure 3 shows the frequency response curves of these two filters measured on the MAX3580 evaluation kit (rev 1).

The duplex filter is used with the MAX3580, and the isolation from interference bands measured on the same evaluation board is shown in Figure 4. When tuned to the VHF band, the signal in the UHF band is attenuated by 15dB (typical value); when tuned to the UHF band, the signal in the VHF band is attenuated by at least 32dB.

This proprietary approach improves IIP2 with minimal impact on overall noise figure. The improvement of IIP2 in the VHF band is attributed to the attenuation of the UHF band, which produces AB distortion in the VHF band. Likewise, the improvement in UHF band IIP2 is attributed to the attenuation of VHF band signals, which produces A+B and AB distortion in the UHF band.

As can be seen from Table 2: For the selected input frequency and LO, the IIP2 of the duplex filtering scheme is improved by more than 30dB, and the noise figure is reduced by less than 0.8dB. It can be seen from Figure 4 that IIP2 has been improved in other aspects, and IIP2 has been improved by isolating out-of-band input signals. When both input frequencies are outside the band, the sum of the attenuation for both is the improvement in IIP2. Therefore, when the input frequency is 707.5MHz, 878MHz, and the LO frequency is 170MHz, IIP2 can still be improved by 25dB.

Figure 2. MAX3580 duplex filter schematic

? Figure 3. Rejection and insertion loss of duplex filter

Figure 4. Isolation between frequency bands when using a duplex filter with the MAX3580

Table 2. What is the impact of duplex filter on IIP2 improvement and NF performance ?

Conclusion

By using a discrete duplex filter in conjunction with the MAX3580 DVB-T integrated tuning IC, the second-order input intercept point (IIP2) can be improved by more than 25dB and the suppression of out-of-band signals is enhanced, at the expense of reducing the noise figure of the receiving system. , but only reduced by 0.8dB.