To verify the operation, a prototype on an FR4 substrate was constructed for use in the 2.45 GHz ISM band (see Figure 6). The hybrid coupler is on the left, with antenna C above it, and ZV is on the right.

Previous work14 used discrete gain- and phase-switched elements for ZV. Although this allowed easy interfacing to a digital baseband controller, it lacked resolution, so analog elements were used in this version. Other work used MEMs tunable elements to tune the variable impedance but had limited tuning range.15

Here, A was implemented using the hybrid coupler with two of its ports terminated in PIN diodes, and θ was implemented using a hybrid coupler with varactor diodes at two of its ports16, both tuned with multi-turn trimmer resistors. These could easily be replaced with digital-to-analog converters for interfacing to a digital baseband controller. A coaxial cable linked A and θ so they could be tested independently. SMA adapters were used between the hybrid coupler and ZV, so the electrical length could be adjusted.

The FD system was evaluated with a Keysight N5172B EXG signal generator, which produces a 20 MHz bandwidth signal for the transmit path, and a Keysight N9010A EXA signal analyzer as the receiver. With antenna C connected, A and θ were tuned for maximum SIC over the entire 20 MHz signal bandwidth. Figure 7 shows the result for a 20 MHz bandwidth Wi-Fi signal at 2.46 GHz; it also shows the transmitted signal and the interference level when ZV was replaced by a 50 Ω load. 53 dB SIC was achieved, which is 30 dB better than with the 50 Ω load. This is important, as it justifies using an actively controlled ZV. For this result, the electrical length between the hybrid and ZV was 110 mm.

Figure 7

Figure 7 Measured performance with a 20 MHz, 2.46 GHz Wi-Fi signal, showing the transmitted signal, maximum SIC (ZV) and interference with a 50 Ω load replacing ZV.

Figure 8

Figure 8 Measured SIC across the 20 MHz ISM band showing effect of phasing unwrapping.

 

Self-interference was also measured across the full ISM band with a 20 MHz bandwidth signal. Figure 8 shows the performance, where the four traces represent different degrees of phasing unwrapping. Phase unwrapping is accomplished by increasing the electrical length between the hybrid coupler and ZV in one wavelength steps with SMA adapters. This ensures ∠VANT =-∠VZ over the entire 20 MHz signal bandwidth. The result of this can be seen in Figure 7, where the best match between VANT and VZ occurs at 2.46 GHz.

A comparison of FD systems is presented in Table 1. This design performs favorably, achieving the second largest SIC compared to other single antenna architectures. Only the work of Bharadia et al.3 performs better; however, this system is significantly more complex, using a multiple tapped delay line, making it impractical for consumer electronics.

Table 1

CONCLUSION

An RF FD front-end achieved a high degree of SIC over a 20 MHz bandwidth. While the target application is Wi-Fi in the 2.45 GHz ISM band, the approach is suitable for many other applications. A new Zv architecture was introduced using a tuneable loop to control the magnitude and phase of reflection. A practical system tested with a 20 MHz Wi-Fi signal achieved up to 53 dB SIC at 2.46 GHz, with a similar level of performance possible over the entire ISM band with this simple architecture.

ACKNOWLEDGMENT

The author thanks everyone at Toshiba’s Telecommunications Research Laboratory, particularly William Thompson, for their support and advice.

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