SIMULATION AND MEASUREMENT
Simulation was performed using ANSOFT HFSS 13.0, and the predicted performance is shown in Figure 3. Measurements made with a Keysight Technologies 8531B network analyzer (see Figure 4) show a center frequency at 580 MHz with |S21| and |S31| = -3.0 dB. For |S21| and |S31| within -3 ± 0.5 dB, the measured fractional bandwidths are 24.1 and 24.2 percent, respectively. Figure 5 shows the phase difference between S21 and S31. With a criterion of ±1 degree around the nominal 90-degree phase difference, the frequency range is 570 to 600 MHz, corresponding to a bandwidth of 5.2 percent.
Figure 4a shows that eleventh harmonic signals are effectively suppressed with |S21| and |S31| below -10 dB. This means the new coupler will protect any following circuitry from interference from 1.1 to 6.5 GHz, such as from the IEEE 802.11 a/b/g standard.
The circuit area of a conventional branch-line coupler at the same frequency is approximately 3900 mm2. By comparison, this prototype represents a surface area of 14.8 percent (see Figure 6). Table 1 compares the performance of the prototype coupler with previous work.
A new microstrip branch-line coupler uses modified radial stub loaded resonators to achieve compact size and wideband harmonic suppression. With eight modified radial stubs placed inside its free area, the occupied area is reduced to 14.8 percent of a conventional design at 580 MHz. The in-band performance is comparable to that of a conventional design and suppresses up to the eleventh harmonic. The measured performance agrees closely with the design simulation.
This work was supported by the Natural Science Foundation of China under Grant Nos. 61377080 and 61302842.
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