The experimental result surpasses the -21-dB input mismatch required by TMAs. Morever, the good impedance matching is maintained over a wide bandwidth; i.e., equal to 83% of the center frequency (fc) at IRL ≤ -20 dB (Fig. 7). The experimental IRL is best around the input coupler’s center frequency. Likewise, the output coupler determines the output-return-loss (ORL) response. The model has semiquantitive agreement with the experimental result over most of the passband. However, the experimental ORL’s unexpected dip at 500 MHz was not predicted by the model, and this could be due to failure to model a component’s parasitics. As previously mentioned, the quadrature couplers enable the extremely wide matched bandwidth. Both input and output return loss are limited by the couplers’ finite isolation and by the microstrip discontinuities.

Balanced UHF LNA Simplifies Cell Towers, Fig. 7

The balanced LNA is unconditionally stable even when its constituent amplifiers are not. Both modeled and measured μ stability factor exceed unity over 50 MHz to 20 GHz (Fig. 8). Therefore, the balanced topology’s self-stabilizing promise is validated because the individual amplifiers are potentially unstable (μ < 1) at several frequencies over the evaluated range. The calculated stability factor has the same general trend as the experimental result, but their peaks do not converge exactly because of  the grossly simplified passive component models.

Balanced UHF LNA Simplifies Cell Towers, Fig. 8

Balanced UHF LNA Simplifies Cell Towers, Fig. 9

Balanced UHF LNA Simplifies Cell Towers, Fig. 10

The author notes that the input third-order intercept point, IIP3, is the highest ever reported (Table 2). The output power at 1-dB gain compression (P1dB) measures about +24 dBm at midband and is relatively constant over frequency, varying less than a 1 dB over a 1-GHz span (Fig. 9). The IIP3 was measured using -20-dBm input signals spaced 1 MHz apart, reaching +21.6 dBm at midband, or about 5 dB better than a competitive solution. But unlike the flat P1dB response, the IIP3 exhibits a pronounced peak (+21.9 dBm) at the approximate center of the couplers’ passband.  Referred to the output, the OIP3 is about +39.6 dBm at midband. The linearity figure of merit based on the ratio of OIP3 to DC power is about 15.7. In the noisy RF environment of a shared antenna tower, this design’s high P1dB and IIP3 values will support immunity to blocking and spurious mixing, respectively.

Activating the MMIC’s shutdown function transforms the amplifier into a nonreflective attenuator which can be used to prevent the overloading of subsequent stages. Signals passing through the shutdown amplifier are attenuated by about 16 dB at midband, while the input and output return losses are better than 20 dB (Fig. 10). Good impedance matching is maintained during shutdown because reflected energies are self-cancelled in the couplers. In comparison, an unpowered single-ended amplifier is highly reflective and must be bypassed when shutdown to prevent detuning of aerial and filter. To the author’s  knowledge, this is the first time this desirable LNA property has been reported and proposed for eliminating the customary LNA bypass switch in cellular towers. The current consumption for the balanced amplifier is a negligible ~176 μA per channel during shutdown.

In conclusion, balanced 900-MHz LNAs can be made more compact by designing around a MMIC with multiple integrated functions. The MMIC’s good noise performance allows smaller but lossier couplers to save space. The TMA’s low impedance mismatch requirements can be met by controlling differences between the amplifier’s S11 performance and the coupler’s isolation. The excellent impedance matching achieved during the LNA’s shutdown mode offers the potential to eliminate the need for LNA bypassing in cellular towers. This new design should enable smaller and better performing tower-mounted balanced LNAs.

Chin-Leong Lim, Engineer

Avago Technologies, 11900 Bayan Lepas, Penang, Malaysia; (604) 610-2525.

Acknowledgments

The author would like to thank Zulfa for the technical discussion, M.D. Suhaiza and S. Punithevati for fabricating the prototype, S.A. Asrul for reviewing the draft, and the management of Avago Technologies for approving the publication of this work. Anaren Communications (Suzhou) provided the couplers at no charge.

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