A Compact W-CDMA RF Front End
Introduction to a complete RF front end assembly that meets environmental requirements for base station service
As the environments in which cell sites are located becomes more constricted, designers of base stations for third-generation networks are faced with challenges such as increased functional integration and other size-reduction techniques similar to those of their mobile phone counterparts. The model FEU-WCDMA1 is a W-CDMA RF front end that measures only 240 x 178 x 56 mm, and forms a complete front end for a W-CDMA transceiver that meets stringent environmental requirements for base station service.
The FEU-WCDMA1 front end assembly comprises a diplexer, a low noise amplifier (LNA), a four-way power divider and a 30 dB dual-directional coupler that can be used to measure the forward transmitted and received power. The coupler measuring the received power is used to monitor the condition of the antenna. If the antenna is damaged, a large reflected transmit signal will be reflected to the received power coupler to indicate a fault. For this reason the received power coupler must have >30 dB directivity so that the normal transmitted signal power is more than 60 dB down at the received power coupler output. In order to achieve a compact LNA design, a single-layer board with surface-mount devices was employed. The three-stage balanced LNA has a 1.8 dB typical noise figure, achieving 34 dB gain in the receive path. The insertion loss in the transmit path is 0.7 dB typical. The return loss is better than -18 dB at all ports. The diplexer provides isolation of 82 dB from the Tx port to the four Rx outputs and 110 dB from the Rx antenna input to the Tx port. The four-way power divider is at the receiver output port.
Loss in the transmit path has a great impact in high power applications, where every 0.1 dB saved is critical to the system range of operation and sensitivity. For instance, at 200 W CW transmitted (+53 dBm), power is reduced to 158 W if the insertion loss is 1 dB, while if the insertion loss is held to 0.7 dB, power transmitted increases to 170 W. To achieve high isolation, the module includes a sixteen-section diplexer, with eight resonators for the receive path and eight resonators for the transmit path. The resulting transmit/receive diplexer isolation is greater than 110 dB. The pass band flatness is 0.7 dB peak-to-peak (receive) and 0.5 dB peak-to-peak (transmit). In order to achieve these specifications, designers used commercial and proprietary electromagnetic modeling software in order to develop the filter mask. The diplexer filters are typically combline designs with cross coupling for sharp rejection while maintaining a flat pass band. Narrow band diplexers employing dielectric resonators with an unloaded Q of 20,000, as required to maintain low insertion loss, are available. Table 1 lists the model FEU-WCDMA1 RF front end's key specifications.
W-CDMA base stations are characterized by demanding environmental conditions due to the heat output of the base station's high power amplifiers. With an operating temperature range of -20° to 70°C, the FEU-WCDMA1 exceeds the requirements that will be encountered in service. The front end can be configured to meet the requirements of specific applications, such as modified frequency coverage, package configuration and filtering.
MCE/KDI Integrated Products, Whippany,
NJ (973) 887-5700,
Circle No. 303