As the wireless industry moves to implementation of 3G standards, wireless service providers are looking for ways to make the transition with the most rapid time-to-market and minimized design cost of new equipment. The TXC Series transmitting converters are designed for this environment, and allow base transceiver station (BTS) transmitters to be upconverted or downconverted from their original operating frequency band to a higher or lower frequency band with no sacrifice in performance. The alternative is to replace or redesign the entire BTS transmitter, which is much more costly and delivers no additional cost-mitigating benefit compared to the converter-based approach.

What It Does

The TXC Series converters take the output of the existing multicarrier CDMA, TDMA, or GSM BTS transmitter and shift it up (or down) in frequency, producing an output that is sent to the multichannel power amplifier (MCPA) and then on to the multiplexers and antennas (Figure 1 ). The unit is integrated with the other BTS electronics located at the base of the tower. The TXC Series implements the transmitter up- or downconversion, with the receiver path handled separately.

KDI has designed converters for a wide variety of customer requirements, from one to nine channels with output frequencies as low as 460 MHz and as high as 2130 MHz (or greater on request). While the performance of each is slightly different, the specifications for a three-channel unit, as listed in Table 1 , are representitive of the product family. Rho is one of the most difficult but nevertheless essential specifications the converters are required to meet. Rho is perhaps the most universal CDMA and W-CDMA specification since it accounts for error mechanisms that can be present in a transmitter that can arise from sources such as modulation error or power amplifier distortion. A perfect value for rho is 1.000, indicating that all of the transmitted power is presented to the receiver as useful information with no transmitter modulation error. The TXC Series converters achieve a minimum rho of 0.990 and typical performance of 0.999 with multiple carriers present.

Another critical specification is rejection of the 2xIF signal, for when the unit upconverts or downconverts the input frequency, the undesired output could fall within the new receive frequency band. The wideband MPCA that follows the converter dutifully amplifies the converter output, making even very low level signals potential interferers. As a result, the converter provides for high levels of filtering, reducing the 2´IF spurious to -75 dBm. Requirements for rejection of LO feedthrough and 2xLO signals are somewhat less stringent (-40 dBm) because potential harmonic and distortion frequencies are well out of the receive band of interest.

A representative transmit converter consists of an input bandpass filter that provides spurious rejection, followed by a low intermodulation distortion voltage-variable attenuator (VVA) that provides gain compensation over temperature, and then a high dynamic-range mixer. A diplexing filter ensures proper termination of the mixer and provides additional spurious rejection. This filter is followed by a balanced amplifier and then another bandpass filter with a very narrow bandwidth that is designed to optimize group delay performance for rejection of the remaining close-in spurious signals without introducing data loss. The resulting signal is then sent to an LDMOS amplifier that can provide 250 mW of drive to the MCPA. It also provides load buffering for the filter, since the MCPA input may not be well matched to 50 .

Constant Monitoring

A wireless BTS is designed to provide near total protection against failure, and the TXC Series meets this requirement by employing redundant systems where appropriate and a comprehensive monitoring system. The redundant systems are always operational, but only one is online. If the monitoring circuits detect that the primary system has failed, the system switches to the backup within 2 ms. In addition, the circuit also monitors the amplifier's bias points and conditions of the internal power supply. The integral DC/DC power converter provides a high level of regulation and signal conditioning, and its performance is monitored as well. If any monitored value drifts out of specified tolerance, the circuit sends a signal through a serial interface to the BTS system where it is routed to the system operator. A visual front-panel indicator is also provided. The protection circuit provides an indication of the severity of the problem, from a hard failure requiring immediate attention to a soft failure that can be remedied at a later time.

The converters are designed to operate within their specific performance requirements within a temperature range of -40° to +65°C. This is achieved by sensing the temperature of the unit and converting the result to a digital signal. A look-up table in the converter's microcontroller then issues an appropriate command to the VVA in each channel. Gain is either increased or reduced, depending on the temperature conditions. This circuit ensures that the converter has a gain variation of only 0.5 dB or less over temperature; without correction the variation could be 2.5 dB over temperature.

A typical TXC Series converter measures 9-1/2" H x 12-1/2" W x 2-1/2" D and mounts in a single rack space. It operates from +15 to +30 VDC at less than 600 mA (24 VDC). A variety of configurations are available including different input and output frequencies, gain levels, and connectors.

MCE-KDI Integrated Products, Whippany, NJ (973) 887-5700, www.mcekdi-integrated.com. Circle No. 300