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Technical Developments Volume 24 March 1995
A RADIO TRANSCEIVER SYSTEM FOR USE IN MULTIPLE FREQUENCY BANDS
by by Alex Hietala and Duane Rabe
INTRODUCTION
SYSTEM #I
Recently the FCC has allocated two frequency bands for PCS usage. The first band will transmit from 1850 to 1890MHz and receive from 1930 to 1970MHz. The second band will transmit from 2180 to 2200MHz and receive from 2130 to 2150 MHz. The system proposed will be much like the Euro- pean DCS-1800 standard which is an eight 577 usec time slot per frame TDMA system.
Since the receive bands will be located in between two transmit bands a new method must be devised to keep the transmitter noise out of the receive band. The method will be new because the traditional approach of using a ceramic filter on the PA output will not be practical with two disjointed bands.
This is a straightforward approach with low-side injection and a 133Mhz IE
Band 1 TxVCO 1850 TO 1890MHz RxSynth 898.5 to 918.5 MHz doubled to 1797 to
1837MHz TxIF 53MHz
Band 2 TxVCO 2180 to 2200MHz RxSynth 998.5 to 1008.5 MHz doubled to 1997 to
2017MHz TxIF 183 MHz
This will require the following signals to be switched between bands.
1) TxIF VCO range
2) Interstage filtering ofthe TxIF IQ Modulator
3) TxVCO range
4) RxSynth VCO range
PROPOSED SYSTEMS
SYSTEM #2
There are two methods by which the above sys- tems can be realized without the use of a transmit- ter duplex filter. This is possible since the proposed system is TDMA and thus the receiver and trans- mitter are not active within the same radio at the same time. Therefore intermodulation products
formed by the transmitter and a strong adjacent trans- mitter (such as a television station) will not inter- fere with the receiver.
Both methods will use ceramic filters for the receiver. If the receive bands are both located between the two transmit bands then one receive filter may be sufficient. If not, then two receive filters would
be needed with a duplex system to avoid interaction between the filters.
The first method uses a translational PLL approach. There are two variations of this approach as detailed below.
In this system the RxSynth VCO range is not switched. This can be accomplished by low side mix- ing Band 2 and high side mixing Band 1. This requires that TxI and Q and Rx1 and Q be swapped for the...
