Motorola Technical Bulletins
Motorola Inc. December 1997
English (United States)
3 pages / 127.2 KB
MOTOROLA Technical Developments
TRANSMITTER STEERING IN VOTING SYSTEMS
by Reinhard Seiler, Jikgen Schmidt, Norbert Roettger,
Bo Filt Jensen and Benny Christensen
This paper describes an improved method for reliable transmitter steering functionality in multi- site RF communication systems.
Total Area Coverage systems are normally con- figured using several simulcast transmitters-which are quite expensive due to the required High Stability Oscillators-,a certain number of receivers, and the voting unit (eg. MOTOROLA Digitac). For nearly all applications, a cheaper transmitter steering technique would be preferred.
It is, however, a problem to vote reliably the transmitter to be activated, if only the received sub- scriber audio signal to noise (S/N) is the criterion for this selection, due to the non-linear relationship of RF signal level to the received audio S/N. Especially under good RF conditions it is an often observed effect that small tolerances in the receiver (RX) adjustment results in any station being voted, but not necessarily the one that is closest to the rele- vant subscriber unit. Hence, the system does not operate optimally.
FUNCTIONAL DESCRIPTION OF VOTER OPERATION
A comparator votes a site depending upon the measured noise energy in the subscriber audio
signal. As the signal to noise (S/N) ratio decreases, the high frequency content increases. The channel with the least amount of high frequency energy is then voted. It is imperative for this method of voting that all input channels show exactly the same behaviour. The Telco lines connecting the receivers to the voting unit must be checked for proper ampli- tude and phase response, the receiver discriminators must be of good quality and properly aligned, and all leased line interfaces in the path must be equalized.
The input stage of the voter (refeer to Figure I) consists of a wireline receiver with automatic gain control (AGC), a high pass filter, a low pass filter, a level detector, and a peak detector. The received signal presented to the voter is impedance matched at POINT A. The gain of the AGC stage is set by the level of a status tone, typically 13 dB below peak audio. Since the AGC output level...