Codan Radio MT-4E Paging - Manual (Page 15)

MT-4E PAGING SYSTEMS | USER GUIDE
Chapter 1: Introduction to Paging Page 7
Minimizing Distortion in Overlap areas
Overlap areas are areas in which two or more RF levels are within 10 to 15 dB of each other. Paging
receivers cannot lock onto the strongest signal when RF levels are this close together and all RF
signals will be received. The Overlap areas are the critical areas where the interference between the
RF signals needs to be controlled. When uncontrolled, the overlapping RF signals can combine to
produce an audible beat frequency in the receiver or even unintelligible audio. Differences in the audio
phase and amplitude from the RF signals cause distortion in the received signal. A high degree of RF
frequency stability, audio amplitude equalization and audio phase delay will minimize the distortion in
these overlap areas.
RF Frequency Stability
Synchronizing the frequencies to ensure that the frequency at all sites is extremely close together can
reduce the beat note in the received signal below the audible level. Codan transmitter modules can be
connected to an external high stability reference using the CI-PM-3 Paging Modulator card. The CI-
PM-3 Paging Modulator uses an on-board 10 MHz OCXO frequency reference source with a standard
stability of 0.35 ppm. For simulcast applications requiring a higher stability, the Paging Modulator may
be confi gured to use an external high stability reference source (typically a GPS receiver or rubidium
oscillator) with a standard stability greater than or equal to 0.002 ppm. This external reference disciplines
the on-board phase-locked loop OCXO oscillator.
Audio Phase Delay and Amplitude Equalizing
Equalizing the audio phase delay so that audio from all sources arrives at the center of the overlap
area at the same time, and equalizing the audio amplitude for variations between different transmission
paths will reduce the distortion of these signals received in the overlap area. Codan Paging Systems
are designed for interfacing to 3rd party simulcast delay and equalizing equipment such as the Zetron
Model 66 (with simulcast delay option) and Zetron Model 600/620. Connection to the Codan transmitter
module is achieved via an external connection.