User's Manual

B-2 July 23, 2002
Appendix B CreataLink2 XT Hardware Integrator’s Guide
Desense Overview
The radio cannot differentiate between wanted and unwanted signals. In frequency-
modulated systems, the radio captures the strongest signal that the receiver
detects. If both a wanted and an unwanted signal are present, and there is no
significant difference in level, the unwanted signal can overtake the receiver, and
block the wanted signal.
Reliable reception occurs when you maintain a safety margin via co-channel
rejection. A typical co-channel rejection is 10 dB. That is, an interference signal
greater than 10 dB below the wanted signal would have little impact on the data
receiver data recovery.
Calculate the level of interference that creates desensitization as follows:
Desense Threshold = Radio receiver sensitivity [dBm] - Co-channel
rejection [dB] + Antenna Factor [dB]
The antenna factor is the ability of the antenna to convert free space
electromagnetic wave energy to power at the characteristic input impedance of
the receiver.
Any interference above this level can create desense, which reduces the radio
sensitivity for reception. Every 1 dB above the threshold level creates 1 dB
of desense.
A worst case scenario is a noise source at the same frequency as the channel center
frequency. This noise source is for any signal that falls within the channel band-
width of the radio receiver. Desense also occurs at IF down-conversion frequencies.
For the CreataLink2 data transceiver, the receiver bandwidth is 10 kHz; the first
IF is at 45.1 MHz, and the second IF is at 455 kHz. Oscillator frequencies are 16.8
MHz, 45.555 MHz, and at the channel frequency minus 45.1 MHz.
990005
Figure B-1. Wanted and Unwanted Signal Levels
Amplitude
F
c
= Radio Receiver Channel Frequency
Frequency
F
c
Wanted Signal Level
Unwanted Signal Level