Codan Radio MT-4E Analog and P25 Digital - Maintenance Guide - Manual (Page 68)

MAINTENANCE GUIDE | MT-4E ANALOG & P25 DIGITAL RADIO SYSTEMS
Chapter 6: Additional Radio System Tests Page 60
Receiver Desense
A Receiver desense test can be performed by connecting an RF Signal Sampler such as the Bird Model
4275 Variable RF Signal Sampler between the receive port of the fi ltering equipment and the receiver
RF IN as shown in Figure 6-3. Ensure the transmitter is connected to the fi ltering equipment and the
transmitter is turned OFF.
Figure 6-3: Receiver Desense Testing
On the IFR 2975, recall Codan setup 1 (Rx_Ana) from your internal drive. Enter the correct RF
frequency, select the MOD TYPE to FM and set it for a 1.0 KHz tone and ensure that the deviation level
is set correctly for your receiver (wide / narrow). Enter the CTCSS tone (if used) and deviation level. In
the System Setup; Jumper Settings area of the Service section on the RSS ensure that the “Subtones
on audio path” selection is set to “Don’t pass”.
Monitor the SINAD meter while slowly reducing the RF carrier level. The RF carrier level can be
reduced on the IFR 2975 and also through the variable RF signal sampler. Note the 12 dB SINAD
point. Now, turn the transmitter to KEY TX and check the SINAD meter for any noticeable drop in
sensitivity. If the sensitivity does drop, increase the RF carrier level on the IFR 2975 until the 12 dB
SINAD point is reached. Receiver desense is the difference between this RF carrier level and the RF
carrier level noted when the transmitter was OFF. The receiver desense gives a relative idea of the
additional isolation required by fi ltering equipment.
Note: other transmitters located at the site can also cause desense.
TRANSMITTER
CNTL
BUS
MIC RF OUT
A D
ANALOG
DIGITAL
NORM
OFF
KEY TX
MIC MODE
REF IN
USB
FREQUENCY (MHz)
MADE IN CANADA MODEL # CODE
RECEIVER
NORM
SQ. DISABLE
OFF
CNTL
BUS
A D
RF N I
REF
IN
USB
FREQUENCY (MHz)
MADE IN CANADA MODEL # CODE
VOL
SYSTEM REGULATOR
METER
+
-
MADE IN CANADA
ON
OFF
SPKR INT EXT EXT
SPKR
OFF
POWER ON
2
1
5
4
3
6 7 8
11
10
9
FUNCTION 12
REPEATER CONTROL
RX B
TX B
RX A
TX A
PULL DOWN
TO REMOVE
9
5 3
7 11
15
9
13 3
7 11
15
SWITCH A SWITCH B
Rx Balanced Audio Output
(from pins B26 and Z26)
T/R port from IFR 2975
to RF signal sampler
HELP MODE
TAB
SHIFT
+
-
.
BKSP
HOLD
RETURN
ESC ENTER POWER
1 2 ABC 3 DEF
JKL MNO GHI 6 5 4
TUV WXYZ PQRS 9 8 7
# 0 *
VOLUME
SQUELCH
GEN T/R ANT
125 W MAX MIC AUDIO I/O DVM CH2 CH1 SCOPE TEST PORT
100V MAX 100V MAX 100V MAX
AUDIO IN AUDIO OUT 1 AUDIO OUT 2
Audio In
RF OUT from Tx
to Duplexer
Duplexer to
RF IN on Rx
To Antenna