Intermodulation - Codan X-2 Service Manual

Adjustments 7-12

7.11.4 Intermodulation

The Test Mode provides 15 channel frequencies to check the PA on the five
bands. One at the start of each band, one in the centre, and finally one at the
top of each band.
A Spectrum Analyser is required to test the Intermodulation Distortion
(IMD). The check procedure is as follows:
1.
2.
3.
4.
5.
6.
7.
8.
9.
10.
Ensure the Exciter output is connected to the PA PCB Assembly
(P1 to P2).
Connect a 50 Ω Dummy Load to Antenna 1 output via a suitable in-
line Tee piece to provide a low level output to connect to a Spectrum
Analyser.
Select the Transceiver Test Mode (refer Section 7.5).
Select first channel position 1 and the USB/LSB to USB. This selects
the channel frequency 2000 kHz, USB, and Antenna 1 O/P.
Apply the two tone audio (700 Hz and 2300 Hz) via the Test Unit to
the microphone socket.
Set up the Spectrum Analyser as follows:
•
Centre frequency to 2000 kHz.
•
Frequency span to 20 kHz (2 kHz/div).
•
Reference to 10 dB/division.
•
Video bandwidth to 300 Hz.
•
Sensitivity will depend on the signal level applied to the Analyser
and will require adjustment when transmitting.
Select PTT (Tx mode) and adjust the two tones for compression. If
this level is unknown it can be checked by observing the output of the
Microphone Amplifier at TP302 using an Oscilloscope for 500 mV pp.
Adjust the two tones for equal amplitude displayed on the Spectrum
Analyser by operating the balance control on the two tone Signal
Generator. Adjust the Sensitivity control on the Spectrum Analyser to
set the two tones at the top of the screen.
The Intermodulation distortion levels at 2000 kHz can be measured
with respect to each tone. Add 6 dB to the reading if referenced to
PEP (refer Specifications Section 1.6.3 for limits).
The Intermodulation distortion can be checked on the other available
14 frequencies by selecting the Test Mode channels detailed in Table
7.1 in Section 7.5. The Spectrum Analyser centre frequency will need
to be set to the channel frequency selected.
Note: By adjusting the Spectrum Analyser frequency span it is also
possible to check the Spurious and Harmonic components during
the IMD checks.
X-2 Technical Service Manual
Publication No: 15-02047