User's Manual

EX7100 Technical Manual - Module Descriptions
©1999 Exicom Technologies (1996) Ltd 7-13 Issue 3 - May 2001
7.3.3 Alignment
Re-tuning Of the VHF Transmitter Module
If the frequency of the Transmitter module is to be changed by more than 250kHz from its
tuned frequency, the transmit filter will require adjustment as follows:
1. Remove the bottom cover, disconnect PL108 from the Transmitter module, and feed
this to the RF input of a scalar network analyser (or spectrum analyser with tracking
generator).
2. Set the analyser’s tracking generator output to –10dBm and feed it into the test
socket PL701.
3. Apply power to the terminal. Turn the Transmitter off by setting the terminal’s
operating mode to ‘On-Demand’. (Refer to Section 3 for details).
4. Adjust the Tx output bandpass filter so that the response is symmetrical about the
new centre frequency, and is >33 dB down at +/- 5 MHz from the centre frequency.
The bandpass filter is made up of five tuneable inductors inside an aluminium block.
Access to these inductors is through five small holes in the block. Tune the filter
using a suitable hex tuning tool, starting from the centre inductor and working
outwards. This procedure may need to be repeated several times. Be careful not to
bottom the cores at either end of travel as the cores will break.
Note: The Duplexer module must also be adjusted to suit the new centre frequency
(refer to Section 7.9.3 for tuning details). Failure to adjust the Duplexer module at
this point may result in damage to the PA module
5. Disconnect the analyser and reconnect the PA’s coax to PL108.
6. Set the new the transmit frequency via the front panel as per Section 3.
7. Check that the VCO Loop Volts measurement on TP301 (or front panel) is within the
limits 4 to 5 Volts. If necessary, reset this by adjusting the VCO trimmer capacitor
through the access hole in the square aluminium block.
Carrier Null
The carrier null should be stable over the long term. If bit error rates climb, carrier
leakage could be the problem.
1. Put the terminal into “Align Mode” with DSBSC mode selected Refer to Section 4.5.1.
(Not to be confused with “Antenna Align Mode” as described in Section 2.11.2)
2. Connect a spectrum analyser via an attenuator to the transmitter’s output to observe
the sampled spectrum.
3. Using a narrow resolution bandwidth observe the on-channel carrier’s level. This
reading should be at least
40 dB (preferably 50 dB) below the two adjacent tones, if
this is not the case, re-null using RV501 and RV502.