Microwave Data Systems MDS 4310, Installation, Operation & Field Maintenance

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MDS 05-2415A01, Rev. A FIELD TESTS & ADJUSTMENTS 4-13
8. Generate an on-channel signal at –60 dBm (225
µ
V), with 1 kHz modulation at
2.5 kHz deviation.
9. Adjust the transceiver’s R25 for 1.2 Vp-p
±
0.05 V at U3 pin 8 (on the main
transceiver PC board) as read on the oscilloscope.
10. Set the service monitor modulation frequency to 1700 Hz.
11. Touch the oscilloscope probe tip to the top of chip resistor R33 on the modem board.
12. Adjust R25 (on the modem) for a square wave on the scope. Remove the probe.
13. Turn off the service monitor and disconnect the coax from the transceiver’s
ANTENNA
connector.
14. Adjust the squelch by observing the receiver audio output signal at Pin 11 of the
INTERFACE
connector using an oscilloscope.
15. Adjust R41 on the transceiver’s motherboard fully counterclockwise, and observe the
noise voltage on the oscilloscope. Now slowly rotate R41 clockwise to a point just
beyond that where the noise disappears. Watch the scope for a few seconds and, if
necessary, continue to advance R41 until the receiver remains fully squelched (the
noise no longer appears intermittently).
R3
SPACE TONE
(2200 HZ)
FREQUENCY
R2
MARK TONE
(1200 HZ)
FREQUENCY
R25
RECEIVE DATA
DETECTOR
THRESHOLD
ADJUST
J6 J8
R3
3
R33
RX TEST POINT
Figure 4-4. 1200 bps AFSK Modem; MDS P/N 03-1815AXX
See Table A-1 in Appendix A for details on switch settings.
Squelch Adjustment
1. Using an oscilloscope set for DC coupling and 2 V or 5 V per division, observe the
Receiver Unsquelched (RUS) signal at Pin 10 of the
INTERFACE
connector.
2. Insert an on-channel signal at –120 dBm, with a 1 kHz tone and 2.5 kHz deviation.
Adjust R41, Squelch Threshold, fully counterclockwise while observing the
oscilloscope display.
3. Set the squelch to be fully closed at this signal level by advancing R41, Squelch
Threshold, until the oscilloscope pattern changes from about 8 V to 0 V. Watch for
noise on the scope display, and advance R41 slightly farther, if necessary, until the
signal level remains at about 0 V.