FM CHANNEL FREQUENCY CHANGE:
The FMU622CA RF output is controlled by a phase lock loop (PLL) automatic
frequency control system. As long as the average frequency of the FM carrier is
equal to the reference frequency, the red “OFF FREQ” LED on the front panel will
not light. To change the frequency, complete steps 6.5.2-6.5.4.
Look up the binary equivalent of the desired operating frequency in Appendix A
of this instruction book, set switch SW1, sections 1-10 (Located inside the top cover)
in accordance with the binary code for the desired frequency. The “OFF FREQ” LED
may illuminate, whether it does or not, continue through these steps.
With a non-metallic flat blade screw driver tune L501, accessible inside the top
cover, until the red “OFF FREQ” LED just extinguishes, then press the pushbutton
switch next to L501 and continue to tune L501 until the red LED again extinguish the
red LED, then release the button. This sets the PLL system to the center of its
control range. Small changes of L501 tuning may cause the red LED to light
momentarily, but is must be out when L501 is correctly set, when illuminated, the
output frequency is not locked to the reference crystal frequency.
Connect a field strength meter to the FM channel output “OUT” and terminate
the “IN” connector with 75 Ohms. Tune C515 and C516 Multi-turn piston capacitors
for maximum output. The output with RF output level control at maximum should be
at least +50 dBmV. Note: Reversing connections to the FM channel “IN” and “OUT”
on the rear panel will reduce readings about 20 dB due to the directional coupler
being reversed.
APPENDIX A:
To change the RF output frequency of FM SYSTEMS 600 series modulators,
two quick steps are essential:
1. Set the new frequency for the PLL (Phase Lock Loop) with the DIP switches on
the circuit board, SW501. See the following tables for the switch positions.
2. Center the AFC (Automatic Frequency Control) range with the variable inductor
(coil) after setting the switches .
If either of these steps is skipped, the frequency will be incorrect and/or will drift.
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