RFL 9780 RFL Electronics Inc.
April 8, 2003 4-13 (973) 334-3100
4.8.2.2 Output Impedance
Once the 9780 has been connected to the line the output impedance should be adjusted to match that of
the actual line. Impedance mismatches can cause signal reflections and other undesirable effects. The
9780 Power Amplifier provides an output impedance adjustment to accommodate small variations in
actual line conditions. When the load and source impedance of a device are matched, the loaded output
voltage is exactly one half of the unloaded output voltage (the internal and external impedances form a
50% voltage divider). This fact is used to set the amplifiers output impedance. However, the power
amplifier is not able to supply twice the rated output voltage when configured for 10 W operation. (The
output impedance is simulated by active circuits in the 9780 and the output compliance voltage is
limited.) For this reason, the adjustment must be made at less than 10 W output power.
1. Set the transmitter for either 3 W or 1 W operation (the “PWR 2” or “PWR1” LED on the
Transmitter Module should be lit).
2. Disconnect all loads from the amplifier, including any hybrids. To do this, remove the Output Filter
Module from the 9780 chassis. Unplug jumper J1 after noting which position it was in (“A”, “B”,
or “C”) and replace the module into the chassis. This will remove the filter and all downstream
components from the transmit path, while retaining the “INPUT” test point on the front of the filter
module.
3. Measure the amplifier output voltage using a FSVM across TP3 (white) and TP1(brown) on the
Output Filter Module and record.
4. Connect the 9780 to the line by replacing jumper J1 on the Output Filter Module.
5. While continuing to measure the amplifiers output voltage at TP3 and TP1 of the Output Filter,
adjust the Power Amplifiers Impedance Adjustment (potentiometer R16 on the rear of the chassis)
to achieve 50% of the previously measured open-circuit voltage.
4.8.2.3 Reflected Power
It is strongly recommended that the reflected power (SWR) on the installed line be verified. This is
most easily accomplished by using a PLC Test Set, such as the Signal Crafters Model 70. Following
the instructions for the test set and the tuning equipment, the line tuning unit should be adjusted to
obtain the lowest possible reflected power.
The RFL 9780 can be upgraded to automatically measure the amount of transmitted power reflected
back to the local receiver. The reflected power can be read locally or remotely using RFL Web
Commander or Hyper-terminal.
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