4.8.2.1
Output Power
The 9785 transmitter is specified to provide 10 W into a 50 ohm load. Adjustments to the output
power are made with a 50 ohm dummy load connected. Any additional devices in the transmit path,
such as hybrids, will reduce the effective output power. This adjustment should be performed at the
rated 10 W level.
1.
Connect the dummy load to the Tx port.
2.
Connect the FSVM across the output of the Output Filter Module at TP2 (blue) and TP1
(brown). Set the FSVM to the transmitter output frequency.
3.
Set the transmitter to output the full 10 W level. Verify that the “PWR 3” LED on the
Transmitter Module is lit.
4.
Adjust the Power Amplifiers Gain control (R2 on the rear of the chassis) to achieve 22.36 Vrms
(40 dBm).
5.
Disconnect the dummy load and restore the line connection.
4.8.2.2
Output Impedance
Once the 9785 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
9785 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 9785 and the output compliance voltage is
limited.) For this reason, the adjustment must be made at less than 10 W output power.
1.
Disconnect all loads from the amplifier. To do this, remove the Output Filter Module from the
9785 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.
2.
Key the transmitter using the reserve input (terminals 1 and 2 on terminal block TB5 on the
rear of the chassis). The “PWR3” and “RESV” LEDs should be lit.
3.
Measure the amplifiers output voltage using a FSVM across TP3 (white) and TP1(brown) on
the Output Filter Module and record.
4.
Connect the 9785 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.
RFL 9785
RFL Electronics Inc.
March 24, 2008
4-13
(973) 334-3100
