User Manual

Contents:
Sec Topic Page
1 RF Pin Attenuator Board Description.......................................................................34a-1
2 AGC/Overpower Protection Setup ...........................................................................34a-1
List of Figures:
Fig Title Drawing Reference
1 Pin Attenuator Board Assembly.....................................................................10A1255G1
2 Pin Attenuator Board Schematic........................................................................20B1287
1. RF Pin Attenuator Board 10A1255G1: Figures 1 and 2.
(Refer also to the RF Metering Board section of this manual)
The function of this board is to limit the output power of the amplifier to about 110% for up to 3dB increase of
input power. The Pin Attenuator Board acts as a variable attenuator controlling the input level to the pre-amplifier
for AGC and cutback purposes. Operational amplifier U2B controls the amount of attenuation in the circuit.
Three inputs are summed at pin 6 from the VSWR and AGC circuits and additionally from U2A which is a
"mute" type signal. R5 controls the threshold of these signals. The circuit configuration comprising CR1, CR2
and CR3 form a matched attenuator, biased by the output of U2B, R2, R3, R7 and R8. As the output of U2B
lowers due to a voltage increase of one (or more) of the three inputs, the attenuation of the circuit increases,
thus decreasing the RF output at J2.
2. AGC/Overpower protection setup
1. Adjust R5 on the Pin Attenuator Board fully clockwise (minimum attenuation).
2. Adjust the input power from the exciter or channel processor for an output power of about 110% (11Wsp).
3. Adjust R5 on the Pin Attenuator board CCW (counter-clockwise) for the output power to just starts to drop,
say by 5%. This adjustment sets the threshold and corresponds to a dc voltage of about 6V at U2 pin 7 of the
Pin Attenuator Board. Check that this is so.
Readjust power to 110% using the output level control in the exciter or channel processor.
4. Next, adjust potentiometer R39 on the RF METERING BOARD for the output to drop to 100%. This sets the
overpower protection limit to 100%.
5. Verify AGC/Overpower protection operation by increasing the input to the amplifer by about 3dB using the
exciter or channel processor control. The output of the amplifier should not increase by more than 10%.
6. Reduce the level of the exciter or channel processor so that the amplifier output is back to 100%.
NOTE: The procedure described above is valid if on the RF METERING BOARD R46 is 100 Ohms, and R39 is
a 1k potentiometer. In earlier versions R46 is 430-560 Ohms, and R39 is a 100 Ohm fixed resistor. If you have
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