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Inside the SB-104(A) Transceiver

Written By: Ron Baker / WB4HFN

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The Transmitter Driver Board

The Transmit Driver Board takes the signal from the transmit IF Board and amplifies that signal to over one watt of power.   This RF amplifier is a push-pull, class AB linear design.  This provides the gain and flatness across the entire operating range of each band.   This board has two switchable outputs controlled with the "High Power" front panel switch.    In the Low Power position the one watt signal is sent directly to the low pass output filter board and to the antenna.    In the High Power position the one watt signal is sent to the RF Power Amplifier board which is the drive signal for the 100 watt RF power amplifier.   The board has separate "High" and "Low" output connectors at the top of the board.

The Transmitter Power Amplifier Board

The RF Power Amplifier Board amplifies the driver output to the level of 100 watts.    This amplifier consists of two push-pull amplifiers circuits in parallel, combined together with a common input and output.    The amplifier is a broadband amplifier with a flat response from 3 to 30Mhz.     The amplifier being broadbanded requires no tuning or adjustments during normal operations.   The output of the Power Amplifier goes to the ALC/Output Filter Board.

The ALC/Output Filter Board

The ALC/Output Filter Board contains a low pass filter used to attenuate the harmonics generated by the driver and power amplifier outputs to an acceptable level.    30dB of attenuation is achieved by passing the amplifier signal through several low pass filters allowing only the operating frequency to pass through the filter to the ALC circuit.   The low pass filter for each band is selected by a board mounted rotor switch ganged to the front panel Band Switch.     The signal is then sent to the ALC (Automatic Level Control) where the signal passes through a bi-directional coupler to the antenna connector through the antenna relay.     As the output signal passed through the coupler,  the coupler produces a DC voltage, ALC Voltage, proportionate to the forward and reflected power resulting from the antenna and feed line match to the transmitter.    The ALC voltage is used, when in the high power mode, to reduce the transmitter power output level if the antenna or feed line presents a high VSWR to the transmitter.     The ALC circuit reduces the transmitter power with high VSWR to prevent damaging the transmitter power amplifier.

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