User's Manual

GMH Series VHF Radio General Information
BK RADIO Page 4-5
4.3.3 High Level PA Board
4.3.3.1 Power Amplifier
The high level power amplifier consists of a driver stage with a nominal output power of 15 watts and a
final stage with sufficient output power to provide 50 watts at the antenna port. Both stages utilize
broadband matching networks and are operated in a class C mode.
4.3.3.2 Harmonic Filter
A 9th order elliptical filter is used to attenuate harmonic components before they reach the antenna port.
4.3.3.3 Directional Coupler
A stripline directional coupler and associated detectors provide DC voltages proportional to the forward
and reflected power at the antenna connector. The forward detected voltage is used for feedback to the
power control loop and the reverse detected voltage is used to detect high VSWR conditions.
4.3.4 Options Board
4.3.4.1 Control Functions
The routing of audio signals through the options board is determined by analog switch I1. The audio path
is controlled by outputs from the control board.
4.3.4.2 Receiver Audio
Low frequency tones are removed from the audio by a high-pass filter of I2A, I2B, I2C, and associated
circuitry. De-emphasis is set by C23.
4.3.4.3 Transmitter Audio
I3D performs pre – emphasis, and I3C performs limiting. I3B is used as a buffer/amplifier and drives a
three pole low pass filter consisting of I3D and associated circuitry. This low pass filter has a cutoff at 3
kHz and provides modulation filtering to within FCC requirements. The output is routed to the systems
board. Signaling tones are generated on the control boar and summed with the transmitter audio at I3B.