ALC – Automatic Level Control

Class A, AB or B amplifiers are circuits that yield gain while maintaining the characteristics of the input waveform. The amplifier configuration, be it grounded grid/base, grounded cathode, etc will determine the inversion or non inversion of the signal but suffice it to say that the output of an amplifier of good design will leave all the characteristics of the input waveform in tact. The design of the amplifier is determined by a set of parameters set down by the designer to meet the requirements of the application and will have a list of items such as size, plate/anode voltage, ventilation requirements, drive requirements, load, input and output impedance, etc, etc.

Lets look at only one of the design parameters; drive requirements. The amplifier needs a signal to process. The signal must be linear to start with and of a character such that the power level does not damage the amplifier input and does not induce distortion through a mismatch or excessive amplitude. All of these potential problems can be accommodated by the use of limiters, filters and/or attenuators.

There is a circuit in your transceiver that controls the final output. The “Automatic Level Control”, commonly called ALC, circuit monitors the amplifier output to provide negative feedback to reduce the input signal in an attempt to maintain the linearity of the signal being transmitted. Within your transceiver there are several monitoring points along the signal path which combine to create this feedback. Check the ALC level on the meter when transmitting and adjust the Mic Gain control to reduce the limiting. The meter gives an indication of the amount of feedback being generated and should not be full scale all the time.

When an external Linear Amplifier is attached, the ALC signal from it should be connected to the transceiver ALC connector so as to include the external amplifier's signal into the equation.

NOTE: Follow the setup procedures for the external amplifier to adjust the internal ALC circuitry before connecting it to the transceiver.

Additional information can be found in “Section 17.8.3” of the 2011 ARRL Handbook.

Distortion of the transmitted wave causes poor audio quality, out-of-band emissions and interference to other services or in the worst case a salutation from the “OO” or the FCC.

73,

de N1TF - Fred

Basic ALC Block Diagram by N1TF

“Reprinted with permission, 2011 ARRL Handbook; copyright ARRL.”