Volume Control Methods

Since the signals intercepted by an antenna may be of widely differing strengths, the receiver should be able to handle both weak and strong signals. It is advantageous to bring both the very weak signal and the very strong signal to approximately the same level of strength for the final reproduction of the intelligence. The process of matching signal strength to desired audio output involves the use of volume controls. Three methods are in common usage manual control of audio

Radio Volume
Manual Volume Control

signal, manual control of RF signal, and automatic control of RF signal.

Manual Audio Control

The control of signal strength in the AF section of a receiver is called manual volume control (MVC). As shown in the circuit diagram above, this type of control is accomplished by using a potentiometer as the detector load resistor. The strength of the audio signal applied to the grid of the first audio amplifier can be regulated by this means. Maximum and minimum points of volume are indicated on the circuit diagram.

For other kinds of coupling, the potentiometer arrangements are made as shown below.

At A, the output is capacitively coupled to a potentiometer. The movable tap on the potentiometer is connected directly to the first audio amplifier grid. Here, the potentiometer is the grid resistor of the audio am-fier. The tap selects a portion of the voltage across the resistor.

At B, the detector stage is transformer coupled to the AF amplifier. The potentiometer is across the secondary winding. The movable tap is connected directly to the grid. Here, the potentiometer is in series with the secondary of the transformer.. The tap selects a portion of the voltage across the potentiometer and applies it to the grid.

Variations of MVC

Manual RF Control

As shown in the illustration at the left, the RF signal amplitude can be controlled in the antenna circuit. At A, the antenna is connected to the movable tap of the potentiometer. At B the movable tap is connected to ground while the antenna is connected to one end of the potentiometer. At C, the movable tap is connected to the grid of the first RF amplifier while a tuned circuit is connected between the grid and ground. In each case, regulation of the potentiometer controls the amount of signal applied to the grid.

RF signal amplitude can also be controlled in the RF amplifier stages by regulating the amount of amplification of the stage. This is known as manual gain control (MGC). As shown on page 25, the variable resistor is in the cathode circuit of the RF amplifier. It is in series with a fixed resistor which provides a minimum amount of cathode bias. Adjusting the potentiometer changes the bias and thus the amount of amplification. This type of regulation is used with variable-mu pentode tubes, since these tubes permit considerable variation of bias without distortion. In actual use, the potentiometer would be in the cathode circuit of several stages. Thus, it would control the gain of several RF stages simultaneously.

Antenna volume control and manual gain control can be accomplished simultaneously by use of a single potentiometer, as shown below. The potentiometer controls both the resistance across the primary of the input transformer and the cathode resistance of the first amplifier.

Dual Antenna and Gain Control

Antenna Volume Control Circuits

Dual Antenna and Gain Control

Manual Gain Control

Automatic Volume Control

A volume control permits the level of output to be kept approximately the same for both a weak signal and a strong one. However, fading causes such variations in signal strength that no single setting of the volume control gives uniform output. Automatic volume control (AVC) corrects this situation and permits uniform output for a signal of varying strength.

In general, AVC controls the gain in several amplifier stages preceding the detector. Such an arrangement is shown below. The parts

VI V2 V3

VI V2 V3

Becs System Chemical Controller Parts

Basic AVC Circuit 25

of the circuit involved in AVC are drawn in heavy lines. When using AVC, these tubes are usually variable-mu tubes.

VI and V2 are RF amplifiers. V3 is the detector. The AVC process starts with the detector and works back to the RF amplifiers. R1 is the load resistor of the detector. The voltage across R1 depends on the strength of the signal being detected by the diode. When the signal is strong, the voltage across R1 is large. Note that the voltage drop across R1 is negative in respect to ground. Note also that this negative voltage drop is applied through R2 to the grids of VI and V2. This means a reduction in the gain of each amplifier stage. The amount of reduction depends on the size of the voltage drop across Rl. If the signal is weak and the voltage across Rl is small, then the gain of the amplifier stages is reduced only a small amount. If the signal is strong and the voltage drop across Rl is large, then the reduction in gain is large. This means that AVC reduces the gain in both tubes. However, since the amount of reduction is proportional to signal strength, the output is kept fairly uniform. The general loss of gain caused by the use of AVC is small compared to the amount of gain provided by the amplifiers.

Of course, the voltage drop across Rl is DC, pulsating at an audio rate. To apply these audio variations to the RF amplifier grids would produce distortion. Therefore, the DC voltage fed back to the grids is filtered to remove the audio component by the filter made up of R2 and CI. The time constant of CI and R2 is such that the AF pulses are filtered out, and a pure DC voltage is fed back to the amplifier grids.

Continue reading here: Typical Trf Receiver

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