To study the operation of AGC and AFC

Aim

To study the operation of AGC and AFC.

Theory

Automatic Gain Control (AGC): To control the gains of the amplifiers of the system, AGC is employed. The AGC voltage is used to keep the volume of the receiver constant to the level set by the listener. The output of the second IF amplifier is also given to the AGC detector.

The AGC detector produces a dc voltage, called the AGC bias voltage, which is proportion to the carrier strength of the received signal. The signal is also amplified by the AG amplifier before being detected to generate the AGC bias voltage. The AGC bias voltage generated is applied to the RF amplifier and first IF amplifier, to control their gains.

In communication receivers, the simple AGC technique discussed in AM receivers is not employed. This is because the simple AGC also reduces the gain of the amplifiers for the weak signals. In communication receivers, improved AGC techniques are used so that the weak signals are satisfactorily processed. The two techniques or AGC that are employed in communication receivers are:

Delayed AGC: In delayed AGC, the AGC remains inoperative below a predetermined input carrier voltage. If predetermined level, it is considered a weak signal. The received signal strength is below the predetermined level, it is considered a weak signal. The AGC bias voltage is applied to the RF and IF amplifiers only if the level or input carrier voltage goes above this predetermined level. In other words, the AGC is delayed in applying the bias voltage to the amplifiers the bias voltage to the amplifier for a certain predetermined level.

A typical circuit diagram of a delayed AGC is illustrated in Figure (b). In delayed AGC, the output of the last IF amplifier, which is the second IF amplifier, is taken through a coupling capacitor, Cc. This is applied to a diode, D1. The cathode of diode D1 is provided with a positive dc voltage, Vdc, through a variable resistor, R. This sets the predetermined level up to which the AGC is not to be applied. If the received signal is weak, the anode voltage of the diode is less than the cathode voltage, and is reverse-biased. This results in the diode not conducting. The received signal thus goes to R1, R2, and C1 networks, as shown in Figure (b). This is an AC signal and passes through C1. Thus, no dc voltage is available for AGC and the amplifiers operate at their usual gains.

When a strong signal is received, the anode voltage goes above the cathode voltage of the diode, and the diode starts conducting. The signal, passed to capacitor C1 in this condition, gets negative peaks and filters them. This results in a constant negative voltage, which is used as a delayed AGC and applied, to the RF and IF amplifiers. The variable resistor, R can adjust the level of delay AGC. This control is given at the panel of the receiver so that the operator can adjust the delayed AGC level according to the signal conditions. If a weak signal is received, then the operator can adjust it so that no AGC is applied.

AGC Characteristics Curves: A comparison between simple AGC and delayed AGC is shown in Figure (c). A curve is also drawn for an ideal AGC.

These curves are also compared with the generated when AGC is not applied. An ideal AGC provides a constant output signal level for all input carrier signals, after a particular input level, marked as A in Figure (c). Up to point A, the output level linearly increases with the input carrier signal. A simple AGC continuously increases with an increase in the input carrier signal. Thus, there is no control on the output signal level for a simple AGC. On the other hand, the characteristic curve for delayed AGC shows that it is very close to the ideal AGC.

Auxiliary AGC:In communication receivers, an auxiliary AGC is provided in addition to the delayed AGC. The auxiliary AGC becomes operative for very strong signals. The auxiliary AGC circuit includes only one diode, which is connected between the collector of the first IF amplifier, Q3, and the collector of the converter, Q4, as shown in Figure (d).

The anode of the diode is connected to the collector of the IF amplifier and the cathode is connected to the collector of the convertor. When the signal is not strong the diode is reverse biased and does not affect the normal operation of the circuit. When a very strong signal is received, the diode becomes forward biased and starts conducting. The diode resistances lowers and it loads the first IFT and capacitor C3, as shown in Figure (d). This circuit is connected to the base of first 1F amplifier, and its gain reduces due to the loading of IFT1 and C3. This accordingly reduces the output signal level. The auxiliary AGC provides another means to reduce the gain of the IF amplifier in the presence of strong signals.

Automatic Frequency Control AFC: circuits are used in situations where you must accurately control the frequency of an oscillator by some external signal. Basically, this type circuit does two things: It senses the difference between the actual oscillator frequency and the frequency that is desired and produces a control voltage proportional to the difference; it also uses the control voltage to change the oscillator to the desired frequency. AFC circuits are used to control the frequency of sinusoidal oscillators and no sinusoidal oscillators. Only sinusoidal AFC circuits will be covered here. AFC circuits are used in radio receivers, fm transmitters, and frequency synthesizers to maintain frequency stability. Figure (e) is a block diagram illustrating AFC operation in a receiver. Let’s run through the applicable parts of this block diagram.

Figure (f) shows another widely used type of AFC and its circuitry. This type is commonly referred to as a BALANCED-PHASE DETECTOR or PHASE-DISCRIMINATOR. This circuit uses fixed capacitors and the varying conductance of the diodes to achieve a variable reactance. As seen in the block diagram, an AFC circuit requires two sections, a frequency detector and a variable reactance. Our detector output is a dc control voltage proportional to the amount of frequency change. This dc voltage is applied directly to the oscillator. The phase inverter input signals are discriminated IF outputs fed to the two diodes 180 degrees out of phase.

A reference voltage is also applied to both diodes. The diodes are biased to conduct only during the peak portions of the input signals. Any change in oscillator frequency will alter the phase relationship between the saw tooth reference voltage and the incoming signals. If this happens, one diode will conduct more than the other and produce a control signal. This system remains unbalanced at all times because any change in frequency is instantaneously corrected. The network between the diodes and oscillator is essentially a low-pass filter. This filter prevents discriminator pulses from reaching the oscillator.

Viva Question

Q1.What is Modulation? What happens in over modulation?

Ans:Modulation is defined as the process in which some characteristics of the signal called carrier is varied according to the modulating or baseband signal. For example –Amplitude Modulation, Phase Modulation, Frequency Modulation. In case of over modulation, the modulation index is greater than one and envelope distortion occurs.

Q2.What is Sampling? What is Sampling Theorem?

Ans:Sampling is defined as the process in which an analog signals are converted into digital signals. It means that a continuous time signal is converted into a discrete time signal. Sampling Theorem is defined as : ’The continuous time signal that can be represented in its samples and recovered back if the sampling frequency (fs) is greater than the maximum frequency of the signal (fm) that is fs >2fm’.

Q3.What is under sampling?

Ans:Under sampling is also known as aliasing effect in which the the sampling frequency is less than the maximum frequency of the signal and therefore the successive cycles of the spectrum overlap.

Q4.What do you mean by Nyquist rate?

Ans:In case of Nyquist rate, the sampling frequency is equal to the maximum frequency of the signal and therefore the successive cycles of the spectrum does not overlap.

Q5.How can be aliasing be avoided?

Ans:Aliasing can be avoided if:

Q6.Define PAM and write down its drawbacks?

Ans:Pulse Amplitude Modulation is the process by which the amplitude of the regularly spaced pulses varies according to the the amplitude of the modulating signal. The drawbacks are:

Q7.What is multiplexing? Name the types of multiplexing?

Ans:Multiplexing is defined as the process in which a number of message signals are combined together to form composite signals so that they can be transmitted through the common channel.The two types of multiplexing are:

Q8.What is Amplitude Modulation?

Ans:Amplitude Modulation is defined as the process in which the instantaneous value of the amplitude of the carrier is varied according to the amplitude of the modulating or base band signal.

Q9.What do you mean by FM and classify FM?

Ans:Frequency Modulation can be defined as the frequency of the carrier (wc) is varied acc. to the modulating signal about an unmodulated frequency.FM are of 2 types:

Q10.State the advantages of superheterodyning?

Ans:The advantages are:

Q11.Explain why modulation is required?

Ans:When we want to transmit electrical signal over an antenna, through free space, it must be converted into electro-magnetic waves. Only electro-magnetic waves have the property to travel through space (vacuum) at the speed of light message signal or voice signals have low frequencies.

Q12.Explain the term companding.

Ans:Companding is the term derived from the combination of two terms Companding= Compression + Expanding. In the process of Companding, the weak signals are amplified and strong signals are attenuated before applying them to a uniform quantizer

Q13.What are the applications of pulse position modulation?

Ans:It is primarily useful for optical communication systems, where there tends to be little or no multipath interference. Narrowband RF (Radio frequency) channels with low power and long wavelength (i.e., low frequency) are affected primarily by flat fading, and PPM is better suited.

Q14.Define ASK, PSK and FSK.?

Ans:All are digital modulation schemes. ASK refers to a type of amplitude modulation that assigns bit values to discrete amplitude levels FSK refers to a type of frequency modulation that assigns bit values to discrete frequency levels. PSK in a digital transmission refers to a type of angle modulation in which the phase of the carrier is discretely varied—either in relation to a reference phase or to the phase of the immediately preceding signal element—to represent data being transmitted

Q15.What is the selectivity of a radio receiver?

Ans:The ability of a radio receiver to select a desired signal frequency while rejecting all others is called selectivity.

Q16.What is envelope detector?

Ans:A circuit containing a diode in series with an RC network, used to perform demodulation. An envelope detector, which demodulates an AM signal, cannot demodulate an SSB signal.

Q17.What is the function of AGC circuit?

Ans:A circuit that maintains the output volume of a receiver, regardless of the variations in the received signal power.

Q18.What is known as Bandwidth?