Diagonal Clipping

The electromagnetic disturbance created by the transmitter is propagated by the transmitter antenna and travels at the speed of light as described in Chapter 2. It is evident that, if the electromagnetic wave encounters a conductor, a current will be induced in the conductor. How much current is induced will depend on the strength of the electromagnetic field, the size and shape of the conductor and its orientation to the direction of propagation of the wave. The conductor will then capture some of the power present in the wave and hence it will be acting as a receiver antenna. However, other electromagnetic waves emanating from all other radio transmitters will also induce some current in the antenna. The two basic functions of the radio receiver are:

(1) to separate the signal induced in the antenna by the transmission which we wish to receive from all the other signals present,

(2) to recover the ''message'' signal which was used to modulate the transmitter carrier.

3.2 THE BASIC RECEIVER: SYSTEM DESIGN

In order to separate the required signal from all the other signals captured by the antenna, we use a bandpass filter centered on the carrier frequency with sufficient bandwidth to accommodate the upper and lower sidebands but with a sufficiently high Q factor so that all other carriers and their sidebands are attenuated to a level where they will not cause interference. This is most easily achieved by using an LC tuned circuit whose resonant frequency is that of the carrier.

Diode Envelope Detector FilterAnode Detectie Buizenradio

Figure 3.1. (a) The envelope detector circuit. The diode ''half-wave'' rectifies the AM wave and the RC time-constant ''follows'' the envelope with a slight ripple. (b) The input signal to the envelope detector. (c) The output signal of the envelope detector. Note that (1) when the voltage is rising the ripple is larger than when the voltage is falling. A longer time constant will help reduce the ripple; however, it will also increase the likelihood that the output voltage will not follow the envelope when the voltage is falling causing 'diagonal clipping'. (2) In practice, the carrier frequency is much higher than the modulating frequency, hence the ripple is much smaller than shown.

Diagonal Clipping Diode Detector

To recover the "message'' we require a circuit which will follow the envelope of the amplitude of the carrier. Such a circuit is called an envelope detector and it consists of a diode and a parallel RC circuit as shown in Figure 3.1(a).

The input signal to the circuit is most appropriately represented by an ideal current source connected to the primary of the transformer. This ideal current source represents all the currents induced in the antenna by all the radio stations broadcasting signals in free space. The signal is coupled to the parallel-tuned LC circuit which selectively enhances the amplitude of the signal whose carrier frequency is the same as the resonant frequency of the LC circuit. In Figure 3.1(b), only the enhanced modulated signal is shown at the input of the envelope detector. Because the diode conducts only when the anode has a positive potential compared to the cathode, only the positive half of the signal appears across the output resistor. Because the capacitor is connected in parallel with the resistor, when the diode conducts the capacitor must charge up to the peak value of the voltage. When the input voltage is less than the voltage across the capacitor, the conduction is cut off and the capacitor starts to discharge through the resistor with the voltage falling off exponentially. With the proper choice of time-constant RC, the output voltage waveform will have the form shown in Figure 3.1(c). This waveform is essentially the envelope of the carrier signal with a ripple at a frequency equal to the carrier frequency. A low-pass filter can be used to remove the ripple.

The circuit shown in Figure 3.1(a) has been used with success as a practical receiver with the resistor R replaced by a high impedance headphone. Needless to say, such a simple circuit has its limitations. The power in the circuit is supplied entirely by the transmitter and naturally it is at a very low level, especially as the distance between the transmitter and the receiver increases. Secondly, the ability of the LC tuned circuit to suppress the signals propagated by all the other transmitters is limited and therefore such a receiver will be subject to interference from other stations. These limitations can be overcome by using the superheterodyne configuration described below.

3.3 THE SUPERHETERODYNE RECEIVER: SYSTEM DESIGN

The superheterodyne receiver takes the incoming radio-frequency signal whose frequency varies from station to station and transforms it to a fixed frequency called the intermediate frequency (IF). It is then easier to do the necessary filtering to eliminate interference and, at the same time, to provide some power gain or amplification to the desired signal.

A normal AM superheterodyne receiver block diagram is shown in Figure 3.2.

The antenna has induced in it currents from all the transmitters whose electromagnetic propagation reach it. The first step is to use an LC tuned radio-frequency amplifier to enhance the desired carrier and its sidebands. The radio-frequency amplifier is tuneable over the frequency for which the receiver is designed by varying the capacitor in the tuned circuit. This capacitor is mechanically coupled or "ganged'' to another capacitor which forms part of the local oscillator circuit. The local oscillator frequency and the frequency to which the radio-frequency amplifier is tuned are chosen in such a way that, as the value of the ganged capacitors change, they maintain a fixed frequency difference between them. The outputs from the local oscillator and the radio-frequency amplifier are used to drive the frequency changer or mixer. The frequency changer essentially multiplies the two inputs and produces a signal that contains the sum and difference of the input frequencies. Because of the fixed difference between the incoming radio-frequency and the local oscillator frequency, the difference frequency remains constant as the value of the ganged capacitor is changed. The output of the frequency changer is then fed into the intermediate-frequency amplifier. The intermediate-frequency amplifier is designed to select the difference frequency plus its sidebands and to attenuate all other frequencies present. Since the difference frequency is fixed (for domestic AM radios the intermediate frequency is 445 kHz) the filters required are relatively easy to

Figure 3.2. The block diagram of the superheterodyne receiver. The capacitor which tunes the radio-frequency amplifier is mechanically ganged to the capacitor which determines the frequency of the local oscillator. In the normal AM receiver, the oscillator frequency is always 455 kHz above the resonant frequency of the radio-frequency amplifier throughout the range of tuning.

Figure 3.2. The block diagram of the superheterodyne receiver. The capacitor which tunes the radio-frequency amplifier is mechanically ganged to the capacitor which determines the frequency of the local oscillator. In the normal AM receiver, the oscillator frequency is always 455 kHz above the resonant frequency of the radio-frequency amplifier throughout the range of tuning.

design with sharp cut-off characteristics. The output of the intermediate-frequency amplifier which then goes to the envelope detector consists of the intermediate frequency and its two sidebands. The envelope detector removes the intermediate frequency, leaving the audio-frequency signal which is then amplified by the audiofrequency amplifier to a level capable of driving the loudspeaker. It is clear that there will be a very large difference between the signal from a powerful local radio station and a weak distant station. To help reduce the difference an automatic gain control (AGC) is used to adjust the signal reaching the envelope detector to stay within predetermined values.

The most interesting signal processing step in the system takes place in the frequency changer or frequency mixer or simply the mixer [1]. There are two basic types of mixers: the analog multiplier and the switching types. The analog multiplier frequency changer simply multiplies the radio-frequency signal and the local oscillator so that when the modulated carrier current is im(t)=A(1 + ksinfflSt) sin<oCt (3.3.1)

and the local oscillator signal is io(t) = B sin<oLt (3.3.2)

the output of the mixer is i(t) = A(1 + ksinfflSt) sin<oCt x B sin<oLt (3.3.3)

i(t) = 1 AB(1 + k sin fflSt)[cos(fflL — <oC )t — cos(oL + fflC)t] (3.3.4) i(t) = 1 AB[cos(<L — <C )t — cos(<L + <C)t + k sin <<St cos(<L — <C)t

— k sin fflSt cos(<L + <C)t] (3.3.5) i(t) = 1 AB{cos(fflL — <C)t — cos(<L + <C)t

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Responses

  • ugo
    When does diagonal clipping occur how can it be removed?
    2 years ago
  • Paul
    What is diagonal peak clipping?
    1 year ago
  • PHILLIPP
    What is digonal clipping?
    1 year ago
  • Gorbaduc Brockhouse
    How diagonal ?
    1 year ago
  • rosarmosario
    What is diagonal clippling?
    1 year ago
  • Liviana
    What is diagonal clipping of envelope detector?
    1 year ago
  • Awet
    What is diagonal clipping in diode detector?
    11 months ago
  • nicolas
    How diagonal clipping can be avoided?
    7 months ago
  • blanda zetticci
    How to reduce diagonal clipping?
    4 months ago
  • Sanna-Leen Soini
    How reduce diagonal peak clipping?
    4 months ago

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