Quantcast Comparison of AM and fm receiver response to an AM signal

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COMPARING FSK AND CW SIGNALS. - A comparison of on-off keyed cw (figure 2-2), (view (A), view (B), view (C)), and fsk (figure 2-3), (view (A), view (B), view (C)), signals will show clearly the principal features of fsk and give us a basis on which frequency modulation can be discussed. Let's use views (A), (B), and (C) of both figures to show the Morse code character "F" for an example. Figures 2-2 and 2-3 are graphic drawings of the two types of keying. Time and amplitude are known dimensions of AM; but to explain fsk properly, we have added the third dimension of frequency.

Figure 2-2A. - Comparison of AM and fm receiver response to an AM signal. THREE-DIMENSIONAL REPRESENTATION OF AN ON-OFF KEYED CW TELEGRAPH SIGNAL

Figure 2-2B. - Comparison of AM and fm receiver response to an AM signal. RESPONSE OF AM DEMODULATOR TO SIGNAL

Figure 2-2C. - Comparison of AM and fm receiver response to an AM signal.RESPONSE OF FM DISCRIMINATOR TO SIGNAL

Figure 2-3A. - Comparison of AM and fm receiver response to an fm signal. THREE-DIMENSIONAL REPRESENTATION OF A FREQUENCY-SHIFT KEYED TELEGRAPH SIGNAL

Figure 2-3B. - Comparison of AM and fm receiver response to an fm signal. RESPONSE OF AM DEMODULATOR TO SIGNAL

Figure 2-3C. - Comparison of AM and fm receiver response to an fm signal. RESPONSE OF FM DISCRIMINATOR TO SIGNAL

CW SIGNALS. - Since cw signals are of essentially constant frequency, there is no variation along the frequency axis in view (A) of figure 2-2. The complete intelligence is carried as variations in the amplitude of the signal. To receive the intelligence carried by such a signal, the receiving equipment must be able to scan the signal along the time and amplitude axes, which carry the information. When scanned along the time and amplitude axes [shown in view (B)], the intelligence appears as large changes in amplitude. If the circuit were perfect, these variations would be from 0 amplitude to some maximum value (established by transmitter power, distance, and so forth) depending on whether the key were open or closed. However, interfering components of energy caused by atmospherics, interfering stations, and electrical machinery appear as additional variations along the amplitude axis. When these amplitude variations approach or exceed the variation caused by the keyed intelligence, the signal is blanked out by interference. We have all heard this happen on our AM radios during storms or when near operating machinery.

View (C) of figure 2-2 represents the same signal when scanned along the time and frequency axes as it would be in an fm receiver. Variations in signal amplitude have no effect on the frequency and no intelligence can be received. Note that the noise and interference components have also been suppressed so that they have little effect on the received signal. Thus, if the intelligence variations were impressed as changes along the frequency axis, and the receiving equipment were designed to respond to this type of signal, then the effects of noise and interference would be practically eliminated. Frequency-shift keyed circuits fulfill these conditions.

FSK SIGNALS. - In fsk the rf signal is shifted in frequency (not amplitude) between "key-open" and "key-closed" conditions. The signal amplitude remains essentially constant. View (A) of figure 2-3 represents the letter "F" keyed as a shift in frequency between mark and space. The normal frequency condition with the key open is a space. Recall that this may be either the lower or higher frequency. When the key is closed, the frequency instantly changes to the mark value and remains constant during the marking interval. Opening the key again returns the frequency to the space frequency. Midway between the mark and space frequencies is the assigned channel frequency.

Also shown in view (A) is the variation along the amplitude axis caused by the same noise and interference mentioned earlier. The right-hand portion of view (A) illustrates the elimination of this noise by the receiving equipment. View (B) clearly shows that scanning the signal along the amplitude and time axes reproduces no amplitude variations from signal interference. However, if the scanning is accomplished along the frequency and time axes, the intelligence is reproduced, as shown in view (C). By this system, the intelligence can be recovered at the receiving station in its original form; it will be nearly unaffected by conditions in the radio path other than fading. As a matter of fact, fsk resists the effects of fading better than cw.

FREQUENCY-SHIFT KEYING. - In its simplest form, frequency-shift keying of a transmitter can be accomplished by shunting a capacitor (or an inductor) and key (in series) across the oscillator circuit. By locking the normal key of the transmitter and operating only the oscillator circuit key, you can change the oscillator frequency. The shift in frequency between mark (key-closed) and space (key-open) conditions is determined by the effect of the additional capacitance (or inductance) on the oscillator frequency. The frequency multiplication factor in the transmitter amplifiers must be taken into consideration when determining the oscillator frequency shift. Thus, if the desired shift is the conventional 850 hertz at the transmission frequency, and this frequency is four times the oscillator frequency (that is, doubled in two stages), then the effect of the additional capacitance (or inductance) on the oscillator must be limited to 212.5 hertz as shown below:

Frequency-shift keyers are, of course, more complicated than this simple illustration would seem to show, but the basic principles are the same. Still, the keyer does change the oscillator frequency by a certain number of cycles. Further, this change must be correlated with the multiplication factor of the transmitter to cause the desired shift between mark and space frequencies.

METHODS OF FREQUENCY SHIFTING. - Frequency-shift keyers operate on either of two general principles. First, the keyer may take the output of the transmitter's master oscillator and modulate it with the output of another oscillator that is frequency-shift keyed. This action will result in two frequencies that are used to excite the first amplifier stage of the transmitter. This system is illustrated in view (A) of figure 2-4. View (B) illustrates the second method of frequency-shift keyer operation. In this method the transmitter's master oscillator is itself shifted in frequency by the mark and space impulses from the keyer unit.

Figure 2-4A. - Two methods of frequency-shift keying (fsk). FREQUENCY-SHIFT KEYING BY MODULATING MASTER OSCILLATOR OUTPUT

Figure 2-4B. - Two methods of frequency-shift keying (fsk). FREQUENCY-SHIFT KEYING IN MASTER OSCILLATOR CIRCUIT

ADVANTAGES OF FSK OVER AM. - Frequency-shift keying is used in all single-channel, radiotelegraph systems that use automatic printing systems. The advantage of fsk over on-off keyed cw is that it rejects unwanted signals (noise) that are weaker than the desired signal. This is true of all fm systems. Also, since a signal is always present in the fsk receiver, automatic volume control methods may be used to minimize the effects of signal fading caused by ionospheric variations. The amount of inherent signal-to-noise ratio improvement of fsk over AM is approximately 3 to 4 dB. This improvement is because the signal energy of fsk is always present while signal energy is present for only one-half the time in AM systems. Noise is continuously present in both fsk and AM, but is eliminated in fsk reception. Under the rapid fading and high-noise conditions that commonly exist in the high frequency (hf) region, fsk shows a marked advantage over AM. Overall improvement is sometimes expressed as the RATIO OF TRANSMITTED POWERS required to give equivalent transmission results over the two systems. Such a ratio varies widely, depending on the prevailing conditions. With little fading, the ratio may be entirely the result of the improvement in signal-to-noise ratio and may be under 5 dB. However, under severe fading conditions, large amounts of power often fail to give good results for AM transmission. At the same time, fsk may be satisfactory at nominal power. The power ratio (fsk versus AM) would become infinite in such a case.

Another application of fsk is at low and very low frequencies (below 300 kilohertz). At these frequencies, keying speeds are limited by the "flywheel" effect of the extremely large capacitance and inductance of the antenna circuits. These circuits tend to oscillate at their resonant frequencies. Frequency-shifting the transmitter and changing the antenna resonance by the same keying impulses will result in much greater keying speeds. As a result, the use of these expensive channels is much more efficient.

Q.1 What are the two types of angle modulation? answer.gif (214 bytes)
Q.2 Name the modulation system in which the frequency alternates between two discrete values in response to the opening and closing of a key? answer.gif (214 bytes)
Q.3 What is the primary advantage of an fsk transmission system? answer.gif (214 bytes)




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