Signals



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Summary

The principles of signal generation and amplitude modulation are covered in Mixers. Those fundamentals are used here to form signals for transmission.

Speech

Fig1

For radio communication purposes we can consider the range of audio frequencies in human speech to extend from 300 Hz to 3000 Hz - a band of 2.7 kHz. This audio spectrum for speech can be drawn as shown in Fig. 1. Here the low end of the wedge represents 300 Hz and the high end of the wedge represents 3000 Hz. This diagram is for purposes of explanation only and should not be taken to represent the voice energy distribution of speech in actual practice. It is usually the other way around - the low notes contain most of the voice energy, whilst the high pitch notes are weak but are very necessary for speech recognition purposes. The band of audio frequencies 300 to 3000 Hz, this audio band, when illustrated as a wedge with each end identifiable, is useful for explanation purposes.

Unmodulated Carrier

Fig2

The signal from a typical simple transmitter is a single-frequency signal, constant in amplitude. Such a signal carries no information. The only information is the deduction that, by its presence at a receiver, it is known that the transmitter is transmitting. Refer to Fig. 2 which represents an unmodulated carrier signal at 3.6 MHz. For our purposes at the moment, such a signal has no bandwidth - this matter will be covered elsewhere in these study notes.

CW Signals (continuous wave)

A fundamental and time-honoured way to transmit information is to turn the transmitter on and off. This can be done by a Morse key (a switch). With the key down, the transmitter is on, with the key up the transmitter is off. The dits and dahs (the dots and dashes) of the Morse code can be sent by careful manipulation of the key. Morse code

KEY

The term CW comes from the constant-amplitude signal transmitted with the key down, compared to damped waves - waves which changed in amplitude - as generated by spark-transmitters in the early days of radio communication and now totally obselete. The term CW can be considered as synonymous with Morse code transmission.

Modulation

The technique to impress information (voice, music, picture, or data) on a radio-frequency carrier wave by varying one or more characteristics of the wave in accordance with the intelligence signal is called modulation. There are various forms of modulation, each designed to alter a particular characteristic of the carrier wave. For our study purposes, the most commonly altered characteristics are amplitude and frequency.

Amplitude Modulation

In amplitude modulation (AM), auditory or visual information is impressed on a carrier wave by varying the amplitude of the carrier to match the fluctuations in the audio or video signal being transmitted. AM is the oldest method of broadcasting radio programs. Commercial AM stations operate at frequencies between 535 and 1,605 kHz and in shortwave radio broadcasts. There is little use of conventional AM systems by radio amateurs today, the SSB and FM modes predominate.

Fig3

When a carrier is amplitude-modulated by an audio signal, the audio signal will increase the amplitude of the transmitted signal during part of the audio cycle, and at other times in the audio cycle the amplitude of the transmitted signal will decrease. See Fig 3.

Amplitude modulating a carrier signal with a constant amplitude sinusoidal audio frequency tone results in a complex signal. It comprises three separate component parts: the carrier (in which most of the signal energy is contained) and two side frequencies. See Fig. 4. (Refer to Mixers where this is explained).

So an amplitude-modulated signal, with a single modulating tone, can be viewed in two different ways:

Fig4

The first plot is Fig. 3. Here the plot is signal amplitude on the vertical axis and with time on the horizontal axis - an oscilloscope diagram. This is sometimes referred to as the time domain.

The second plot is Fig. 4. The plot is still signal amplitude on the vertical axis but with frequency on horizontal axis - a Spectrum Diagram. This is sometimes referred to as the frequency domain.

Remember that with NO audio input signal, there is then no modulating signal, so the modulated output reverts to being an unmodulated carrier only.

A speech-input amplitude modulated (AM) transmission can be shown as Fig. 5 using the diagrammatic wedge symbol. Speech is made up of many different audio frequencies - a band of audio frequencies. Here is a radio frequency carrier signal with two adjacent sidebands - the wedges shown above and below the carrier frequency - each indicated here as USB (upper sideband) and LSB (lower sideband). The energy of the AM signal is contained in these three components - the lower sideband, the carrier, and the upper sideband.

Fig5

It can now be seen that with no input speech to the amplitude-modulated transmitter, there are no output sidebands - but the carrier continues on unchanged.

The bandwidth of an AM signal can be seen to be: Twice the highest modulating frequency. So for our 300 to 3000 Hz audio modulating signal, the bandwidth will be 6 kHz.

Distortion Considerations

There is a limit placed on the level of audio signal that an AM system can accept. Referring to Fig. 3 above, if the amplitude of the modulating audio signal is further increased, a point is reached at which the level of modulated signal output can no longer be a replica of the input signal. The audio-shaped envelope of the modulated wave exhibits peak flattening - introducing distortion. The maximum amplitude audio signal is the point at which the envelope shapes meet at the zero-output axis. This level is given the expression 100% modulation.

Power Considerations

It is important to note the energy distribution in the carrier and the two side frequencies of the resulting AM signal. At the 100% modulation level, the amplitude of the two side frequencies can add together to equal the carrier in amplitude and either add to or subtract from the carrier signal. If the carrier is transmitting 100 watt, and if the amplitude of each single side-frequency is half that of the carrier (full modulation), then the power in the upper side-frequency and the power in the lower side-frequency will each be one-quarter of the carrier power.

The power distribution at full modulation then becomes 100 watt in the carrier, 25 watt in the upper side-frequency and 25 watt in the lower side-frequency. The total radiated power is 150 watt, of which only 50 watt - or 33% - is from the modulating intelligence. With no modulation, the output power is the unmodulated carrier alone - 100 watt. Modulation changes the output between 100 and 150 watt, from no modulation to full modulation.

So, depending on the modulation level, more than two-thirds of the radiated energy from an amplitude-modulated transmitter is carrier power - which does not contribute to the intelligence of this system.

AM Reception

An AM signal can be successfully demodulated at the receiver in several ways but the easiest is to use a simple diode rectifier followed by a filter smoothing circuit. Refer to Fig. 6.

Fig6

Single-Sideband (SSB)

In a SSB transmission, only one sideband is radiated - both the carrier and the other sideband (of AM) are suppressed. So, with SSB, with no input speech, no signal at all is transmitted! With no carrier signal at all being transmitted, all the radiated energy is related to the input modulating signal. The total transmitter output is useful.

You will now recognise that SSB transmissions are a particular category or variant of amplitude-modulated transmissions.

The convention generally followed by radio amateurs is that on amateur bands above 10 MHz the Upper Sideband will be used and on bands below 10 MHz the lower sideband will be used.

So on the 80m band, it is customary for amateur stations to use the lower sideband for a SSB transmission. Amateurs use the (suppressed) carrier frequency when referring to the frequency of a SSB signal. So the Fig. 7 applies. The position for the re-inserted carrier (your receiver does this), needed as the reference to restore the signal during demodulation in your receiver, is shown in Fig. 7.

Note that the LSB signal appears inverted. The 300 Hz component of the speech is now the higher frequency component in the transmitted signal. The 3000 Hz component is the lower frequency component.

Fig7

The bandwidth of a SSB signal is the same as that of the modulating signal. For our speech band 300 to 3000 kHz, the bandwidth is 2.7 kHz. Compare that with the equivalent for the AM signal to see the spectrum-conserving value of SSB - just one of its advantages. Remember too that all the radiated energy from a SSB transmitter is useful energy (there is no power-consuming carrier).

Reception of SSB Signals

A local carrier is re-inserted at the receiver and is used as the reference for the demodulation process. You can resolve an SSB signal by carefully adjusting your SSB receiver. This inserted carrier can be the receiver's beat frequency oscillator (BFO). In effect, the BFO beats with the incoming side-frequencies to produce the restored audio frequencies. You can hear the full 300 to 3000 Hz range of the transmitted audio in the speaker.

A receiver without a BFO cannot resolve SSB signals. So a receiver fitted with a BFO for reception of CW signals can receive both CW and SSB signals. AM signals can also be received on this receiver when the BFO is switched off - the BFO then being unnecessary.

Sometimes a receiver is fitted with a mixer to demodulate SSB signals. See Mixers for the product detector.

The oscillator in the receiver which provides the re-inserted carrier for SSB reception is sometimes called the carrier insertion oscillator (CIO). This is especially so if a product detector is used and the receiver is specifically designed for SSB reception. For most amateur radio purposes, receivers are designed for CW and for SSB reception and the terms BFO and CIO are interchangeable.

Fig8

Depending on the characteristics of the filter in the receiver, at times you may also hear an interfering signal. If there are two quite separate but adjacent SSB signals - shown as SSB1 and SSB2 in Fig. 8, and you are listening to the lower frequency one (SSB1), you may hear the higher frequency one as 'inverted speech'. The 3000 Hz component of that higher-frequency SSB signal will be heard by you as a low-pitch audio signal and its 300 Hz component as a high pitch! Fortunately this interference is almost indecipherable by the human ear. Your ear will tend to discard it as noise and will receive and listen to the 'natural-sounding' wanted signal. Of course this ear-discrimination characteristic also depends upon the relative levels of the two signals.

Frequency Modulation

In frequency modulation (FM), unlike AM, the amplitude of the carrier is kept constant, but its frequency is altered in accordance with variations in the audio signal being sent. See Fig. 9. Note how compression and stretching of the modulated signal is shown in the diagram of the modulated signal - indicating increase and decrease of the carrier signal frequency.

Fig9

The frequency of the frequency-modulated signal deviates up and down in frequency. The extent of the frequency sweep is called the deviation and it depends upon the loudness (i.e. amplitude) of the modulating signal. The rate at which deviations are made depends on the frequency of the modulating signal.

The deviation is usually given the symbol Δf (delta f).

FM is less susceptible than is AM to certain kinds of interference, such as random electrical noise from machinery and other related sources. These noise-producing signals affect the amplitude of a radio wave but not its frequency, so an FM signal remains virtually unchanged.

A frequency-modulated signal is passed through a limiter stage prior to demodulation. The limiter clips off the tops of the FM signal, removing any amplitude changes, restoring the signal to a constant-amplitude signal. In this way, noise-spikes are clipped off, unwanted interfering noise - principally appearing as amplitude-changes - is reduced.

Commercial FM broadcasting stations are assigned higher frequencies than are AM stations. The assigned frequencies range from 88 to 108 MHz. Amateurs use FM principally on VHF and higher bands for speech communication.

It appears from these diagrams that the amplitude of the frequency-modulated carrier signal remains unchanged. This is not so. Energy from the carrier signal is distributed across a range of frequencies adjacent to the carrier frequency in a complex system of sidebands. So the carrier itself reduces in amplitude as its energy is distributed to the adjacent side-frequencies. The spectrum diagram of a frequency-modulated signal is complex and is beyond the requirements for the amateur radio examination.

The bandwidth of a frequency-modulated signal is approximately = 2(Δf + fa). This is an empirical formula. So the bandwidth is approximately twice the sum of the deviation plus the audio modulating frequency. You can now see why FM is generally constrained to the VHF and higher bands.

The typical deviation for an amateur hand-held FM transceiver is about 5 kHz - a transmitted signal bandwidth of about 12 kHz. This depends on how loud you talk into the microphone. Shout, and over-deviation can take place, with the received signal being distorted because the receiver bandwidth has been exceeded.

Reception of Frequency-Modulated Signals

A circuit known as a discriminator is used to demodulate frequency-modulated signals. It can take many forms but the general type involves a tuned circuit with a pair of diodes. As the frequency of the input signal moves up and down across the resonant frequency of the tuned circuit, a rectified output voltage varies positive and negative to provide the output audio signal. The exact details are not required for the amateur radio examination but it is recommended that you look up a typical discriminator circuit in a radio textbook.

A phase-locked loop (PLL) can be used as a demodulator for frequency-modulated signals . See Oscillators

Phase Modulation

The phase of a carrier wave can be varied in response to the vibrations of the sound source in phase modulation (PM). This form of modulation is a variation of FM. The two processes are closely related because phase cannot be changed without also varying frequency, and vice versa. Also, the rate at which the phase of a carrier changes is directly proportional to the frequency of the audio signal. For the purposes of the amateur radio examination, PM can be ignored!



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Compiled Thu Jan 20 2011 at 9:07:41pm


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