AMPLITUDE MODULATION (AM) BASIC DEFINITION AND TUTORIALS

This is the method used in medium-wave and short-wave radio broadcasting. Figure 1.3 shows what happens when we apply amplitude modulation to a sinusoidal carrier wave.

Fig. 1.3 Amplitude modulation waveforms: (a) modulating wave; (b) carrier wave; (c) modulated wave


Figure 1.3(a) shows the modulating wave on its own.6 Figure 1.3(b) shows the carrier wave on its own. Figure 1.3(c) shows the resultant wave. The resultant wave shape is due to the fact that at times the modulating wave and the carrier wave are adding (in phase) and at other times, the two waves are opposing each other (out of phase).

Amplitude modulation can also be easily analysed mathematically. Let the sinusoidal modulating wave be described as

vm = Vm cos (ωmt ) (1.2)
where
vm = instantaneous modulating amplitude (volts)
Vm = modulating amplitude (peak volts)
ωm = angular frequency in radians and ωm = 2πfm where
fm = modulating frequency (hertz)

When the amplitude of the carrier is made to vary about Vc by the message signal vm, the modulated signal amplitude becomes
[Vc + Vm cos (ωmt )] (1.3)

The resulting envelope AM signal is then described by substituting Equation 1.3 into Equation 1.1 which yields
[Vc + Vm cos (ωmt )] cos (ωct + φc) (1.4)

It can be shown that when this equation is expanded, there are three frequencies, namely (f c – fm), f c and (f c + fm). Frequencies (f c – fm) and (f c + fm) are called sideband frequencies. These are shown pictorially in Figure 1.4.

Frequency spectrum of an AM wave

The modulating information is contained in one of the sideband frequencies which must be present to extract the original message. The bandwidth (bw) is defined as the highest frequency minus the lowest frequency. In this case, it is (f c + fm) – (f c – fm) = 2fm where f m is the highest modulation frequency. Hence, a radio receiver must be able to accommodate the bandwidth of a signal.

IF NOTCH REJECTION BASIC ELECTRONICS

WHAT IS IF NOTCH REJECTION?


If two signals fall within the passband of a receiver they will both compete to be heard. They will also heterodyne together in the detector stage, producing an audio tone equal to their carrier frequency difference.


For example, suppose we have an AM receiver with a 5 kHz bandwidth and a 455 kHz IF. If two signals appear on the band such that one appears at an IF of 456 kHz and the other is at 454 kHz, then both are within the receiver passband and both will be heard in the output.

However, the 2 kHz difference in their carrier frequency will produce a 2 kHz heterodyne audio tone difference signal in the output of the AM detector.


In some receivers, a tunable high-Q (narrow and deep) notch filter is in the IF amplifier circuit. This tunable filter can be turned on and then adjusted to attenuate the unwanted interfering signal, reducing the irritating heterodyne.

Attenuation figures for good receivers vary from –35 to –65 dB or so (the more negative the better). There are some trade-offs in notch filter design. First, the notch filter Q is more easily achieved at low IF frequencies (such as 50 kHz to 500 kHz) than at high IF frequencies (e.g. 9 MHz and up).

Also, the higher the Q the better the attenuation of the undesired squeal, but the touchier it is to tune. Some happy middle ground between the irritating squeal and the touchy tune is mandated here.

Some receivers use audio filters rather than IF filters to help reduce the heterodyne squeal. In the AM broadcast band, channel spacing is 9 or 10 kHz (depending on the part of the world), and the transmitted audio bandwidth is 5 kHz. Designers of AM broadcast receivers may insert an R–C low-pass filter with a –3 dB point just above 4 or 5 kHz right after the detector in order to suppress the audio heterodyne.

This R–C filter is called a ‘tweet filter’ in the slang of the electronic service/repair trade. Another audio approach is to sharply limit the bandpass of the audio amplifiers. Although the shortwave bands typically only need 3 kHz bandwidth for communications, and 5 kHz for broadcast, the tweet filter and audio roll-off might not be sufficient. In receivers that lack an effective IF notch filter, an audio notch filter can be provided.

-1dB COMPRESSION POINT AND THIRD ORDER INTERCEPT POINT

WHAT ARE -1dB COMPRESSION POINT AND THIRD ORDER INTERCEPT POINT?


–1 dB compression point

An amplifier produces an output signal that has a higher amplitude than the input signal. The transfer function of the amplifier (indeed, any circuit with output and input) is the ratio OUT/IN, so for the power amplification of a receiver RF amplifier it is Po/Pin (or, in terms of voltage, Vo/Vin).



Any real amplifier will saturate given a strong enough input signal (see Fig. 3.16). The dotted line represents the theoretical output level for all values of input signal (the slope of the line represents the gain of the amplifier).

As the amplifier saturates (solid line), however, the actual gain begins to depart from the theoretical at some level of input signal. The –1 dB compression point is that output level at which the actual gain departs from the theoretical gain by –1 dB.

The –1 dB compression point is important when considering either the RF amplifier ahead of the mixer (if any), or any outboard preamplifiers that are used. The –1dB compression point is the point at which signal distortion becomes a serious problem. Harmonics and intermodulation are generated at high levels when an amplifier goes into compression.


Third-order intercept point
It can be claimed that the third-order intercept point (TOIP) is the single most important specification of a receiver’s dynamic performance because it predicts the performance as regards intermodulation, cross-modulation and blocking desensitization.

Third-order (and higher) intermodulation products (IP) are normally very weak, and don’t exceed the receiver noise floor when the receiver is operating in the linear region. As input signal levels increase, forcing the front-end of the receiver toward the saturated nonlinear region, the IP emerge from the noise and begin to cause problems.

When this happens, new spurious signals appear on the band and self-generated interference begins to arise. Look again at Fig. 3.16. The dotted gain line continuing above the saturation region shows the theoretical output that would be produced if the gain did not clip.

It is the nature of third-order products in the output signal to emerge from the noise at a certain input level, and increase as the cube of the input level. Thus, the third-order line increases 3 dB for every 1 dB increase in the response to the fundamental signal.

Although the output response of the third-order line saturates similarly to that of the fundamental signal, the gain line can be continued to a point where it intersects the gain line of the fundamental signal. This point is the third-order intercept point (TOIP).


Interestingly, one receiver feature that can help reduce IP levels is the use of a front-end attenuator (or input attenuator). In the presence of strong signals even a few dB of input attenuation is often enough to drop the IPs back into the noise, while afflicting the desired signals only a small amount.

Other effects that reduce the overload caused by a strong signal also help. Situations arise where the apparent third-order performance of a receiver improves dramatically when a lower gain antenna is used.

This effect can be easily demonstrated using a spectrum analyser for the receiver. This instrument is a swept frequency receiver that displays an output on an oscilloscope screen that is amplitude-vs-frequency, so a single signal shows as a spike. In one test, a local VHF band repeater came on the air every few seconds, and one could observe the second- and third-order IPs along with the fundamental repeater signal.

There were also other strong signals on the air, but just outside the band. Inserting a 6 dB barrel attenuator in the input line eliminated the IP products, showing just the actual signals. Rotating a directional antenna away from the direction of the interfering signal will also accomplish this effect in many cases.

Preamplifiers are popular receiver accessories, but can often reduce rather than enhance performance. Two problems commonly occur (assuming the preamp is a low noise device). The best known problem is that the preamp amplifies noise as much as signals, and while it makes the signal louder it also makes the noise louder by the same amount.

Since it’s the signal-to-noise ratio that is important, this does not improve the situation. Indeed, if the preamp is itself noisy, it will deteriorate the SNR. The other problem is less well known, but potentially more devastating. If the increased signal levels applied to the receiver push the receiver into non-linearity, then IPs will emerge.

When evaluating receivers, a TOIP of +5 to +20 dBm is excellent performance, while up to +27 dBm is relatively easily achievable, and +35 dBm has been achieved with good design; anything greater than +50 dBm is close to miraculous (but attainable).

Receivers are still regarded as good performers in the 0 to +5 dBm range, and middling performers in the –10 to 0 dBm range. Anything below –10 dBm is not usually acceptable. A general rule is to buy the best third-order intercept performance that you can afford, especially if there are strong signal sources in your vicinity.