Radio Receiver Architectures

No matter how simple or fancy the system may be, the basic function of a radio receiver is the same: to distinguish signals from noise. The concept of “noise” covers both human-made and natural radio frequency signals. Human-made signals include all signals in the pass band other than the one being sought. In communications systems, the signal is some form of modulated (AM, FM, PM, on-off telegraphy, etc.) periodic sine wave propagating as an electromagnetic (i.e., radio) wave. The noise, on the other hand, may be a random signal that sounds like the “hiss” heard between stations on a radio. The spectrum of such noise signals appears to be Gaussian (“white noise”) or pseudogaussian (“pink noise,” or bandwidth limited noise).

Every electronic system (even a simple resistor) generates thermal noise, even if no power is flowing through it. One goal of the system designer is to minimize the noise added by the system, so that weaker signals are not obscured. A basic form of noise seen in systems is thermal noise. Even if the amplifiers in the receiver add no additional noise (they will), thermal noise will be found at the input due to the input resistance. If you replace the antenna with a resistor matched to the system impedance and totally shielded, noise still will be present. The noise is produced by the random motion of electrons inside the resistor. At all temperatures above absolute zero (about −273.16ºC), the electrons in the resistor material are in random motion. At any given instant, a huge number of electrons will be in motion in all directions.

Equation 2.2 shows the basic problem of radio reception, especially in cases where the signal is very weak. The signal in Figure 2.2A is embedded in noise of relatively high amplitude. This signal is lower than the noise level, so it is very difficult (perhaps impossible) to detect. The signal in Figure 2.2B is easily detectable because the signal amplitude is higher than the noise amplitude. Detection becomes difficult when the signal is only slightly stronger than the average noise power level.

How high an SNR is required depends on a lot of subjective factors when a human listener is present. Skilled radio operators can detect signals with an SNR of less than 1 dB— but the rest of us cannot even hear that signal. Most radio operators can detect 3 dB SNR signals, but for “comfortable” listening, a 10 dB SNR usually is specified. For digital systems, the noise performance usually is defined by the acceptable bit error rate (BER).

A number of strategies can be used to improve the SNR of a system. First, of course, is to buy a receiver that has a low internal “noise floor” and do nothing to upset that figure. High-quality receivers have very low noise, but sometimes some creative spec writing in the advertisements uses different bandwidths for the measurement, and only the most favorable value—which may not be the bandwidth that matches your needs—is reported. By common sense, we see that there are two approaches to SNR improvement: either increase the signal amplitude or decrease the noise amplitude. Most successful systems do both, but they must be done carefully.

One approach to SNR improvement is to use a preamplifier ahead of the receiver antenna terminals. This approach may or may not work and under some situations may make the situation worse. The problem is that the preamplifier adds noise of its own and will amplify noise from outside (received through the antenna) and the desired signal equally. If you have an amplifier with a gain of, say, 20 dB, then the external noise and the signal both are increased by 20 dB. The result is that the absolute numbers are bigger but the SNR is the same. If the amplifier produces any significant noise of its own, then the SNR will degrade. The key is to use a very low-noise amplifier (LNA) for the preamplifier. Using an LNA for the preamplifier may actually reduce the noise figure of the receiver system.

Another trick is to use a preselector ahead of the receiver. A preselector is either a tuned circuit or a bandpass filter placed in the antenna transmission line ahead of the receiver antenna terminals. A passive preselector has no amplification (uses L-C elements only), while an active preselector has a built-in amplifier. The amplifier should be an LNA type. The preselector can improve the system because it amplifies the signal by a fixed amount, but only the noise within the passband is amplified the same amount as the signal. Improvement comes from bandwidth limiting the noise but not the signal.

Yet another practical approach is to use a directional antenna. This method works especially well when the unwanted noise is other human-made signal sources. An omnidirectional antenna receives equally well in all directions. As a result, both natural and human-made external noise sources operating within the receiver’s passband will be picked up. But if the antenna is made highly directional, then all noise sources not in the direction of interest are suppressed. Highly directional antennas have gain, so the signal levels in the direction of interest are increased. Although the noise also increases in that direction, the rest of the noise sources (in other directions) are suppressed. The result is that the SNR is increased by both methods.

When designing a communications system, the greatest attention usually should be paid to the antenna, then to an LNA or low noise preselector, and then to the receiver. Generally speaking, money spent on the antenna gives more signal to noise than the same money spent on amplifiers and other attachments.

  Radio receivers are at the heart of nearly all communications activities. In this chapter, we discuss the different types of radio receivers on the market. We learn how to interpret receiver specifications in Chapter 3. Later, we look at specific designs for specific applications.

The very earliest radio receivers were not receivers at all in the sense we know the term today. Early experiments by Hertz, Marconi, and others used spark gaps and regular telegraph instruments of the day. Range was severely limited because those devices have a terribly low sensitivity to radio waves. Later, around the turn of the 20th century, a device called a Branly coherer was used for radio signal detection. This device consisted of a glass tube filled with iron filings placed in series between the antenna and the ground. Although considerably better than earlier apparatus, the coherer was something of a dud for weak signal reception. In the first decade of this century, however, Fleming invented the diode vacuum tube and Lee DeForest invented the triode vacuum tube. The latter device made amplification possible and detection a lot more efficient.

A receiver must perform two basic functions: (1) It must respond to, detect, and demodulate desired signals; and (2) it must not respond to, detect, or be adversely affected by undesired signals. If it fails in either of these two functions, then the design performs poorly. Both functions are necessary. Weakness in either function makes a receiver a poor bargain, unless there is some mitigating circumstance. The receiver’s performance specifications tell us how well the manufacturer