Radio System

A radio transmitter must produce a signal that has enough power, has generally a very accurate frequency, and has a clean enough spectrum so that the transmitter does not disturb users of other radio systems. Information to be transmitted, the baseband signal, is attached to a sinusoidal carrier signal by modulating the carrier amplitude, frequency, or phase either analogically or digitally (see Section 11.3).

Low-power transmitters are usually based on a semiconductor device, a transistor or diode oscillator. When a transmitter power of hundreds of watts is needed, power is generated with electron tubes or so-called microwave tubes. Tetrodes are used from LF to VHF, and klystrons at UHF and SHF. In radar transmitters, a magnetron oscillator is the most common. In klystrons, magnetrons, and other microwave tubes, microwave oscillation is generated by an electron beam interacting with a resonance cavity or a slowwave structure [1].

In order to have a sufficiently accurate and clean signal, the oscillator frequency must be stabilized and the signal must be bandpass filtered before transmitting. Oscillators may be stabilized using a resonator with a sufficiently high quality factor. Often the accurate frequency is based on a quartz crystal oscillator at a frequency of 1 to 40 MHz and with a frequency stability of 10−9 to 10−10 per day. The signal of the quartz oscillator is frequency multiplied to the transmission frequency and then amplified, or it is used to injection lock or phase lock another oscillator to the correct frequency.

Figure 11.1 presents a direct-conversion transmitter. A digital baseband signal modulates the carrier in an IQ-modulator (see Section 11.3.2). The modulated signal is then filtered and amplified. In a superheterodyne transmitter the signal is further upconverted to the final frequency. The carrier frequency is stabilized by phase locking [2]. A basic phase-locked loop (PLL) is a feedback system consisting of a VCO, a phase detector (for example, a double-balanced mixer), and a low-pass filter. In the loop of Figure 11.1 there is also in the feedback branch a digital circuit that divides the frequency of the VCO by N. The output of the divider is compared with the signal from the reference oscillator in the phase detector. 

A possible difference in frequency (in phase) is transformed into a voltage proportional to the phase difference, and after lowpass filtering this voltage is used to control the VCO frequency until the frequency difference is zero. The output frequency of the locked loop is Nfref . The loop also stabilizes within its bandwidth the random phase variations of the VCO and, thus, the reference oscillator determines the phase-noise characteristics.

Random fields and voltages, that is, noise, disturb all radio systems. The antenna receives noise from its surroundings, and all receiver components, which are either active or lossy, generate noise. We call the former the antenna noise and the latter the receiver noise; their sum is called the system noise. In a radio system (e.g., a communication link) the system noise power in the receiver bandwidth may be stronger than the signal to be received. The ratio of the signal power to the noise power at the receiver bandwidth, that is, the S /N often determines the quality of a radio link. However, noise signals may also be useful, as is the case in radiometry, for example, in remote sensing and radio astronomy (see Sections 12.7 and 12.8). In system considerations, a radio channel, where white noise corrupting the signal is the only nonideality, is called the additive white Gaussian noise (AWGN) channel. In addition to noise, in practical radio channels there are other nonidealities. When the small-scale fading or Rayleigh fading in multipath propagation conditions is the limiting factor for the channel performance, we call it a Rayleigh fading channel.

In a receiver, many kinds of noise are generated, for example, thermal noise, shot noise, 1/f noise, and quantum noise.

Thermal noise is generated by the thermal motion of charge carriers. The warmer the material is, the more electrons collide with the crystal lattice of the material. Each collision causes a change in the kinetic energy state of the electron, and the energy difference is radiated as an electromagnetic wave. Similarly, collisions are also the reason for resistivity of a material and, therefore, thermal noise is generated in all materials and circuits absorbing RF power. Thermal noise is directly proportional to the absolute temperature of the medium, but its power density is independent of frequency—it is socalled white noise.

Shot noise is often the most important noise mechanism in semiconductor devices and electron tubes. Shot noise is caused by the fact that charge is not a continuous quantity but always a multiple of an electron charge. For example, a current going through the Schottky interface is not continuous but is a sum of the current impulses of single electrons. The power density of shot noise is directly proportional to the current. At low frequencies there is 1/f noise (flicker noise) in all semiconductor devices. It is caused, for example, by the fluctuating amount of electrons in the conduction band. Its power density is inversely proportional to frequency. Quantum noise is due to the quantized energy of the radio wave. It is important only in cases of submillimeter and shorter waves, because their energy quantum W = hf is large.

where Nin is the available noise power in a bandwidth df from a matched resistive termination (here ‘‘matched’’ means that the termination is matched to the characteristic impedance of the line) at temperature T0 = 290K connected to the input of the device, and Nout is the total noise power available at the output port in a bandwidth df when the input power is Nin . Ga is the available power gain of the two-port for incoherent signals from an input bandwidth of df to an output bandwidth of df. The noise factor indicates how many times larger the output noise power of the device is compared to that of a noiseless device, when both have in the input a matched resistive termination at the absolute reference temperature of T0 = 290K.

You have two low-noise amplifiers, LNA1 and LNA2, with characteristics T1 = 100K, G1 = 13 dB, and T2 = 90K, G2 = 7 dB, respectively. You want to use these LNAs together in a series connection in the input of a lownoise receiver. Which one should be placed as the first stage in order to obtain the best possible noise performance of the receiver?

Let us first calculate the corresponding noise factors: F1 = 1 + T1 /T0 = 1 + 100/290 = 1.34, and F2 = 1 + 90/290 = 1.31. Gains of the amplifiers in absolute values are G1 = 20.0 and G2 = 5.0. Now we can calculate the noise measures: M1 = (F1 − 1)/(1 − 1/G1 ) = 0.34/0.95 = 0.36, and M2 = 0.31/0.8 = 0.39. Therefore, LNA1 should be placed as the first stage.

Besides the useful signal, an antenna also receives noise power from its surroundings. The antenna noise temperature TA is defined as the temperature of such a matched resistive termination, which provides the same noise power as the noise power available from the antenna terminals, which is equal to the noise power received by the antenna in case of a lossless antenna. In the following we assume a lossless antenna. 

Thus, TA = T, and the received noise power is independent of the antenna gain and is directly proportional to the temperature of the black surface and to the bandwidth. If the temperature of the black surface depends on the direction within the radiation pattern of the antenna, the received noise power is calculated by integrating from (11.25). The antenna receives noise from everywhere, including from space and the atmosphere. For these it is possible to define an equivalent black surface temperature, which depends on frequency and direction, as shown in Figure 11.8.

At frequencies that do not penetrate the ionosphere, that is, in the HF band and at lower frequencies, noise from electric discharge in the atmosphere (lightning) is dominant. The amount depends on the season and day, location, and frequency. Noise from space dominates at frequencies from 20 MHz to 1 GHz. The Milky Way produces RF noise, which is at its maximum in the plane of the Milky Way and decreases as the direction goes away from this plane. The Milky Way noise also decreases as frequency increases. 

At all frequencies there is a 3K cosmic background radiation, which has its origin in the Big Bang, that is, it is a remnant of the birth of the universe. Thermal noise due to the atmospheric attenuation is the dominating noise source above 1 GHz. It depends on the atmospheric humidity and elevation angle. The atmosphere can be considered as an attenuator at a physical temperature of about 270K. Noise due to human activity may be considerable, especially near densely populated areas. In the VHF band and at lower frequencies, noise from the spark plugs of cars and power lines may be stronger than that from nature.

Information to be transmitted in a radio system, such as voice or music, is first transformed to a low frequency, for example, an audio frequency, electric signal. This baseband signal cannot be directly transmitted through a radio channel, or at least that would be very inefficient. The signal is first fed into a modulator, which modulates some property (amplitude, frequency, phase) of a high-frequency carrier according to the baseband signal. The highfrequency signal obtained is then transmitted by a transmitting antenna. A receiving antenna receives the high-frequency signal and feeds it into a receiver. In the receiver the signal is often downconverted to an intermediate frequency and then demodulated, that is, the original baseband signal is detected; for example, in the case of voice radio, the original voice signal is recovered. In other words, with a modulator the information is attached into a carrier, and with a demodulator it is detached.

There are a number of different modulation schemes, which can be divided into analog and digital methods. Modulation is important not only in communication (radio broadcasting, radio links, mobile phone systems) but also in radar, radionavigation, and so on. Modulation is treated in many communication textbooks, for example, [8–10].