Internal noise of radio reception and processing devices

Noise passive circuits.  

Let an arbitrary passive linear circuit formed by conductors with metallic conductivity be at temperature T. If we consider this circuit as a two-terminal circuit whose resistance

then between its ends the emf of thermal noise acts, the rms value of which

where r is the active part of the two-pole impedance;

        k = 1.38 10 ^-23 J / deg (J / K) is the Boltzmann constant;

        T is the absolute temperature of the circuit;

        P is the band within which the emf of noise is measured.

The equivalent noise scheme of such a two-pole device is shown in Fig. a) where - noise voltage generator. You can imagine another version of the equivalent two-pole noise scheme.

Figure 4.1.

Here, Y = G + jB is the total conductivity of the two-port network;

        - Noise generator

If the resistance of the two-pole is active (x = 0), then for circuit b)

Equivalent schemes a) and b) , as well as all the considered formulas can be used to determine the noise of passive two-terminal networks containing an arbitrary number of elements L , C , r . In this case, the values ​​of r and x or G and B in schemes a) and b) will be difficult to depend on frequency. Formulas for determining and while true within the band within which the change in r and G can be neglected .

As an example, let us turn to the noise of a parallel LC circuit , the loss resistance r _to which we will assume to be independent of frequency.

Figure 4.2.

Determine the noise voltage on the circuit in a narrow lane near the resonant frequency . In this case we proceed to the equivalent noise circuit of the parallel circuit. Considering narrow bandwidth , many times smaller than the bandwidth contour at the level of 0.707, we can neglect the detuning of the contour for the noise components lying inside the band . Then the voltage generated by these components will be Q times their emf ( Q is the quality factor of the circuit). Then

Where - the resonant resistance of the parallel circuit at points 1-2.

If you are interested in the noise of a parallel circuit, choosing a narrow strip frequency at an arbitrary frequency, it is necessary to consider the dependence on frequency. With increasing detuning value and therefore the voltage are decreasing.

Thus, the noise source of passive circuits is thermal noise of loss resistance.

Noise antenna.  

Thermal noise occurs in the antenna and in the ground; however, their role is usually insignificant, since The loss resistance in these devices is small. Much more important is the colossus ***, induced in the antenna by electromagnetic waves coming to the Earth from space, by thermal electromagnetic radiation from the atmosphere of the Earth and the Earth itself. The fluctuation nature of these disturbances allows us to consider them together.

Antenna noise characterizes antenna noise temperature which means temperature in which thermal noise resistance equal to the radiation resistance of the antenna appear to be the same as the actual noise of the antenna. This allows you to express the antenna emf noise according to the Nyquist formula

The higher the noise level in the antenna, the higher its noise temperature. Such an assessment of the noise level is convenient in that it is not possible to decide on the bandwidth P , which would be inevitable when using EMF or power. It is convenient to consider as the sum of the components

Where noise temperature of the sky, determined by the level of cosmic noise;

        - noise temperature of the atmosphere;

        - Earth's noise temperature.

Sources of intense electromagnetic radiation are stars and, above all, the nearest star to the Earth - the Sun. It is considered that the maximum corresponds to the equator of the galaxy, and the minimum - its poles. For frequencies of 30-120 MHz, the average value

On cm waves, cosmic noise is insignificant. The noise temperature of the atmosphere increases as the reception direction approaches the horizon. These noises are most significant at frequencies above 10 GHz. The noise temperature of the Earth is considered to be close to 250 K. All three components of the noise temperature of the antenna, thus, depend on the frequency range, the antenna pattern, the direction of reception.

Noise active elements of receivers.  

The noise of a pn junction is caused by four factors:

              1) the resistive resistance of the pn-junction layers is thermal noise;

              2) the uneven flow of charge carriers through the pn junction - shot noise;

              3) the unevenness of the process of carrier recombination in the distribution of current between the electrodes, for example, in the distribution of the emitter current between the collector and the base — current distribution noise;

             4) macroprocesses on the surfaces of the pn-junction layers, creating atrial (flicker) noise.

At shot noise spectral density uniform at frequencies lower than ~ 0.5H ^. 10 ¹¹ Hz. It drops at higher frequencies, because shot noise is caused by a random sequence of current pulses created by a charge carrier span, the time of which (pulse duration) is approximately 10 ^-11 s.

Thermal noise at frequencies less than 3H ^. 10 ¹² Hz.

Have atrial noise . Atrial noise affects frequencies below a dozen kHz and is not considered further in the analysis.