Sensitivity of the receiving device

One of the most important indicators of the quality of the reception path is the sensitivity of the receiver. It characterizes the receiver's ability to receive weak signals. Receiver sensitivity is defined as the minimum input level of a device necessary to ensure the required quality of the information received. Quality can be estimated by a given bit error probability (BER), error message reception probability (MER), or signal-to-noise ratio (SNR) at the input of the receiver demodulator. If the sensitivity of the receiver is limited by internal noise, then it can be estimated by the real or ultimate sensitivity of the receiver, noise figure or noise temperature.

The sensitivity of the receiver with a small gain, the output of which is almost no noise, is determined by the emf, (or nominal power) of the signal in the antenna (or its equivalent), which provides a predetermined voltage (power) of the signal at the output of the receiver.

The sensitivity of the receiver is determined by its gain KUS. The receiver must provide amplification of even the weakest input signals to the output level necessary for the normal functioning of the device, however, at the input of the receiver there are interference and noise, which are also amplified in the receiver and can degrade the quality of its operation. In addition, the amplified internal noises appear at the receiver output. The less internal noise, the better the quality of the receiver, the higher the sensitivity of the receiver.

The real sensitivity of the receiver is equal to the emf. (or nominal power) of the signal in the antenna at which the voltage (power) of the signal at the receiver output exceeds the voltage (power) of interference by a specified number of times. The maximum sensitivity of the receiver is equal to the emf. or the nominal power of the RAP signal in the antenna, at which at the output of its linear part (i.e., at the detector input), the signal power is equal to the internal noise power.

When setting the receiver sensitivity in the form of emf, it is measured in microvolts. Modern mobile communication receivers have a sensitivity of a few tenths of a microvolt. The way to set the receiver sensitivity in the form of emf leads to the fact that at different input impedance of the receiver, we will receive a different value of emf. Therefore, despite the fact that all modern receivers of mobile communication systems have an input impedance of 50 Ohms, the sensitivity of receivers is set in terms of the signal power at the input of the receiver. Sensitivity is defined as the ratio of the power at the receiver input to the power level of 1 mW and is expressed in a logarithmic scale in dBm.

(dBm)

The maximum sensitivity of the receiver can also be characterized by a noise factor of N 0 equal to the ratio of the noise power generated at the output of the linear part of the receiver by the equivalent of the antenna (at room temperature T 0 = 290 K) and the linear part to the noise power generated only by the equivalent of the antenna. Obviously

, (one)

where k = 1.38 • 10–23 J / deg - the Boltzmann constant;
Pr - noise band of the linear part of the receiver, Hz;
R AP - signal power, watts.

From (1) it can be seen that the signal power corresponding to its ultimate sensitivity and referred to the unit of the frequency band can be expressed in units of kT 0:

, (2)

The maximum sensitivity of the receiver can also be characterized by the noise temperature of the receiver T pr, to which the antenna equivalent should be additionally heated so that at the output of the linear part of the receiver the power of the noise generated by it is equal to the noise power of the linear part. Obviously from where

(3)

The real antenna is affected by external noise, the rated power of which ,
where TA is the noise temperature of the antenna. Therefore, at the output of the linear part

To obtain equal signal and noise power, power is needed.

The sensitivity of the receiver characterizes its ability to receive weak signals.

Quantitatively, sensitivity is evaluated in two ways:

1) Minimum EMF modulated signal equivalent of receiving antenna or minimum field strength . Such a case is characteristic of LF-VHF receivers operating with an open antenna. used to assess the sensitivity when using magnetic and whip antennas;

2) Minimum signal strength at the receiver input. This case is typical mainly for UHF and microwave receivers.

As an antenna equivalent, two-pole networks are used with averaged parameters close to the probable parameters of a real antenna. Figure a) shows the equivalent of an open antenna of broadcasting LF-HF receivers, where R1 = 50 Ohm; R2 = 320 Ohm; C1 = 125 pF; C2 = 400 pF; L = 20 μH.

At LF - MF, because of the smallness of the reactance XL, a simplified scheme can be used (Fig. B), and at HF, due to the large XL and small and the equivalent contains only the resistance R0 = R1 + R2 (Fig. c). The antenna equivalent in the form of a half-wave vibrator is a resistor of 75 Ohm.

Figure 3.1.

Sensitivity is distinguished: limited by gain, real and threshold sensitivity of the receiver.

a) Sensitivity limited by gain.

Sensitivity limited by gain is characteristic of receivers with relatively low gain, receiving strong signals, i.e. in conditions where interference has little effect on reception. It is determined at a given power at the output of the receiver.

For analog signal receivers (for example, sound broadcasting), there is a distinction between nominal and normal output power. Rated power pc. there is the greatest power corresponding to the 100% modulation depth of the input signal with nonlinear distortions not higher than the specified norm. The normal power Pcp corresponds to a 30% modulation depth of the input signal and is 10% of Pcc.

Let us determine the sensitivity of the receiver, limited by the gain connected to the equivalent of an open antenna when receiving AM signals.

Receiver gain

where m is the modulation factor of the signals;

- the effective carrier voltage of the signal in the equivalent of the antenna;

- voltage signal at the output of the receiver.

Denote by voltage required to create a voltage output at the receiver .

Then

(*)

Consequently, the sensitivity, limited by the gain, increases with increasing K ( decreases).

When it comes to evaluating sensitivity, it is often under understand the voltage at the detector input then

-

where K is the gain of the RF path of the receiver located before the detector.

b) Real and threshold sensitivity.

The real sensitivity of the receiver takes into account the effect of its own noise and is determined by the minimum signal level at the input given the output level of the signal and its excess over noise.

The threshold sensitivity is determined when the signal-to-noise ratio at the output = 1.

To determine the real sensitivity It is necessary to determine how the " K " effect on the noise level at the output ( ). To do this, a real noisy receiver connected to a noisy equivalent of an antenna will be replaced by a noiseless receiver with a generator of intrinsic noise. given to its input, which, together with the noise generator equivalent of the antenna forms a generator of total noise voltage reduced to equivalent antenna with effective receiver bandwidth

Figure 3.2.

If a , then taking into account (*)

With a given the equivalent of the antenna is necessary to ensure excess signal . Hence the real sensitivity. . (+)

Figure 3.3.

The figure shows dependencies (*) and (+). The point O of their intersection corresponds to the critical gain coefficient Kkr.

When K gain is small, , but and real sensitivity limited by gain, i.e. matches with .

When K> CRC and if then that does not meet the definition of the concept of real sensitivity. To ensure equality need to increase up to value .

This means that the real sensitivity does not depend on K and is determined by the receiver’s own noise. With the increase of K> Kkr grows like , but persists constant.

Receiver sensitivity depends on tuning frequency. In this case, the nominal real sensitivity corresponds to the highest value .

For broadcast receivers, depending on the quality class, the values ​​of the nominal real sensitivity in the LF and MF range lie within the range of 50 ... 300 μV; in the HF range - 50 ... 200 µV; in the VHF and UHF bands - 3 ... 30 µV; for professional auditory receivers of telegraph signals of the HF range - up to 0.1 µV; for TV receivers - 200 ... 500 µV.