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Radio receivers

Radio receiver is a device that is capable of using an antenna to capture high-frequency modulated wave, the information contained therein removed and converted into useful signal. There are two basic properties which determine the quality of the receiver. They are the sensitivity and selectivity of the receiver.

Sensitivity

Sensitivity expresses the lowest input voltage high frequency modulation captured with a normal antenna, which causes the receiver output normal test power of 50 mW when all controllers ordered at the highest sensitivity. The sensitivity depends on the construction, it is expressed in mV. Under normal amplitude modulation is considered the modulation depth m = 30% and modulation frequency fm = 1000 Hz.

Selectivity

Selectivity gives the receiver the ability to choose from a whole spectrum frequency band, which occupies one transmitter and suppress other signals.

Types of radio receivers

The basic division of radio receivers results from a method of amplifying the received signal. So we divide the receiver without amplification, the receiver with direct amplification and finally receiver with indirect amplification. It should be noted that the first two mentioned types of receivers are definitely outdated and they are not used in any case.

Indirect amplification

Receiver indirect amplification is characterized in that the received signal is translated into the mixer in the frequency band around the intermediate frequency and the adjusted signal is amplified in the IF an amplifier which is precisely tuned to an intermediate frequency. Another approach with receiver with direct amplification is analogous. Receivers of this type are also called superheterodyne.

Benefits superheterodyne:

  • Thanks MF amplifier has excellent selectivity
  • It can be re-tuning in a large bandwidth
  • The Automatic Adjustment Sensitivity

Disadvantages superheterodyne:

  • In the amplifier leads to undesirable combinations of components f
  • There is a so-called mirror frequency that we must suppress (signal fO + FMF)
  • needs sufficient selectivity
  • At high signal is difficult to suppress → FMF is chosen as the highest
  • To increase the selectivity - high freq band-pass filters
  • Or up-converter - FMF is above the highest frequency.

For the input circuit is the quality and FM radio receivers connected amplifier. It is responsible for improving the sensitivity and selectivity of the receiver. Do frequency of about 30 MHz, the involvement, the FM receiver is most commonly used wiring SB. The reception quality is the range VHF amplifier connected in cascade (SE + SB). This connection is characterized by good stability, high gain and very good noise characteristics.

Mixer

Mixer is responsible for mixed signal from the antenna with a frequency fa and the oscillator frequency fosc, create their frequency difference thus creating the so-called intermediate frequency FMF. The amplitude modulated intermediate frequency signals is elected FMF = 460-470 kHz, the frequency mode-lated signals FMF = 10,7 MHz, the accompanying sound for television broadcasting, which is modulated frequency is our elected FMF = 6,5 MHz, in western and southern Europe it is 5.5 MHz.

Mixers distinguish between multiplicative and additive. Multiplicative mixer uses for its function active element with two control electrodes, eg vacuum tube with two control grids or FET transistor with two gates. The disadvantage of tubes is their great noise, their use for VHF is excluded multiplicative mixer for FM can be realized only with FET transistor with two gates. The advantage of multiplicative mixers is the fact that both input frequencies affect each other. The additive mixer to bring the two signals to one control electrode active element. This has the disadvantage of interaction signal. This can be prevented so that the signal from the antenna is in the implementa-tion to the base, while the signal from the oscillator is fed to the emitter of the transistor. The advantage is simplicity additive mixer involvement and its easy implementation. For a range of DV, SV and KV is an additive mixer common emitter configuration, the range VHF is connected to a common base. Additive mixer can also be realized as self oscillating in which one transistor acts as the mixer and local oscillator at the same time.

Multiplicative mixer

The received signal fV, and the signal from the oscillator fO are connected to two control electrodes and the output is proportional to the product of two voltages. Additionally, it contains ingredients fO and fO + fV - fV. As an amplifying device is a FET transistor.

  • The long, medium and short waves
  • High noise figure, gives mixer slope

Additive mixer

Both signal processing Fo, fV are fed to one electrode, the resulting current is proportional to the sum of the two signals. For a sum-and difference frequency signal must bring an element with nonlinear haracteristic.

  • Greater mixing slope (gain even more)
  • Lower noise figure
  • FM
  • disadvantage of insufficient segregation of VST → oscillator circuit and the possibility of radiation fO antenna

Amplifier

There is usually connected amplifier after the input circuit in quality FM radio receivers. It is responsible for improving the sensitivity and selectivity of the receiver. To frequency of about 30 MHz they are connected as SE, for higher (VHF) frequencies they is most commonly used SB wiring. At quality reception the range VHF amplifier is usually connected in cascade (SE + SB). This connection is characterized by good stability, high gain and very good noise characteristics.

If amplifier is a high-frequency amplifier, a classic design is multistage amplifier with bound resonant circuits. Today we meet by making an integrated circuit. At this amplifier is concentrated all amplification and it determines the resulting sensitivity and selectivity of the receiver. If the receiver is designed to receive, for example, SV and FM, it is necessary to duplicate intermediate amplifier frequency.

  • Amplifier tuned to a fixed frequency
  • Has the largest share of the profits and selectivity of the receiver
  • Mostly involvement, use transform. ties
  • Due to the selectivity of the 1st stage circuits using concentrated selectivity
  • MF amplifier gain controlled signal

Automatic adjustment of sensitivity

  • due to gain control
  • to avoid differences between close and distant transmitter

Detector

The role of the detector is to take carried low frequency information from high frequency wave and remove remaining modulated high frequency remains. DC component of the output signal of the detector is often redirected back to their amplification stages, where it regulates the amplification according to its size. This process is called automatic alignment sensitivity - AVC.

Low-frequency information is amplified in a classic audio amplifier and reproduces the relevant acoustic converter.

Regarding the choice of intermediate frequency, there can't be any more powerful transmitter, since then the signal would went down to the detector and it would cause unwanted interference with signal.

Intermediate frequency acts to suppress the mirror frequency. Mirror frequency is higher than the frequency of the oscillator, which is the receiver currently tuned to. Mirror frequency is dangerous, because if it combines with the input mixer with frequency oscillator, than it creates a folder that has also IF frequency, so that passes through the detector and cause interference. Suppress mirror frequency is only possible in RF amplifier before the mixer. The need to suppress mirror frequency suppression occurs on the extent of HF.

Antenna

Antenna brings energy to the input circuit of the receiver. There are these basic types:

  • Wired - for long and medium wave
  • Ferrite
  • frame

For FM

  • YAGI
    • About 75 - 300Ω
    • May not be around metal parts

Parabolic antenna

  • Circular - very directional
  • Offset - converter from below

Transmitting antenna is essentially an open RO circuit or a part of VF line. If the load is on short, energy is reflected and the wire arises stagnant current. Maximum current and zero voltage is at distance of λ/4 from the end of the line.

Technology


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