en:principle
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+ | ====== Principle ====== | ||
+ | For all RF communication devices a make a short referral to it's technical principle. | ||
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+ | ===== Receiver ===== | ||
+ | Devices used to convert radio frequency signals back to audible information are called (radio) receivers. | ||
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+ | After the introduction of the earliest wireless communication sets in the years of World War I, many signals of different transdmitters appeared at the same time in the headphones of a radio operator, so the need of a receiving equipment that is capable of separating signals coming from different transmitters active on different wavelengths was evident. | ||
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+ | Usually, the radio frequency signals are amplified and then a small portion of the radio frequency spectrum is "cut out" and feed to a demodulation circuitry, which converts the RF signal to an audio frequency signal. | ||
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+ | ==== TRF ==== | ||
+ | {{: | ||
+ | In **[[wpen> | ||
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+ | THe most simple form of a TRF receiver is a " | ||
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+ | In more complex sets, one or more radio frequency (RF) amplifier stages or tuned RF stages are added, these can be tuned by means of a ganged variable tuning capacitor or manually, in this case, in every stage a variable capacitor has to be tuned individually to the desired frequency. | ||
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+ | A special kind of a TRF receiver is a regenerative receiver (" | ||
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+ | ==== Superhet ==== | ||
+ | {{: | ||
+ | In the **[[wpen> | ||
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+ | The intermediate frequency usually is amplified in several IF amplifier stages and finally fed to the demodulator, | ||
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+ | The engineers choose the intermediate frequency depending on the frequency range of the receiver and the selection of IF filters. In a VLF or longwave receiver a low intermediate frequency is more efficient, as cheaper LC (coil - capacitor) filters can be used with good results and the risk for mirrors (the tuned signal can be heard a second time on the dial with a distance of the double of the intermediate frequency) is small. On shortwaves, this is more a problem, as coil filters offer not such a good selectivity and disturbing " | ||
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+ | To prevent " | ||
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+ | A typical intermediate frequency used in broadcasting receivers is 455 kHz. | ||
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+ | ==== Double Conversion ==== | ||
+ | {{: | ||
+ | Since for the choice of the intermediate frequency, always a compromise between mirror frequency rejection (preventing strong signals of appearing as "ghost stations" | ||
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+ | In a double conversion receiver, the antenna signal is mixed with a much higher oscillator signal, so that a first high intermediate frequency, for exemple at 5 MHz, is generated. This IF has to pass through a bandpass filter, then it's amplified and converted in a second mixer stage to generate the second intermediate frequency. The final filtering and amplification, | ||
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+ | The high first intermediate frequency protects against " | ||
+ | On the second lower intermediate frequency, high quality IF filters can be constructed with reasonable effort and less IF amplification is necessary: a low second IF results in an easier achievable good selectivity. | ||
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+ | ==== CW Reception ==== | ||
+ | {{: | ||
+ | Morse code signals, which are generated by simply switching on and off a unmobulated radio frequency signal (A1) cannot be heard in the headphones or only a slight noise can be perceived. To make these Morse code signals audible, an oscillator signal must be mixed to the antenna signal, which differs by a small amount from the antenna signal' | ||
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+ | Usually, a BFO pitch control cused to regulate the beat note of the BFO can be found next to the BFO switch. | ||
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+ | In single-sideband (SSB) transmissions, | ||
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+ | To demodulate such a signal, a subcarrier instead of the carrier signal from the transmitter must be added in the receiver. For this, the BFO is used in cheaper receivers for simple SSB reception. With the BFO pitch control, the difference between the subcarrier signal an the received single sideband signal must reduced to zero as much as possible so that the voice of the speaker sounds natural, and doesn' | ||
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+ | ==== SSB Reception ==== | ||
+ | {{: | ||
+ | High end receivers use a more sophisticated but more expensive approach: | ||
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+ | The IF signal of a single sideband, which is not intellegible in AM mode, is mixed with two oscillator signals with a fixed spacing higher or lower then the IF. Then specific asymmetric IF filters are used to select the upper sideband (**USB**) or lower sideband (**LSB**). | ||
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+ | With high quality narrow band single sideband IF filters, a signal suffering from adjacent channel interference can be made intelligible. | ||
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+ | ==== FM Reception ==== | ||
+ | {{: | ||
+ | Frequency modulation, transmissions in which not the amplitude of the sidebands carry the modulated voice or sound information, | ||
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+ | For FM reception, usually a superheterodyne receiver is used: the antenna signal is converted to an intermediate frequency of typically 10.7 MHz in FM receivers. | ||
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+ | The signal is then amplified in the IF amplifier stages. Rests of amplitude modulation are eliminated by amplifying the signal and then clipping it in the limiter stage. The demodulation takes place in the discriminator stage, a special demodulator circuit. | ||
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+ | ===== Transmitter ===== | ||
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+ | ==== Further Information ==== | ||
en/principle.txt · Zuletzt geändert: 2018/10/29 19:17 von 127.0.0.1