For all RF communication devices a make a short referral to it's technical principle.
Devices used to convert radio frequency signals back to audible information are called (radio) receivers.
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.
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.
In Tuned Radio Frequency Receiver, the radio frequency signal coming from the antenna is selected in one or several tuned RF stages and is fed directly to the demodulator. IN most sets, the audio frequency signal after demodulation is fed to an audio amplifier to make it audible not only in headphones but also in a loudspeaker system.
THe most simple form of a TRF receiver is a „crstal set“ or a „crystal detector receiver“ in which a signal is selected by a resonant stage consisting of a coil and a variable capacitor an then demodulated by a crystal detector which acts as rectifier.
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.
A special kind of a TRF receiver is a regenerative receiver („Audio“ in german literature), in which a regenerative stage oscillating on a desired frequency is used for amplification and demodulation of the signal. The regeneration control has to used very carefully, as with to much regeneration, the set starts to oscillate and to radiate on radio frequencies.
In the Superhet oder Superheterodyne receiver, also known as single conversion receiver, the RF signal coming from the antenna is mixed with the signal of an oscillator in a mixer stage to generate an „intermediate frequency“ on a fixed frequency higher or lower then the original RF signal. As this frequency is fixed, filters and amplifier stages for this single frequency or easier to design without the risk of distortion or unwanted signals.
The intermediate frequency usually is amplified in several IF amplifier stages and finally fed to the demodulator, which generates the audible signal.
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 „mirror signals“ are appearing near the wanted signal on the dial. So crystal or mechanical IF filters have to be used for good selectivity.
To prevent „mirror signals“ or „ghost stations“ appearing on the dial just 900 kHz away (twice the standard IF for domestic radio receivers), high intermediate frequencies (higher then the upper end of the shortwave bands) habe been used, e.g.IF around 40 - 50 MHz. To prevent the necessity of the development of expensive filters on high IF, often a double conversion design is used: a first step, the signal is mixed to a high first IF and in a second mixer stage converted to a low second intermediate frequency, for which cheaper and standard IF filters can be used.
A typical intermediate frequency used in broadcasting receivers is 455 kHz.
Since for the choice of the intermediate frequency, always a compromise between mirror frequency rejection (preventing strong signals of appearing as „ghost stations“ a second time on the dial) and selectivity must be made, quite often in high-quality receivers, a double superheterodyne or double conversion design is used.
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, which determines receiver selectivity, can now be done with standard filters which are easier and cheaper to be constructed for standard IF frequencies.
The high first intermediate frequency protects against „mirror signals“ that can appear in a distance of twice the IF on the dial - in our example of an IF of 5 MHz in a distance of 10 MHz, these can easily be filtered out: the high first IF provides good rejection of „mirror signals“.
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.
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's frequency. As a result, an audible frequency is generated in the headphones. This oscillator used for telegraphy reception is called a BFO (Beat Frequency Oscillator).
Usually, a BFO pitch control cused to regulate the beat note of the BFO can be found next to the BFO switch.
In single-sideband (SSB) transmissions, only one sideband of a standard AM signal (consisting of two identical symmetrical sidebands and the carrier signal in between) is radiated, so that transmitters can be constructed which more efficiently use the output power from the final output stage (no energy is used to transmit a carrier signal which does not contain any information).
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't sound grumbling or squeeking like „Mickey Mouse“.
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).
With high quality narrow band single sideband IF filters, a signal suffering from adjacent channel interference can be made intelligible.
Frequency modulation, transmissions in which not the amplitude of the sidebands carry the modulated voice or sound information, but the transmission frequency itself through slight variations, were used in military communications and for broadcasting not before the end of World War II. In the VHF frequency range, wider channel spacing is possible, the FM broadcasts are less subjected to atmospheric disturbances and the speech intelligibility is much better.
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.
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.