Bandwidth of a CW signal
The way in which information can be passed on to an RF signal can be very different. In the early days of radio telegraphic communication, Morse code was transmitted by simply pressing a signal on and off, but radio telephony was used as early as the 1930s. Spoken information and sounds were modulated onto a carrier signal.
in the Channel can with more modern devices the Modulation type can be selected in receiver must accordingly the Operating mode properly chosen to demodulate the signal.
Under Continuous wave or CW the operating mode is selected in which an unmodulated carrier is broadcast at the transmission frequency in the rhythm of the Morse code; this operating mode is sometimes called Telegraph soundless designated.
The transmission technology is through simple keying resp. Interruption of the transmission signal is very easy, but for reception the signal has to be mixed with a telegraphic local oscillator and thus made audible. In a simple AM radio receiver, at most a noise in the rhythm of the Morse code is audible.
With the switch Telegraph overlayer or BFO If the signal of the auxiliary oscillator is switched on, a controller usually allows the frequency of the BFO and thus the pitch of the Morse code in the headphones to be adjusted.
As a temporary measure, CW signals can also be made audible with an audio receiver, which is made to vibrate with excessively tightened feedback.
As Telegraph sounding is an operating mode in which a carrier that is modulated with an audio frequency (in the early days of transmitter technology, the frequency of the voltage generator was used for this purpose) is transmitted to the rhythm of Morse code.
In contrast to the unmodulated carrier of a CW transmitter, the signal in A2 can be made audible by a normal AM receiver, even with a simple detector receiver, which made it possible to build very simple receivers in the early days of radio telegraphy.
For the radio reception on medium and long wave and also in the telephony of the early military radios, the Amplitude modulation used, which can be received with simple receivers from detector receivers to complex superhetes without the need for a telegraph overlay. The modern name for this type of modulation is A3E.
The width of the envelope curve of the transmission signal is modulated by the modulated audio frequency; the Degree of modulation is calculated from (a-b) / (a + b) * 100%. A simple detector, respectively. a tube or semiconductor diode is sufficient for demodulation.
The energy and bandwidth requirements are extremely high with amplitude modulation: A large part of the transmission energy is in the carrier signal, but with which no information is transmitted, due to the identical audio information in both sidebands, the signal is broader in the frequency spectrum, it covers more than twice the width the uppermost transmitted audio frequency, ie a signal with good transmission quality requires more bandwidth, respectively. Channel spacing, a narrow-band signal becomes dull and difficult to understand.
Amplitude-modulated signals are susceptible to propagation interference (fading), crackling and crackling caused by static discharges and flying sparks on generators, motors, etc.
The contemporary designation of the type of modulation Single sideband or Single sideband would actually be J3E.
For more efficient use of the transmission energy and bandwidth, the Single sideband modulation used: Here only a sideband is transmitted, in which the modulated audio frequency information is contained, the transmission of a carrier and the identical second sideband (with actually redundant information) is dispensed with. The entire transmission power of the transmission tube can thus be put into the information transmission.
By not broadcasting the second sideband, bandwidth in the frequency spectrum can also be saved.
The shortwave station SE-222 from Zellweger, introduced in 1959, was a leading technology with the consistent use of single sideband modulation at the time of its introduction. SSB signals are less prone to selective fading and crackling interference from sparks.
In a broadcast in Single sideband modulation If the upper or lower sideband can be transmitted, if the wrong setting is selected on the receiver, the transmission is incomprehensible.
In military and commercial communication, only the upper sideband (USB) used in amateur radio on frequencies below 10 MHz in LSB and at frequencies above 10 MHz in USB Posted,
For reception, an auxiliary carrier must be mixed in in the receiver instead of the carrier signal. In high-quality receivers, an oscillator with a fixed frequency difference to the intermediate frequency is used; in simpler devices, the BFO (telegraphic overlay) is used for this. In order to optimize speech intelligibility, the mixed carrier signal must be at the correct distance from the received single sideband signal, otherwise the frequencies are too low or too high and the speaker's voice squeaks like Mickey Mouse. Fine tuning is therefore a prerequisite for single sideband reception.
A special form of play is the transmission of a signal with different audio information in both sidebands of a signal (usually with a suppressed carrier), known as ISB, "Independent side band". This form of modulation was used on shortwave feed routes from international shortwave broadcasting services.
The Frequency modulation, modern name F3E was only used with the American devices working in the VHF area towards the end of the Second World War.
On the higher frequency ranges above 30 MHz, respectively. the upper end of the shortwave range plays the signal bandwidth, respectively. the channel spacing plays a subordinate role. FM signals have a significantly better audio quality and intelligibility, it is easier to work with a squelch, and fading is no longer relevant as a transmission problem.
In the case of a frequency-modulated transmitter, the transmission frequency must fluctuate within a narrow range in time with the modulation.In the receiver, demodulation requires the use of a limiter (to eliminate amplitude-modulated signal components that convey interference) and a discriminator; the circuit complexity is significantly higher.
In contrast to VHF broadcasting, which works with a channel spacing of 100 kHz (sometimes 50 kHz), in military communication one works with a channel spacing of 50 kHz, which in new generations of devices is reduced to 25 kHz or even 12.5 kHz could be. With the frequency response resp. the voice transmission quality must, however, with the so called NBFM (Narrow Band FM) compromises are made.
The FSK procedure ("Frequency Shift Keying", former name) is usually used for teletype operation F1, modern F1B), in the simplest modulation method the transmission frequency for the two states “Mark” and “Space” (corresponding to 0 and 1) is shifted by a certain amount (“Shift”, can usually be preselected on the radio teletype demodulator).
Alternatively, a single sideband signal can be modulated with an audio signal with two different pitches in order to transmit the teletype signals. This operating mode is known as "audio frequency shift keying".
A similar operating mode is the transmission of image data in a frequency-modulated signal as a "facsimile"; F4 resp. modern as F * C designated.
A completely different telex method is that named after Rudolf Hell Bright-Procedure. With the facsimile method, all letters are converted into a 7 × 7 dot matrix and these pulses are broadcast as audio signals with an AM transmitter.
On the receiver side, the Hell signals are fed to the Hell writer, where a writing screw electromagnetically presses a color ribbon onto a strip of paper and thus prints the text as plain text on a telegram strip.
In Switzerland, Hell-Schreiber commercially available from Siemens were on the major stations in the corps resp. Army frames in use, for transmission to the troops as with the German Feld-Hellschreiber, the Hell procedure was never used in Switzerland.
The names for the different modes of operation and modulation have changed over the years (decades), new ones have been added, the nomenclature has become more and more confusing ...
The designations can be found in different forms on the front panels of the device, depending on whether the English or German spelling of the abbreviations was used, etc.
The one valid today ITU nomenclature was introduced in 1982 and consists of a nine-digit BBBBMSIDX designation, from which some components are usually left out. It stands BBBB for the bandwidth of the signal, M. for the type of modulation, S. for the type of signal (e.g. 3 = analog), I. for the type of information (e.g. E = telephony), D for detailing (optional), X for multiplexing (optional).
Usually only the short form MSI, i.e. a three-digit designation, is used.
|CW||Telegraph soundless||A1||A1A||Morse code with unmodulated carrier||Amateur radio|
|MCW||Telegraph sounding||A2||A2A||Morse code with sound-modulated carrier||Aeronautical radio beacons|
|AT THE||Telephony||A3||A3E||Telephony, amplitude modulated||Broadcast LW, MW, KW; Aeronautical radio|
|SSB (LSB / USB)||SSB (single sideband)||A3J (A3A)||J3E||Telephony single sideband, suppressed carrier||Amateur radio, commercial radio services|
|SSB (LSB / USB)||SSB (single sideband)||A3J||R3E||Telephony single sideband, reduced carrier||Shortwave broadcasting (experimental)|
|FSK||-||F1||F1B||Radio teletype with keyed carrier (frequency shift keying)||Radio telex|
|FM||Telephony FM||F3||F3E||FM telephony||Radio FM, military often NBFM (narrow band - FM)|
|FAX||facsimile||F4||F1C (F3C)||FM facsimile (analog facsimile)||Weather maps KW facsimile|
|ISB||Independent side band||B8||B8E||Independent side band||Shortwave playback (feeder) (obsolete)|
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