Operating a modern shortwave receiver III

 
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überarbeitet am 25.9.2007

The signal breaking in on the frequency of 6 0 7 5 kHz of the European service of the German external service "Deutsche Welle" can sound nearly as nice, as the first channel of Your nationwide FM service, but it could also be hardly identifiable because of very much hiss and squeeking noise. In these cases probably the reception mode is not set correctly.

  • AM (amplitude modulation): a carrier signal and the audio signals in both sidebands are usually radiated by broadcasting stations, the tuning is quite uncritical.
  • USB/LSB (Upper Side Band / Lower Side Band): Amateur radio and commercial speech communications arte radiated usually in single-sideband mode (SSB).
    In this mode, only one sideband with audio information is radiated, the radiated high frequency power is much more economically used. The carrier signal mixed together with the received sideband in the receiver. As the reception does not depend on the signal strength of the carrier signal, very weak stations can still be made audible.
    In these cases, You will need very fine tuning steps to make the voices or music intelligible; if the carrier is inserted a few Hertz too high or too low, the speaker gets a bassy or Mickey Mouse voice and the music sounds completely strange. Too large tuning increments will make it impossible to understand the spoken word, You will hear the so-called musical scale effect when You tune the receiver across a station's signal.
  • CW: "Dot-and-dash signals" used for transmitting messages in morse code. In this(so called A1) mode, only the unmodulated carrier signal is switched on and off. Only by the means of adding of a carrier signal in the receiver You get audible morse code out of Your speaker. The pitch of the tone is changed with the fine tuning capabilities of the receiver, many sets have a knob to change the pitch of the BFO (beat frequency oscillator).
    Most receivers have a special position of the mode switch for the receiption of CW - signals, whilst in others, SSB must be selected.
  • FM: the frequency modulation is used in the highes ranges of the short wave spectrum by CB hobbyists, and ist the typical mode in VHF and UHF communications.
    Professional communication receivers usually offer FM mode only in the shortwave band, some travel world band receivers also cover the frequency range of 87,5 - 108 MHz, the receiver can be used thereby as "kitchen radio".
  • RTTY/FAX: certain receivers offer these modes of SSB operation with special IF filter bandwidths frequency offset to operate of radio teletype or FAX converter. In other receivers providing single sideband reception, You can chose the SSB mode for RTTY reception in conjunction with a suitable RTTY decoder.
8 The operation modes switched in different ways. Sometimes, a rotary switch is used, sometimes You might find push buttons for AM, for USB, LSB, CW and FM. Usually You find a hint to the activated repeption mode by light emitting diodes or letters.
In some receivers, certain operation modes are coupled to certain IF filters to facilitate the operation for untrained listeners; independently adjustable reception modes and IF bandwidths are however preferred.
9 The key SYNC or ECSS activates the synchroneous detector. In this mode, only the sideband information is used and an internal carrier signal added for demodulation. In contrast to manual tuning of a single sideband signal until the interference beat note disappears by careful fine tuning, in this mode the internal carrier is added automatically in the correct position and allows automatic demodulation of the sideband signal.
The demodulation doesn' have to rely on a carrier signal deriving from the transmitter, that is dependent on fading with varying signal strength and propagation conditions, but a stable int the receiver internally generated carrier replaces the external one.
While switching on this circuit, You will usually perceive a short whistling until You will heard an intelligible demodulated audio signal.
10 Intermediate frequency (IF) or bandwidth filters can be selected out of a choice of two ore more filters with a NARROW / WIDE - switch or rotary switch, to change the IF bandwidths. After an incoming signal is mixed to an intermediate frequency, it has to pass a more or less narrow filter that will only allow frequencies very near the intermediate frequency to pass through, this will make up the selectivity of a receiver, the capability to select one out of a few signals on very nearby frequencies in a shortwave band.
Using a narrower IF-filter allows to fade out unwanted signals from interfering signals on a frequency nearby more effective. But with using a narrow IF bandwidth / filter, audio becomes bassy or dull.
A second important technical parameter of an IF filter is it's skirt selectivity, with very steep shoulders or "skirts", interfering signals can be cut off easier, wile the information - bearing parts of the audio signal still can pass.
For standard broadcast, i.e. AM - reception a IF filter with a bandwidth of 4 - 6 kHz will do a fine job, for SSB reception, You need a 1.8 - 3 kHz, that will be broad enough to let the single sideband pass through. For CW - reception of Morse code signal, sometimes filters with a bandwidth of 250 - 500 Hz are provided, these allow to virtually cut out the signals from one Station of a mix of several interfering transmitters. For audio transmissions, these filters are not suited, You won't understand the spoken word nor music.
Some sets like the NRD-535 provide a variable bandwidth (10a), this technology is nowadays replaced by digital processing of the HF signal, with a virtually unlimited number of IF filter bandwidths using DSP technology.

After tweaking all these controls, the "Deutsche Welle" should be audible in good quality, like a nearby medium wave transmitter.

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