Ham Radio – How Does the Digital Voice Access Point Work?


The D-VAP used to connect a handheld transceiver to the International d-star network uses G.M.S.K. technology, it stands for Gaussian minimum shift keying, or to give the technology its correct explanation Gaussian minimum filtered minimum shift keying. G.M.S.K. is a form of modulation, modulation means to change, and in this case we adapt the carrier wave in the signal by adding digital tones to alter the carrier wave. Many communication systems use G.M.S.K. it has an advantage over analogue systems G.M.S.K. is a more efficient use of the radio spectrum. A problem occurs when using other forms of phase shift keying, side-bands extend outward from the main carrier wave; they can cause interference to other nearby communication channel systems.

G.M.S.K. is an efficient modulation which conserves the radio spectrum bandwidth. The most widely used communication system employing the system is cellular radio or G.S.M. which stands for global system of mobile, in other words the mobile telephone network, it is estimated that over three billion people use the G.M.S.K. technology contained inside the mobile phone.

Gaussian minimum shift keying is a derivative of M.S.K., which is actually a form of continuous-phase shift frequency keying, one of the problems with phase shift keying is that side-bands extend out from the carrier wave, this problem can be overcome using minimum phase keying and its derivative Gaussian minimum phase shift keying

Both M.S.K. and G.M.S.K. modulation technologies are classed as continuous phase schemes; there are no phase discontinuities because the frequency changes occur at the carrier zero crossing points. An interesting point to note the unique factor of M.S.K. is that he frequency difference between the logical one and logical zero states is always equal to half of the data rate, this is expressed as the modulation index and it is always equal to 0.5

The way the G.M.S.K. is modulated is to filter the modulated signal with a Gaussian filter; this is then applied to a frequency modulator where the modulation index is set to 0.5. This method is very simple but effective, however there are drawbacks, the modulation index must be exactly 0.5. 0.5 is difficult to achieve because the analogue signal is not suitable because of drift that occurs between the tolerances of components, thus making the index very difficult to set exactly.

You can use an external aerial on your D-VAP to increase the transmitted and receive range of the 10 mill watt transceiver which is contained within the D-VAP, however avoid an amplifier to increase transmitted range; the D-VAP was not designed as a stand-alone transceiver and thus is not as clean as a conventional transceiver.

Source by John Allsopp

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