PLL - Hardware decoders using Phase Locked Loop demodulatorsare not at all suitable to decoding HF keying systems and will not be discussedany further here, they are that bad!
 

'AM detectors' - Many demodulators (sometimes also called TuningUnits) use two matched filters and a Slicer. They are usually fitted witha limited selection of shifts, e.g. just 170 and 850 Hz. Another limitingfactor is that the low pass filter after the slicer is fixed at cuttingoff any keying components above 100 Baud. For signals that fall withinthe specification of these 'tuning units' they work very well, often ableto receive signals that are almost inaudible to the human ear! Howeverthey have been designed with a limited number of shifts and Baud speedspossible, which means that any other kinds of signal are undecodeable orat best, decoded below the systems normal performance capability. The equipmentis effectively limited to those signals for which it was specifically designedto decode. Some more expensive demodulators offer a larger choice of shiftsand post detection filters, but they are still limited in that the operatorcan only select those settings which have been provided by the manufacturer.For example, shifts in increments of 10Hz and either a 200, 400 or 600Baud low pass filter.
 

Periodic Convertors - The CODE3 interface (called theLF3) uses a simple but extremely effective means of demodulation called'Periodic Conversion'. With this process one converts the audio frequenciesbeing received into digital numbers rather like a frequency counter. Samplingat just a couple of thousand times a second is sufficient for nearly allkinds of FSK signals to be decoded. One is not limited to any particularshift (from about 20 to 1500 Hz) and the baud speed demodulated can beanywhere between approximately 5 and 480 Baud.
 

However, HF technology moves ever onward, and manufacturers of transmittingequipment are making more and more use of other modulation methods suchas, MFSK, 2DPSK and 4DPSK. For these keying systems a Periodic Convertoris not enough to extract the data being modulated. Enter the DSP CODE30with A-D convertor card.