Threshold Improvement inanFM Subcarrier System* addition ofanideal limiter infront ofthis discriminator will innowayaffect theperformance ofthediscriminator, Tisthepurposofths trtod sse r s since an ideal limiter bydefinition doesnotaffeet the madeinarecent paper1 onFMAthresholdls which seem
J. J. Spilker · 1961
erroneous results, forinthissituation changes inS/N nator. Thebases forhiscalculations aretheworksoft 3 attheinput toa zero-crossing typeofdiscriminator Stumpers' anid Davenport. resulting fromtheinclusion ordeletion ofa limiter can Inhispaper, Martin states thathiscurves oflimiter < . ,, e~~~~~ause nochange intheoutput S N whatsoever. input vsdataoutputS-N forthezero-crossing type Secondly, theoutputS/Nofallnoncoherent FMdis- ofdiscriminator preceded bya limiter differ fromthe . . curesresntebyStuperinthatheefecoftecriminators (andAIMdetectors aswell) knowntothis curvespresdbreader varies inproportion toatleast thesecond power limiter ontheinput S/Nratio hasbeentakeninto ac- oftheinput S/Nforsmallinput S/N.(Forexample, see count. Thesecurves, Figs. 7and8,seemtobeinerror.Middleton.. )Martin, ontheotherhand, showstheout- First ofall, Stumpers' article rigorously treats theputS/Nvarying linearly withtheinput S/Nforsmall operation ofthezero-crossing counting typeofFM input S/N. Thecorrect behavior wouldhavebeen ob- discriminator withanFM sine waveplusGaussian noisetamed bycombining theresults showninFigs. 3and9 astheinput. Thisdiscriminator measuresinstantaneous inStumpers' paper, and thereby accounting forthesmall frequency bvaveraging overa short timeinterval the numberofzerocrossings oftheinput waveform. The