April 1999 - American Bonanza Society

with the radio's tuning dial or channelselector. The signal comi ng in from theamenna and the output of the local oscillatorare mixed in a stage once calledthe fi rst deteclOr (because it is a non-linearcircuit, and old radio engines calledany such circuit a detector). but now usuallyknown as the converter. The mixingresults not only in the two original frequencies,but al so in a frequency thai isthe difference between them.In audio work such a difference iscalled a beat note. but the RF people callit a heterodyne. Thus, a superheterodynereceiver is one in which tuning is accomplishedby scui ng the local oscillator 10a frequency thai differs from thai of thedesired tation by the IF, so that the conveneroutput can be amplified by theamplifier.An interesting point here is that itdoesn't mauer whether the local oscillalOrfrequency is above or below the desiredstation frequency. It's only the differencebetween the two that counts,which means that a superheterodyne isreally always tuned 10 two freq uenciesat once.However, these two are separated bytwice the IF, so that the undesired one(called the image) is easily rejected bytuned circuils at the amenna. Such a receiverhas great selecti vi ty, for the narrowbandwidth of the IF amplifier istransferred algebraically (i.e., not multiplied)10 the receiver input by the heterodyningprocess and the relatively lowfrequency,fixed-tuned IF amplifier canbe made to have a very narrow bandwidthindeed.ote, however, that the lower the intermediatefrequency, the narrower theactual bandwidth can be made, but theseparation of the desired signal and theimage may become too small for theluned circuits at the antenna to reject theimage adequately. To overcome thi sproblem, some expensive communicationsrecei vers are "double-conversionsuperheterodynes," meaning that theyhave two converters. local osci ll ators.and IF amplifiers. The first IF is relativelyhigh , making image rej ection acinch, but there is then a second converterleading to a very low (sometimesas low as 50 kH z, rea ll y a supersonic fre ­quency) IF, which can produce a verynarrow bandwidth with no worries aboutan image. this having already been disposedof.ABS Apri I 1 999BUI to return to the problem of suchreceivers causing interference: For somereason, manufacturers have elected 10 putthe local oscilialOrs in their sets on thehigh side of the desired station frequency.This means that, as the FM band extendsfrom 88 to 108 MH z. the local osci llatorin your typical FM set gets tuned from88.0 + 10.7 MHz. the local oscillator inyo ur typical FM set gets tuned from 88.0+ 10.7 = 98.7 MH z to 108.0 + 10.7 =118.7 MH z.Now observe that our VOR stationslie in the range 108.0 to 11 8.0 MHz andyou will see that the FM set's local oscillatorgets tun ed right through th atrange. For example. the Naples. Florida(where I'm based) VOR is at 108.6 MHz.An FM set tuned to 97.9 MHz wi ll thenhave its local oscillator transmitting ri ghton the Naples VOR's frequency. Andagain, the Boston. Massac huseus, VORis at 11 2.7 MH z. so that the local oscil ­lator of an FM set IUned 10 102.0 MHzwi ll fall right on that frequency.No wonder there's a problem for theaverage AM/FM radio, especially thelinle portable ones usually do nO! incorporatesufficient shielding to prevent thelocal oscilialOr's signal from gening oul.This Can be demonstrated quite dramaticallywith just a couple of AM radios.They can both be linle portableones, or one can be ponable and broughtnear your big living room FM receiver.Place the two radios near each other andtune one (either one) 10 the low end ofthe band, say in this AM case 10 600 kHz.Then turn the other one 465 kHz higher.that is, in our example, 10 1065 kH z.You'll find a station there that was neverthere before. obody will be talking onit, i.e., it will be an un modulated carri er,but it will be a spurious station nevertheless.And it's coming from yo ur firstradio. the one tuned to 600 kHz, as youcan readily show by simply turning th atradio off.That should be enough to convinceyou not 10 use your FM set in an airplane.AM sets cause less trouble because thereare no aircraft electronics operating onfrequencies just above the AM band,whose top end is at 1600 kHz. the DBsall being at lower frequencies. Buttherc'sno guarantee th at at some point, youwon 't sw itch yo ur FMIAM radio to FM .Finally, in view of all the foregoing,here's an interesting question: Why isthere no Channel I on American television?You will note that the switch dialson older television sets having channelselector switches all start with 2 and thatalthough you can command Channel Ion modern digital TVs, you won't gelanything. Why is this?Is there anyone out there who knowsthe answer and is interested enough tosend it in? I will offer a small prize to(he first one to send me the ri ght answer.- Dean S. Edmonds, Ir10 19 Spyglass LaneNaples, FL 34102Overhead instrumentpane/lightJohnny Johnson, owner of Sparrow.has been very accommodating and hasdeveloped a great solution for aircraftwith no panel lights. His single overheadlamp is now my back-up light source, ona separate circuit. The "Feds" have giventheir blessing with a PMA. Sparrow'snumbers are 800-4 16-8548 and FAX903-839-1613.- Bob EdmondsonThe Woodlands, TexasABS Magazine stays "dry"Just wanted you 10 know that I gO!my February issue of ABS Maga~ille inth e mail yesterd ay during a goodrainshower. About half of the contentsin my mailbox were soaking weI. but theABS Magazine in its plastic mailing bagwas dryas a bone. Thanks again for thatliule extra quali ty.-Ron DavisNewport Beach, CaliforniaPage 5&58