... -J to send radio signals through the air, how modulation , IJ."~ UNITED STATES~~ FREQUENCY ALLOCATIONS

Transcription

1 ..., J."..J What would lfe be lke wthout rado and televson? Only a hundred years ago the fastest way to send a message between Amerca and Europe or Asa was a fast shp. Now we get lve televson coverage of news and sports events from anywhere n the world. Whle the telegraph and telephone allowed nstant : communcaton at great dstances, these requred : wres. The development of rado made nstant communcaton possble wthout wres. ntally the equpment requred was expensve, so the frst man use was n large shps at sea. of rado transmssons for thngs such as televson, cellular phones, arcraft navgaton, communcaton wth probes n outer space, radocontrolled toys, and thousands of other uses. The Federal Communcatons Commsson (FCC) makes sure that all of these uses operate on dfferent frequences so that they don't nterfere wth each other. UNTED STATES FREQUENCY ALLOCATONS. &:: ;, ',; "... L""J 'l r;;;;] '!/,l;"; D!l D m"' ".. ",..,(A, n ths chapter you wll learn how antennas are used -J to send rado sgnals through the ar, how modulaton s used to encode the nformaton about transformers. You wll also use Snap Crcuts? to buld rado crcuts. beng sent, and -68-

2 le Ae _ The electrcty suppled to your home and school by your local electrc company s not a constant voltage lke that from a battery. t averages about 2V but s constantly changng, due to the desgn of the generators that produce t. Ths s not a problem, snce all equpment that uses t accounts for ths change. ts frequency s 6 Hz. An electrcal sgnal that s changng s called an alternatng CLH"Tent, or AC. Because of ths, the power from the electrc company s also called AC power. An electrcal sgnal that s constant and unchangng s called a drect current, or DC. The power from a battery s also called DC power. le2t -----_._ n a motor, electrcty can make mechancal moton: an electrc current flowng through a col of wre can make a magnet rotate on a shaft. But what f the small magnet was nstead a large, heavy, ron bar and was held n poston? The current n the wre magnetzes the ordnary ron bar, and t becomes an electronc magnet. The ron bar stores electrcal energy as magnetc energy. What f another col of wre from a dfferent crcut was also wrapped around the ron bar? The magnetzaton of the ron bar would create a current n t. Ths s a transformer, whch allows one crcut to create a current n another crcut usng magnetc felds. For a demonstraton of ths, consder ths smple crcut (whch s projects 55-56): Orgnal Current ron Bar Make a paper dsc wth lnes on t lke the one shown here (a sample for cutout s on page 46 of project manual #). Tape t to the fan blade and place t on the motor. Place ths crcut under a whte fluorescent lght n your home or school (don't use an ordnary ncandescent lamp). As the speed changes you w" notce the whte lnes frst seem to move n one drecton then they start movng n another drecton. Do you know why t does ths? The reason s. because the lghts are blnkng 6 tmes a second and the changng speed of the motor s actng lke a strobe lght to catch the moton at certan speeds. To prove ths, go nto a dark room and try the same test wth a flashlght. The lght from a flashlght s. constant, so you won't see ths effect and wll always see the lnes move n the same drecton. 'mportant Note: Transformers only work wth. changng voltages (AC). Unchangng voltages. (DC) have no magnetc propertes and don't work [' wth transformers. Many motors (lke the one n! Snap Crcuts") have a mechancal desgn that allows them to use the DC voltage from batteres. Thnk of a transformer as a magnetc brdge n electroncs, snce we use magnetsm to cross an ar gap that electrcty cannot cross by tself. Snap Crcuts" does not nclude a transformer, but a typcal one s shown here wth ts symbol: The fluorescent lghts are blnkng because they use the AC power from the electrc company. A flashlght, uses DC power from batteres. Note: some new fluorescent lghts use an electronc ballast and they also produce a constant lght. -69-

3 ; f the second col n the transformer had twce as, many loops of wre as the frst, t would have twce j the voltage but half the current as the frst. The.eason s that power s not lost across a transformer, and Power = Voltage x Current. A transformer can use a small voltage to create a very large voltage by usng a many-loop col wth a few-loop col. Transformers are very mportant n electroncs for two reasons. Frst, they allow crcuts to be solated from each other, snce the connecton between them s magnetc and not electrcal. Second, they can Change the voltage wthout wastng power (by usng cols wth more or less loops of wre). Hgh Voltage Power lnes When electrc power companes transport electrcty across great dstances (lke between power generatng plants and ctes), they use very hgh voltages and low currents snce ths reduces power loss n the wres. Large transformers convert ths to 2V, whch s suppled to homes and offces. Many products (lke computers) then use small transformers to convert ths to smaller or larger voltages as needed. For example, most crcuts n computers use 5V. Transformer 2V House Power Lne Flash camera: A flash camera needs to make a lery brght flash, but ts small battery cannot supply ths much power at once. So the camera lets the battery charge up a large capactor (typcally 6lF) to a hgh voltage usng a transformer. t takes a few seconds to charge the capactor (to about 3V); the flash tube wll dscharge ths n an nstant. 6llF Capactor Flash Tube Battery 3mNJWaA\_ nductance s a measure of one col's ablty to create a current n another. t s expressed n henrys (H, named after Joseph Henry who developed electromagnetc nducton) or mcrohenrys (lh, mllonths of a henry). The more loops n a col of wre, the more nductance t has. Placng an ron bar nsde a col of wre magnfes that col's nductance. Cols can be desgned wth shapes and other characterstcs that wll maxmze ther electromagnetc felds for specfc frequences. f the orgnal ("transmtter') col and current through t were large, then the electromagnetc feld from t could stll be pcked up by another ("r@cehrsl"") col and produce a small current even f the dstance was many mles. _ Transformers use electromagnetc felds through 'ron bars to "brdge" a gap between crcuts. f the ron bars through the two cols were close but not connected to each other, ther electromagnetc felds would be reduced but would stll affect each other. -7- Ths s the concept of rado, whch uses electromagnetc waves to send nformaton through the ar. The cols used for transmttng and recevng these sgnals are called antennas.

4 Cols lke the antenna are also called nductors. Ther magnetc propertes enable them to oppose changes n electrcal current, and to store electrcal energy as magnetsm. Ths allows nductors to be made to block hgh 'f"equencv Sfjn2S;vhnepassng low fr'ecu;ncysqnals. Ths s opposte to rovt' capactors act on hgh and low frequency sgnals, so nductors and capactors are used together to make complex frequency flters. ntroducng New Parts Snap Crcuts" ncludes an antenna: ---. c!...,.., _,"._, Antenna Col (A) \...".,..",., C 'JZ\ ;. J.'. An nductor frequences, frequences. be calculated value: Antenna Col Symbol The antenna you wll use s a 25H col wrapped around an ron bar, whch ncreases the nductance to 3H. Although t has magnetc effects smlar to those n the motor, those effects are tny and may be gnored except at hgh frequences (lke n AM rado). At low frequences the antenna acts lke an ordnary wre. Rnductor ", --- = 6.28 x Frequency x nductance For example, your 3H antenna wll have a resstance of only.884q at Hz, but a resstance of,884q at OOO,OOOHz ( MHz). A wde range of schemes are used for encodng the rado sgnals wth the nformaton beng sent. These are called modulaton. Modulaton uses one m has lower resstance at lower but hgher resstance at hgher The resstance of an nductor may from the frequency and nductor sgnal to modfy another. You've probably heard of AM and FM rados. These stand for Ampltude Modulaton and Frequency Modulaton. ","'", -,..., " ffor a smple demonstraton usng Snap Ctrcults=, onsder ths crcut (whch s project 258): " Usng the fan outlne as a gude cut a 3" crcle out of a pece of paper. Then, cut a small trangle n t as shown. Tape the crcle onto the fan and then place t onto the motor. Set the adjustable resstor to the center poston and turn on the swtches. The fan spns and the lamp lghts. As the trangle openng moves over the photoresstor, more lght strkes t. The brghtness of the LED changes, or s modulated. j-;d;am:;n; "message") s used to modulate the ampltude of another (the "carrer"). n FM rado transmtters,, one sgnal (the "message") s used to modulate the frequency of another (the "carrer"). The! "message" wll be talkng or musc, whle the "carrer" wll be an oscllator crcut tuned to the, desred transmt frequency. Here s an example: Audo RF Carrer Frequency Modulaton -7- Ampltude Modulaton l_.

5 AM rado was developed before FM rado, because the transmtter and recever crcuts are not as complex. FM's greater complexty means t s better rrotected from nterference (such as storms). FM also has wder channel bands (25kHz vs. 7kHz for AM), whch gves t better musc qualty. AM rado uses a carrer frequency range of 5 to 6kHz whle FM rado uses 88 to 8MHz. There are many dfferent rado sgnals floatng around, but we only want to lsten to one. Thnk of ths as beng n a large, crowded room and tryng to talk to someone on the other sde. Connectng the antenna to a capactor n sort of an antennacapactor oscllator solves ths. Together, these two components "flter" out a small range of frequency that you lsten to. Adjustng the varable capactor changes the range of frequency that you are lstenng to. The hgh frequency C amplfes and decodes the modulaton nto the orgnal sgnal (voce or musc). Ths s amplfed by the power amplfer C. Varyng the adjustable resstor makes the sound louder or softer. Ths smple rado has the same types of crcuts as AM rados sold n stores, but does not have as much flterng and amplfcaton crcutry. Take a look nsde an old AM rado n your house; you'll see a lot more components. The tunng dal on all AM and FM rados s a varable capactor just lke yours. complex than the crcut wth the C, but there s actually a lot more crcutry hdden n the C. Snap Crcuts" project 289 s smlar to project 242 but wthout loudness control. Snap Crcuts" project 288 shows another AM recever crcut, but usng a 2-transstor amplfer nstead of the power amplfer C. Ths crcut has smlar performance and appears to be more r,' Place the crcut next to an AM rado n your home. Tune the rado so no statons are heard. Turn on the swtch. You should hear the song on your rado. Adjust the varable capactor for the loudest sgnal. Cosder ths AM rado transmtter crcut (whch s project 23): A ;! Ths crcut uses the antenna to transmt electromagnetc energy to your AM rado. The antenna-capactor combnaton tunes the transmt frequency. The musc C provdes the ampltude modulaton.! a +! J, G Notce that ths crcut transmts across a wde part of the AM rado band, not just one staton. Ths crcut has just two components tunng the transmt frequency; a commercal AM rado staton wll have a complex flterng crcut dong ths. ) -72-

6 Snap Crcuts" projects 22, 45-5, 24, and 255 are varatons of ths basc AM transmtter crcut, usng the alarm C, space war C, or the photoresstor. These crcuts also show how to use ths crcut as an alarm. Snap Clrcuts" project 98 (n most manuals) s smlar but transmts your voce to a rado. t also shows how varatons n ar pressure (caused by your talkng) can make an electrcal sgnal n E. speaker - lke a mcrophone, and opposte to how a speaker s normally used. Replace the speaker wth the mcrophone n ths crcut (+ sde to Q2) and compare the performance.,. Lst all the products n your home that use some form of rado or remote control. Summary of Chapter 8: Chapter 8 Practce Problems. Why can't DC currents transfer energy across transformers? A. DC currents have no magnetc propertes. B. DC currents don't have enough power to overcome the resstance of transformers. C. Transformers block the transfer of energy from both AC and DC currents. D. They are not dgtal crcuts.. An electrcal current that s changng s called an alternatng current (AC). An electrcal sgnal that s constant and unchangng s called a drect current (DC). : 2. The electrcty n homes s AC power, wth a voltage of 2V and a frequency of 6Hz.. 3. Transformers allow one crcut to create a current n another usng magnetc felds. Ths can change the voltage wthout wastng power. 2. The followng are true about transformers except: A. They allow crcuts to be solated from each other. B. They allow electrcty to be efflcently transported over great dstances. C. They allow a small voltage to create a large voltage. D. The cols used can never have the same number of loops. 4. nductance s a measure of one col of wre's ablty to create a current n another, and s expressed n Henrys. nductance can be ncreased by addng more loops of wre or by placng an ron bar nsde the col., 5. Rado uses electromagnetc waves to send nformaton through the ar. The cols used for transmttng and recevng these sgnals are called antennas. 3. The followng are true about rado except: A. Frequency modulaton crcuts are smpler than ampltude modulaton crcuts. B. The FCC regulates rado transmsson frequences. C. FM has better musc qualty than AM. D. For DC currents, antennas act lke ordnary wres. 6. Cols are nductors, whch have lower resstance at lower frequences but hgher resstance at hgher frequences. nductors and capactors are often combned n rados to flter out a range of frequences. 4. At low frequences, the Snap Crcuts" antenna acts lke a _ A. Kn resstor B.. flf capactor C. 3-snap wre D. whstle chp 7. Modulaton uses one sgnal to modfy another. 8. n AM rado, a musc/voce sgnal ampltude modulates the transmt carrer frequency. n FM rado, frequency modulaton s used nstead v 'V "8 '.c: ''f. :SJaMSU'V