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Radio Electronics, July 1986 with an Article That 7A 3v7 Most us believe that the age of solid- MARTIN CLIFFORD DID YOU KNOW: TI IAT SOLID-STATE ELEC- of tronics can trace its roots back to 1835? state electronics began with the invention In this, the first installment That radio signals can be demodulated of the transistor; Bardeen and Brattain of using sulfuric acid or nitric acid? That oil - Bell Laboratories produced that first crys- of our new, occasional series filled variable capacitors were once used tal triode in 1948. However, lost in the about the early days of radio, in radio receivers? That a single crystal mists of history is the fact that true solid- detector can be used as a radio receiver? state receivers have been with us since we look at the original That there are some radio receivers that about 1918. "solid-state" receivers. never need to be turned off, have no on/off In more recent times, the term solid- switch, and do not require battery or AC state has been so firmly associated with power? Or that lenzite, zincite, bornite, germanium, and subsequently with sil- tellurium, and chalcopyrite are all semi- icon, that no one should be blamed for conductors? thinking that those are the only materials The Early Days o 1 i I y m y,o- .. J' -, - suitable for use in semiconductors. Yet least some improvement over those with there are numerous materials that are just no tuning at all. as suitable. Among them are carhorun- Another problem was that the output of dum (silicon carbide): galena (a crystal the crystal detector consisted of both an sulphide of lead); molybdenum: lenzite: audio signal and an RF carrier: both were ¿incite (an oxide of zinc): tellurium: bar- passed directly to the headphones. Subse- nite (a sulphide of iron and cooper): chal- quently, a small capacitor was placed copyrite (also known as copper pyrites): a across the headphones to bypass the RF and cerusite. Except for carborundunt, a HEADPHONES carrier. manufactured material also used as an Also, it was found that selectivity could abrasive. all are materials that are found in U he further improved by adding a second nature. slider to the tuning coil. The radio shown CRYSTAL DET. in Fig. 3-a incorporates both those im- Early solid state SINGLE -SLIDE provements: the schematic for that circuit The fact that certain materials have rec- TUNING COIL is shown in Fig. 3-b. tifying properties (that is. they allow cur- rent to flow in one direction only) has been FIG. 2-TO IMPROVE SELECTIVITY a slide Improved designs known since 1835. thanks to the research tuned coil can be used (see a). The schematic of As time went on, various methods were a crystal set that used such a coil is shown in b. of one Munk Al. Rosenshold. At the time used to improve tuning. In one arrange- a laboratory curiosity, his discovery was ment, shown in Fig. 4-a, the single -slide largely forgotten until it was unearthed tuner was used as the primary winding of again by F. Braun in 1874. an RF transformer. The secondary wind- However. a practical use for that discov- 2 SLIDE ing, which was tapped, was wound on a ery was not found until many years later. It TUNING COIL form that could travel into the primary by - -0 came at at time when interest in radio was means of a pair of rods. That resulted in heating up and many early experimenters triple tuning. The primary was tuned by had radio setups in their attic or basement the single slider: coupling was adjusted by workshops. III III II III Illllllllllllllillllllllllll!Illllllllilllu>oo moving the secondary' in or out of the Rectifiers are key components in sim- primary, and finally, the appropriate sec- ple radio detectors. Along with vacuum ondary' tap could he switch selected by the tubes, early experimenters turned to sol- operator. The schematic for the circuit is id-state rectifiers made from one of the shown in Fig. 4-b. substances that exhibit natural rectifying Since most crystal radio receivers are properties. Since most of those sub- "powered" by the radio signal itself, they stances are crystalline in nature, such rec- require no voltage supply or battery. (They tifiers were called costal detectors; radi- os that used such detectors were called radio sets, cn.cta/ or simply crystal sets. a The simplest radio Early vacuum -tube rectifiers, such as the UX-201A and UV -199 triodes, cost about $15. Since $15 a week was consid- ered good pay for a workman back then. CRYSTAL DET. ImIiiliiiiiUiIIVIIIIIIIIIIIIIIpII911111Vi and since crystal detectors cost only a small part of that amount, many experi- TUNING menters turned to crystal sets. Such radios COIL were among the simplest possible, con- HEAD- qp sisting of just a crystal detector and a PHONES- headset. See Fig. I. b The crystal set of Fig. 1 has its virtues. but it also has plenty of faults. Selectivity FIG. 3-FOR EVEN GREATER selectivity, re- is non-existent; after all, that radio has no ceivers with double -slide -tuned coils were de- veloped (a). The schematic of such a is tuning circuits. So whatever the radio re - set shown in b. Note the inclusion of a capacitor across the headphones. Its purpose was to by- pass the RF carrier. AERIAL ceives is what you hear-the strongest LEAD-IN signal dominates, and all the rest provide background noise. CRYSTAL DIODE DETECTOR The earliest effort at improving selec- tivity was to add a tuning coil like the one shown in Fig. 2-a. EARPHONES (HEADSET) That coil consisted of enamel insulated wire wound on a round form: experimenters often used cylin- b drical oatmeal boxes as the coil form. The FIG. 4-IN THE CRYSTAL SET SHOWN in a the GROUND amount of inductance could he selected slide -tuned coil is used as the primary of an RF transformer; the secondary had multiple taps using a metal slider. The tuning method FIG. 1-THE SIMPLEST CRYSTAL RECEIVER and was wound on a form that could be moved consists of an antenna, a detector, headphones, was crude. but the radio circuits that in- in to or out of the primary. The schematic of the and a ground. cluded that coil (see Fig. 2-h) did offer at circuit is shown in b. 62 was weak. Those two factors eventually caused the carborundum detector to fall out of favor. but not before it gave rise to N some interesting circuit innovations. CARBORUNOUM One of those was the concept of the DETECTOR amplifier. The early amplifier shown in Fig. 6 was a mechanical one known as the Brown amplifying relay. The leads identi- lied as A and n were connected to the output of the detector. The signal current through relay coil Ml caused relay arma- FIG. S-BATTERY POWER was needed in crys- ture v to vibrate. thus varying the magnet- tal sets that used carborundum detectors. ic field around winding R2 of the second relay. That caused the current supplied by the battery to be varied at an audio rate. That current flowed through the head- phones. p oducing sound. In some in- stances a series arrangement was used: the current from the battery drove still another mechanical amplifier. the need for clumsy tuning coils was finally eliminated through the use of a FIG. 8-A VARIO -COUPLER, or variometer, con- variable capacitor. then known as a con- sisted of two coils mounted in such a way that one :ould be rotated within the other. denser. For a while, however. variable ca- pacitors were used in conjunction with ADJUSTING single- and double -slider variable coils. KNOB PIVOT Two examples of capacitor -coil -tuned cir- CAT cuits are shown in Fig. 7. WHISKER FIG. 6-BECAUSE OF THEIR LOW OUTPUT, In some of the more advanced sets. a some sort of amplification was often used with TERMINAL TERMINAL carborundum sets. Here, a popular mechanical vario coupler, sometimes called a varí- amplifier of the time is shown. ometer (see Fig. 8), was used. That con- sisted of a pair of coils mounted in such a way that one could he rotated within the other. a Ordinarily, the variable capacitors used were air types; that is. the dielectric he- CRYSTAL tween the stator and rotor plates was air. CONTAINER CAT WISKER Air. though. has a dielectric constant of I. To increase the capacitance without adding more plates. one design had the variable capacitor positioned in a leak- 'i~INSULATING BEAD _ proof. transparent case, filled with oil. Oil WAX SEAL has a dielectric constant of five. so the LEADS capacitance of such a capacitor is live b times that of one with an air dielectric FIG. 9-TWO CATWHISKER DETECTORS. A a variables to be the \in a; a sealed, (assuming all other variable contact type is shown same). One such capacitor had 17 plates ftxec unit is shown in b. and a total maximum capacitance of 0.(K04 µF. while another had 43 plates we've not looked at the detector itself and and a maximum capacitance of 0.0011.1.F. how it was made. It is time now to correct Anyone who has had any contact with that oversight. radio knows that the antenna plays an im- Crystal detectors were available in two portant role in determining the quality of basic forms: contact and combination. Of the received signal. Antennas were even those, the contact detector was cheaper more important in the days of the crystal and more popular. set. That's because that design made no A single crystalline substance was used provision for amplification.
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