japan's hush-hush rushbox - .japan's hush-hush rushbox ... sbe shown in the
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-f;E' , . . . . ,Photo B. Top view of the radio. Inside the lid are the circuitdiagram and frequency calibration curves for the threeranges. On the radio, top row left to r ight: tun ing capacitorCS(see Fig. 4), frequency ra nge switch 51, top o f the tube,and antenna-current meter. Bottom: filament-current con-tra /R2, variable grid leak R1 , and ;acks fo r two handsets.
Fig. 3. Received signal, anamplitude-modulated car-rier, and the variation o f therectif ied current through thetube. Start ing from the am-p litude of the rec ieved sig-na l , the signal bu i lds updur ing interval 1and stays atthe maximum level duringin terval 2. The sum of inter-vals 1and 2 is r, the period ofthe quenching signal.
- '-- -,,-- .
proved to be em inen tly suc-cess ful."We can go a bit further
into the theory by looking atFig. 2. Here isshown how thesignal in the tuned circuitgrows exponentially from ast art i ng voltage (Ym) to amaximum vo ltage (Ymax)'Ymax depends on the anodevoltage and maximum an-ode current the circuit al-l o w s t o fl ow , When t hevo ltage in the tuned circuitis higher at the moment thequenching signal makes thedetector osci llate. the Vmaxlevel is reached sooner, asshown in Fig. 2 for a starting
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Fig. 2. Oscillating vol tagesin the superregenerat ive reo.ce i ver ensuing from twostart ing levels, Vin and V 'jnrespectivel y, and fi na lJybuilding up to the same vol t-age, Vmax- Lower sec t ion:Bias vol tage a s dependento n t ime, c au sed b y th equench ing os c iflato r. Inreal it y the voltage is not rec-tangular but sinusoidal.
suive. An elementary form ofa superregenerat ive c ircu it isshown in [ the illustra t ion].Suoerregenerenon is rela-tivel y difficult to atta in atord ina ry frequenc ies, anddoes not possess the prop-ert y o f discrim inat ing be-tween signals o f differentfrequencies charac teristicof oth er t yp es of detec-tors - in other words, the se-lectivi ty is poo r. For thisreason the superregenere-rive circui t finds its chieffield in the recep tion of ul-tre-b ign-treauencv signals,for wh i ch purpose it ha s
Photo C At the right center, transmit/receive switch 52;above it, sw itch S3 to interrup t the microphone circuitwhen using telegraphy. Beh ind 52, the Morse "key" can beseen. At the bottom, the cable to the generator is pluggedin. Inside the rad io, transformer T1 is fa stened to the bot-tom. M icrophone transformer T2 is in a screened box below52. The two round coils below filament-control R3 are L4and LS, the tuning coils fo r the quenching oscilla tion.
amp l if i ca t ion ceases. Toovercome this l imitation andgive still greater amplifica-t io n, the supe rregenerativecircuit has been devised. Es-sentially, the superregenera-tive detector is similar to theordinary regenerat ive t ypebut with a low, but super-au-dible(aboveaudibility)signalin troduced in such a way asto vary the detector's operat-ing point at a uniform rate.Asa consequence of the intro-duct ion of th is quench or in-terrupt ion freq uenc y t hedetector can oscillate at thesigna/ frequency only whenthe operating point is in a re-g ion suitable for the produc-tion of osciffations. Becausethe oscillations are con-stantly being interrupted, thesignal can build uoto relat ivetremendousproportions,andthe suoenegeneretive detec-tor therefore isextremelysen-
The Rushbox ExplainedAs mentioned, the super-
regenerative receiver wasvery popular for recept ionon VHFand UHF in the earlythirt ies. To explai n the prin-ciple, I therefo re quote theARRL Rad io Ama teu r 'sHand book o f 1936 . Fig. 1was taken from that source.First, a " no rmal" tri ode
with a grid detection and re-generation (positive rf feed-back) is exp lained . TheHandbook then carrieson as tL _follows:" The limit to which regen-
erative amplification can be Fig. 1. An elementary super-carried is the point at which r egenerat i ve circui t a ssbetubestensto oscil lete. be- shown in the ARRl RadiocausewhenosciJfationscom- Ama teu r's Handbook ofmence, fu rther regenera tive 1936.10 73 for Radio Amateurs - March, 1986
modulated by t he secondtriode.O lder readers no doubt
will remember the superre-generative receiver or " rush-box." Bu t to t he youngergeneration this elect ronicmarvel may be new. So westart with an explanation ofthis rema rkable type of re-ce iver, invented by th atAmerican geniusMajor E. H_Armstrong, who presentedthe system in 1922. (A rm-st rong also gave the worldth e supe rhe te rod yne re-ceiver and the system of f re-quency modulation for YHFbroadcast ing!)
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" -l ooooe ," " "t,----.- S IB' ,/ 0 .(104, .. ".C>. 0>...
0' I,,.". """ r- ..'" =..,. ), -"."T 2
The transmit/receive switching is somewhat sim-Fig. 4. Circu it d iagram o f radio t ype 94-6.otiiied.
VHF, where automobi le ig-n it ion noise ca n be ve rytro u blesome . Nowad aysevery car ha s good suppres-sion, but before WWII th iswa s mo re t he ex cept ionthan the rule.So it seems that the rush-
box has on ly desirable fea-t ure s . Ar e t he re nodisadvantages then? Sure -selec tivity is very poor, as a l-ready ind icat ed by theHa ndbook. . But even thismight be ca lled an advan-tage : The superregenera tivereceiver often was used inconjunct ion with very sim-ple. self-excited, single-staget ra nsm it t e rs . These haveve ry poor frequency stabil-ity. So poor that the signalsfrom these transmitters onVHF are mostly unreadableo n a superhe te rodyne re -ce ive r. No problem fo r therushbox! Its passband is sowide t ha t t he se s igna ls ,wh ich swi ng a ll over theplace, are coped with eas ily.Now tha t we ha ve re -
freshed our knowledge onth is remarkable invent ion byMa jor Armstrong, it is timeto return to ou r Japane setransce iver.
mu ch over the years that Iwas unsuccessful in mak inga sa tisfactory photographicreproduction, so the circuitwa s red rawn for Fig. 4. (Forthe sake of cla rity, the trans-mit/ re ceive switc h ing issomewhat sirn phf ted.) Thisswitch is visible in Photos Aand C.It is a classic telephone
kevswitch w ith three posi-tion s. In the middle position,the set is turned off . Thelower position activates thereceiver and the upper posi-tion act ivates the transmit-t e r. If bo t h the ha ndgenerator and batteries areconnected, the transm it/re-ceive switch au tomatica llyse lec ts the batteries for re-ce ption and the generatorfor tran smission. (These fea-tures are deleted in Fig. 4.)S2is the T/R switch with on lytwo posit ions shown, T fortransmit and R fo r rece ive.The radio is tuned by co ils
L1, l2, l3, and variable ca-pacitor C5. The frequencyrange is 25-45.5 MHz, andthis is divided into three sub-bands selected by S, .On thelowest range, a ll three coi lsare u sed ; o n the m id dl eband. II is short-circuited,and o n the highest band ,both II and l 2 are sho rt-ci r-cu ited so that onl y l3 re-mains. The coils are threese pa ra te o nes o n formersthat can be seen in Photo D.Ea ch co il is d ivided intoequal halves, A and B. Twoantennas can be used, ind i-cated as ANT.l and ANT.2 .Perhaps one was meant asan antenna and the other asa counterpoise? A thermo--co up le ammete r ind ica tescurrent in ANT.2 .l e t us now look at the cir-
cuit when the radio is usedas a super regenerative re-ceiver. S2 is in position R,The left triode is the detec-tor. ( 6 is the grid capaci to rand Rl is the variab le gridle a k . But where is thequenching signa l comingfrom ? From the same triode,playing a double role. It os-
Circuit Analysis c il ia tes at the receive fre-The ci rcu it d iag ram o n quencv and the quenching
the inside of the lid faded so frequency at the same time!73forRadioAmateurs March,1986 11
of the superregenerative de-tec tor we find the audio thatwa s tran sm itted . W he nthere is no signa l vo ltage inthe tuned circu it there still isthermal no ise, which is am-plified in the same way as asigna l vo ltage.This explains the hissing
no ise tha t is so cha racteris-tic of the superregenera tivereceiver -lovingly ca lled arushbox by amateurs of thethirties . As soo n a s a re -ceived signa l o verrides thenoise suff iciently, the hi ss-ing sound d isappea rs. In thatrespect. tuning a rushbox isvery similar to tuning a mod-ern FM recei ver with thesqu elch turned off . The fac tthat circuit noise, perhaps amicrovolt o r so, is amplifiedto the full output level of avolt or more in theanode cir-cuit ind icates the enormousga in, wh ich can be of the o r-de r of a million times. Andtha t in a single stage !In this respect. the super-
regenerative re ceiver is are al marve l. but there ismore. The rece iver. featuresvery good automat ic ga incontro l (age) so that all sta-tions are reproduced at al-mo st the sa m e le ve l.Su pp re ss io n of im pul senoise is excel lent, too . This isa very desirable fea ture on
vo l t age of V ' in. The mo-ment t he signa l decays isa lso go vern ed b y thequenching signal, but inde-pendent of the starting sig-na l. So the tota l surfa ce thatis hatched in the ill ustrationva rieswith the magnitude ofthe voltage on the tuned cir-cui t at the beginning of eachquenching cycle.As the rf signal is rectified
in the dete ctor val ve, t hehatched area is a lso a mea-sure for the anode current.Th is pr in cipl e is f u rt h e rworked out in Fig. 3. whichdepicts an amplitude-modu-lated grid of the de tec tor.The part of the quenchingcycle during which osci ll a-tions build up has the dura-tion 1. So with interva ls of tseconds. the anode currentbuilds up as shown in the up-per part of Fig. 3.