A Simple Receiver

8/12/2019 A Simple Receiver

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A Simple Receiver - the DC40Home A Simple Receiver - the DC40

A receiver project's diary

Usual construction write-ups only deal with a finished desin that is loically e!plained and the performance isdiscussed clinically" Such te!ts overloo# the unpredicta$le path that an e!perimenter ta#es durin a pro%ect" &he

final form is often a result of initial specifications' personal choices' accidents and availa$ility of time and

material"

Hence' this article( a %ournal that follows a receiver)s $irth" &his receiver was made over several evenins'spendin less than an hour each day"

*or those real hams who never read the entire articles' + follow my convention of #ey caveats and some tips"

Here is the entire circuit(

Remem$er this while assem$lin the receiver(

," Shield the *." +t should $e inside a metal $o!" + soldered pieces of copper clad $oard /$lan# C1

material2 around the *. circuit"

3" + don)t thin# you understood how important it is" So' + am sayin it aain' SH+5D &H *." &his is amaic $ullet cure for common ills of a direct conversion receiver"
http://www.phonestack.com/farhanhttp://www.phonestack.com/farhan


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6" &he receiver layout is uncritical" 1ut #eep it clean" 7,' 73 and 76 form a tric#y circuit" Dou$le chec#

the connections"

4" +t miht $e temptin' $ut don)t use a $attery eliminator" +t will produce hum" Use a reulated power

supply with ade8uate filterin" A ,9 volt supply with 3300 u* capacitor followed $y a :;,3 reulator isrecommended"

9" + have included the voltaes to $e e!pected at all the transistor leads" Use that as a uide to trou$le-shootin the receiver"


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A simple definition of a ood receiver is that a ood receiver consistently' clearly receives only the intended

sinal' such a definition hides a wide rane of re8uirements" &he receiver has to $e sensitive enouh to pic# upthe wea#est sinal imaina$le /note( clearly2' it has to $e selective enouh to eliminate other sinals /only2' it

has to $e sta$le enouh /consistently2"

*or a ham or an enineer' $uildin a usa$le receiver is a personal landmar#" +t esta$lishes a personal

competency to $e a$le to understand the very fundamental operation of the radio and mastery over it"

A direct conversion design

A direct conversion desin is simple and pure" A sinal arrives from the antenna it is converted to audio $y

simply mi!in it with a local R* source and played to your ears throuh an audio amplifier" &he principle can

$e e!plained to anyone on a piece of paper in a few minutes" 1ut havin the principle e!pounded so simply' theissues of sensitivity' sta$ility' dynamic rane' etc" that confront a $uilder are e!actly the same as those that will

confront anyone $uildin a far more comple! system"

?e chose a direct conversion desin $ecause(

," +t could $e assem$led easily from parts already availa$le in the %un# $o!"3" A direct conversion receiver needs as much desin consideration as superhet does to achieve accepta$le

performance"

6" + hadn)t e!perienced a direct conversion receiver for a lon time"

4" +t would $e an innovative pro%ect su$mission"

!nitial Design

?e didn)t plan the receiver minutely" ?e #new that we re8uired somethin li#e in the fiure $elow(

So' the initial desin was to fill up these four $o!es in the followin way(

R* *ilter( Dou$ly &uned Circuit centered at :"090 Bh@

roduct Detector( a two diode sinly $alanced detector

A* amplifier( discrete transistors used as audio amplifier to drive ?al#man headphones"

*.( 1ipolar transistor $ased' low noise oscillator"


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?e reasoned that a dou$ly tuned circuit will prevent stron out-of-$and sinals from AB $rea#throuh" &he

sinly $alanced detector would $e simply enouh /%ust one trifilar coil to wind2 and an audio amplifier $ased ondiscrete transistors would $e $etter than the 5B6;< /that + was $einnin to hate the 5B6;< for hih noise2"

Havin read in the BR*D /Ref" ,2 that the local oscillator radiation can cause tuna$le hum and microphonics'

it was contended that an R* amplifier miht $e re8uired"

Day "# $eginning at Audio %nd

?e $ean the receiver construction $y fishin out a copper clad $oard that was a$out ; inches $y 3-,E3 inches"

Usin an old ra@or $lade it was scrapped until the copper loo#ed $riht" &his was to $e our new receiver)s $ase"

?e had decided to use discrete transistors in our desin instead of the standard 5B6;


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&he #ey issue plauin us at this time was( what #ind of tunin mechanism should we useL By %un# $o! has a

fast depletin stoc# of varia$le capacitors /%ust four more to o2" .n the other hand' varactor tunin would meanoin out and $uyin a decent ,00J linear potentiometer" aractor tunin also adds noise to oscillator" +t was

decided to par# this issue for the time $ein and et on with the rest of the oscillator"

A Hartley oscillator $ased on a *& was decided upon" +t is easy to et a Hartley oscillator wor#in" &he

num$ers of crucial fre8uency determinin components are few"

?e started the wor# with windin a coil" A nylon tap washer was used as toroidal former to wind the oscillatorcoil" A toroidal coil)s inductance in micro-henries is iven as /# ! s8uare of num$er of turns2" &he # for these

nylon toroids varies $etween ,nH per turns s8uared to ,"9 nH per turns s8uared" A coil with ;0 turns /with a tapat 30 turns2 was wound to ive an estimated inductance of 6"


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selected such that it would not cover the oscillator components and #eep the shaft at mid heiht from the $ottom

of the panel to allow the $iest #no$ possi$le"

Usin a hand drill' three holes were drilled for the tunin capacitor' the volume control and the ear-phones %ac#"

&he tunin capacitor re8uired two screws to affi! it to the panel' these were mar#ed after insertin the shaft

throuh the main hole and they were also drilled out" *rom a small piece of scrap copper clad $oard' three riht-

anled trianles a$out an inch to a side were cut and smoothened with a flat file" &hese were soldered instandin position on the main $oard and the front panel was in turn soldered onto these anles /see the pictures2"

&he main tunin capacitor was screwed in and so were the volume control and the phone %ac#" &he oscillator

was found to $e oscillatin at :"


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?e 8uic#ly fashioned a coil and cap tan# for the front end that was coupled lihtly throuh 33pf capacitors to

the antenna lead and the diode mi!er" A :090 BH@ crystal was soldered into a test oscillator and used as asinal enerator"

?e found that we could tune in the sinal" So the receiver was $asically wor#in" At this point' my cousin was

losin courae" &his was not the way she saw it" *rom her point of view' we had desined and $uild the circuit"

All $loc#s were wor#in as e!pected $ut due to some evil spirit we were not havin a wor#in receiver"

At this point' we decided to sleep over the pro$lem and revisit it the ne!t day" ?e needed a plan for the ne!tday to #eep the spirits up"

Day +# Completion

.ur pro$lems were clearly centered around our product detector' we decided to do somethin a$out it" +t could

have $een any of the followin(

," +nsufficient drive from the *.( ?e had a simple emitter follower as a $uffer to the oscillator" &heoscilloscope showed that it had insufficient $ias and it was clippin on the down-swin"

3" Sinly $alanced( &he R* input was not $ein $alanced out at the detector output" ?e needed a dou$ly

$alanced detector"

6" Accordin to Ric# Camp$ell)s te!ts' *. lea#ae into the R* input is responsi$le for tuna$le hum and

microphonics"

A trip was made to the mar#et and G", Mener was procured that was soldered $etween the power line of the

*. and the round" &he supply resistor was chaned from 4:0 to 330 ohms so that the Mener had reasona$le$ias current"

&he *.)s $uffer amplifier was chaned from the s#impy sinle stae desin to usin a two stae amplifier

with feed$ac# with unity ain and the *. was completely shielded from all sides $y solderin copper clad

$oards all around it" /See the pictures2"

A dou$ly $alanced detector was fa$ricated usin two trifilar coils and four diodes" A simple diple!er of a 0",ufin series with a 90 ohms resistor was added to the detector output to properly terminate the mi!er"

A simple low pass filter was added on a trial $asis to the input of the detector" &he receiver was fired up"

Success at lastK &he receiver was now wor#in %ust as well" Connected to a lon wire /actually' the shield end of

the 30 meters inverted 2' it pulled sinals from all around" &he noise floor was sufficiently low to mar# adramatic %ump in the noise when connected to the antenna"

,erormance

?e do not have the tools to ma#e performance measurements on the receiver" Hence' we rely on our actual

receiver e!perience instead" &he receiver is clearly far more sensitive than is re8uired for this $and"


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?e used a reular disc ceramic capacitor in the *.' $ut still' it is 8uite sta$le" &he local SS1 nets could $e

monitored continuously without needin to retune" ven the initial warm-up too# less than a minute and thedrift was less than , JH@ durin this period /not measured2"

&he initial audio $andwidth was much reater than 6 JH@" A 0", u* at the output of the audio preamp $rouht

the $and-width down to accepta$le levels" +deally' an active first or second order low pass filter $ased on op-

amps should have $een used" However' the current arranement is sufficient for us" + personally prefer a widerthan normal $andwidth"

At rare instances' a sliht trace of AB $rea#throuh can $e heard' if we had an R*C' we could have inserted it

$etween the A* amplifier and the detector to #eep the R* out of audio staes" &he AB $rea#throuh' when ithappens' it hardly percepti$le and it ta#es a $it of concentration to discern it a$ove the $and noise"

Conclusion

?e had started with an over-enineered desin that included an R* amplifier' etc" .ur first cut was a su$-

optimal desin' and the final version was a reasona$le compromise $etween performance' effort and componentavaila$ility"

&he receiver performance is very clean and the sta$ility is 8uite ood' it is now the standard niht-stand receiver

$y the $ed"

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A Simple Receiver