RADCOM. MAY2010 HOMEBREWEAMON SKELTON, EI9GQ . E-MAIL: [email protected]
HomebrewThe microphone: your voice transmission starts here. . .
PHOTO 1: Typicaldynamic microphone(left) andelectret condenser mic (right). The electret mic is9.8mm diameter.
ESSENTIALITEM.Themicrophoneisoneof the mostimportantcomponentsofatelephonytransmitter.Asthe microphoneis theveryfirst stagein a phonetransmitter,anydistortionornoisefromthe microphoneor itsamplifier issureto degradethe qualityof the transmittedsignal.Theamateurradioshackcan bea harshenvironmentforamicrophone.RFsignalsat highfield strengthand strongmagneticfieldsfrompowersupplytransformersareeasilypickedup bythe mic.Acousticnoisefromcoolingfans isanothersourceof unwantednoise.
A microphoneisa transducerthat convertssound(vibrationsin air) intoavaryingelectricalvoltageorcurrent.Thiselectricalsignalcanbeamplifiedand usedto modulatea speechtransmitter.'DynamicMike' is notnecessarilythe nameofa particularlylivelyradioamateur;theword 'microphone'isoftenabbreviatedto 'mike'or 'mic'.Therearemanydifferenttypesof microphonein commonuse.. Someoldermilitaryequipmentused
carbonmicrophonesof the typethat usedto befound in telephones.Thecarbonmicrophoneusesa looselypackedcapsuleof carbongranulesthat arein closecontactwith thediaphragm.Vibrationofthediaphragmcausessmallchangesinresistanceofthe carbongranules.. Crystalmicrophoneshavea piezoelectricmaterialin contactwith thediaphragm.Stressescausedbythevibratingdiaphragmgenerateavoltageacrossthe piezoelectricmaterial.. Condenser(capacitor)microphonesuseone plateofacapacitorasthe diaphragm.Thesecondplateisfixed in position.A polarisingbiasvoltageisapplied
acrossthetwo plates.Movementofthe diaphragmplaterelativeto thefixedplatecausesthe capacitanceto vary.Thisproducesachangingoutputvoltage.. Thedynamicmicrophoneisanelectromagneticdevicethat works inthe samemannerasa movingcoilloudspeaker.Thediaphragmisconnectedto acoil that issuspendedin a magneticfield. Movementof the diaphragmwillproducean electriccurrentin thecoil.Carbonmicrophonesarerarelyused
todaybecausetheytend to sufferfrom higherlevelsof distortionthan othermicrophonetypes.Thecarbonmic isalsoverysensitiveto humidity.Crystalmicrophonesarenotaspopularastheyusedto be, butyouwill stillhearmanyolderexamplesontheamateur'bandstoday.Thetwo mostpopulartypesof microphonein usetodayarethedynamicmicand oneparticulartypeofcondenser.microphoneknownastheelectretcondensermic. Bothtypesareinexpensive,readilyavailableandcapableof producinghighqualityaudio.
CONDENSERMICROPHONE.Figure1showsthe configurationofa condensermicrophone.Thetwo capacitorplatesareonthe leftof the schematic.Thefixedplateisconnectedto ground;the diaphragmplateisconnectedto a polarisingvoltagesupply.Movementof thediaphragmvariesthecapacitancebychangingthe spacingbetweenthe plates.Asthe capacitorhasa fixedchargeprovidedbythe polarisingvoltage,variationsin capacitancewillcausevoltagechangesat the inputof thehigh impedancesource-followerbufferamplifier.Regularreaderswill rememberhow I accidentallymadea condensermicrophonewhen I wastestinganaudioamplifierin the February2010 Homebrew.
Theelectrettypeofcondensermicrophonedoesn'tneeda polarisingsupplyacrossthecapacitorplatesbecausethecapacitorismadefroma special,permanentlychargedmaterialcalledan electret.Modernelectretmicrophonesaresmall, light, inexpensiveand capableof producingveryhighqualityaudio. Electretmicsareusedin mobilephones,camcorders,telephonyheadsetsand anywhereelsethat a smallhighqualitymicrophoneis required.Electretmicrophonesusuallyhavea built-in FET(fieldeffecttransistor)bufferamplifierwith a veryhigh input impedance.Thebufferamplifierrequiresa DCsupply,typicallyin the range1.5 to 12Vat acurrentof lessthan 1mA.
DYNAMICMICROPHONES.Figure2 showsthe configurationofa movingcoil dynamicmicrophone.Thistypeof microphonehasalightweightcoil that isattachedto the centreof the diaphragm.Thecoil issuspendedina magneticfield that is usuallyprovidedbya permanentmagnet.Air pressureactingonthe diaphragmcausesthe coil to movethroughthe magneticfield, which inducesasmallelectriccurrentin thecoil. Thedynamicmic actslikea loudspeakerin reverse.Somesmall loudspeakerscan makereasonablygoodmicrophones.
Photo1 showsa typicaldynamicmicinsert(left) andan electretcondensermic.Thedynamicinsert isa DM-9type(MaplinQN39N), the electretisa typeEM-60B(Maplin FS43W).SSBis notverydemandingin termsof microphonefrequencyresponse.Thebandwidthof an SSBtransmissionisusuallylessthan3kHz. AtypicalSSBexciterwill requirea reasonablyflat responsefromabout200-300Hz at the LFendup to 3kHzat the HFend. Bothof thesemicrophoneshaveaspecifiedfrequencyresponsefrom50Hzto morethan 12kHz. Dynamicmicrophoneshaveaspecifiedloadimpedance,which is usuallyin the 200-6000 range.The DM-9specifiesanoptimum loadimpedanceof 6000. It is usuallyacceptableto usea loadimpedancethat isa bit higherthan the specifiedvalue.Manymicrophoneamplifierinputshavean impedancesomewherebetween1kO and 50kO. Usinga loadimpedancethat is muchlowerthanthe specifiedoptimumimpedancemay leadto lowoutput,excessivedistortionand poorfrequencyresponse.
Electretmicrophoneelementsrequireaveryhigh loadimpedance,in the orderof
Polarisingvoltage v+
~tFIGURE 1:
Basic condenser
microphone.The left hand
'plate' is the diaphragm, which is moved inrelation to the grounded plate by sound waves.
This results in a voltage output that isimpedance matched and buffered by the FET.
19
I HOMEBREW
\ Om
~ Magnet
-:I~/ri7
FIGURE2: Basic prD:ipIes ofa dynamicll ~IOI~AcO!attached to the ciapm~.moves in sympathj wiIh sound
waves and diredJy geII.."dLt.::.an
electrical signal iT;:ndudion..
output is taker> from IDe C;.~
end at the juncOOr ~ medrain of the F2" a~rfrn.er. ~two-termhal capsu.es~, don-::have this resisior b-";=~:'1. so <t
is necessap.. to use an ~...er'1a~resistor oor"l€d:ed ~ ~ X
supply. "1}e exterrIal :esistoris typica::, aOO:..:t2.2c;'c for a5-12V suppl.J' and about::tI.C
for a 1.5\ suppl j.. 'he exl:Sf'1a~
ooupling capcritor is ~ypiC2;.,about ::j1L..A dose 'r:specti(r ofthe back of the capsu 1e".1 show
that it is Ierj ~ -::0idePill:, a-aenegative/gnd tef"""'1a! 0ec:a.JSe
it is always OOP~ direct:j
to the meta. case. :..oremat a
three terminal electret car JSUa'~J' be ~to replace a two terminat ~'P8 tr, si'1'1pt}leaving the V+ terminal ..mcoonected.
PHOTO2: My originaldeskmicrophone,which hasgivenmorethan 20 years'goodservice.
severalmegohms.Mostif notall electretcapsules(asin the photo)havea built-in
. FETamplifierthat meetsthis requirement.Theoptimum loadimpedancefor the outputof the FETamplifierwill bea morereasonablevaluerangingfrom 1kOto about10kO.
Asmostdynamicmicrophonesdon'thavea built in amplifier,no DCsupplywillberequired.Takecareto ensurethat DCis notaccidentallyappliedacrossthe terminalsof adynamicmic. Thiscouldeasilyburnout thecoil ordamagethediaphragm.Acouplingcapacitorofabout 1OfJFbetweenthe micinsertand theamplifier inputwill isolatethemic coil fromthe DCbiasof theamplifier.Dynamicinsertswith plasticbodiesareusuallysymmetricalsothat theycan beusedwith eitherbalancedor unbalancedamplifier inputs.Somemetal-bodiedinsertsmayhaveoneendof the coiI attachedtothe caseground. Electretinsertshavethenegativesupply/micgroundterminalattacheddirectlyto the metaloutershellof the capsule.
Therearetwo typesof electretcapsulein commonuse.Onetypehasjust twoconnectionterminals,the otherhasthree.Figure3 showstypicalconnectionsforbothtypes, Figure4 showstheelectricalconfigurationof bothtypes.Thethree-terminalcapsules(a)havea built-in loadresistorforthe FETamplifier;TheDCsupplyisfedtooneendof this resistorand theamplifierAF
20 ,.
MICROPHONEAMPUFIER.~ outputfroma typical dynamicmicrophoneis justa 'fewmillivolts(mV)whentheoperatorisspeakingdirectlyintothe micand eI."ef1lessw'lePthemouth is more than a fewcentimetres awayfrOJ11thediaphragm.Electretmicsproduceslightlymoreoutput:I measuredabout20mVp-pat normalspeakinglevels.Thismaybeenoughto fullymodulatesometransmitters,
but my homebrewrigandsomecommerciallymaderigswill requirea higherlevelfor properoperation.Themicrophoneamplifierin thismonth'sconstructionprojectis basedonmystandarddesignasusedin severalpreviousprojects[1].I usedthe surfacemountversionofthis amplifierasshownon p59 ofthe October2009
MAY 2010 + RADCOM I
RadCom.Youcan, if you prefer,usea DILdual op-amp insteadof the SMTcomponents.;t]e choice of op-ampis not toocritical. Igotgoodresultsusingthe LF353dualop-amp.Evenstandardtypes likethe humbleLM4558can be expectedto performwell in this circuit.~ schematic of this amplifierisshown inFigure5. Someslight changesmustbe"1adeto the circuitdependingonwhether't is to beusedwith the electretinsertor thedynamicinsert.Forusewith theelectret'nsert, a 2k2 resistorwasconnectedfromtl1emic input to the DCsupplyrail.A IfJFcapacitorwas initially usedforCl. TheoriginalRl/R2 valuesof lOk/lOOk were,",sed.On-airtestingof this configurationresultedin reasonablygoodreportson theaudioquality.Somereportssuggestedthatthe audiowasa little bit too 'bassy'comparedto myusualmicrophone.ReplacingCl witha 100nF capacitorgivesa slightbasscutatfrequenciesbelow200Hz. On-airreportsonthisconfigurationwereuniversallygood.
Forusewithadynamicmicrophone,the 2k2 resistorshouldberemovedfromthe microphoneamplifierinputand Clshould bereplacedbya capacitorwithamuch largervalue:about lOfJFworkswellwiththe DM-9 insert.IfyouuseanelectrolyticcapacitorforCl, connectthepositivesideto the amplifierinputandthe negativesideto thedynamicmic. Tooompensatefor the low outputof the dynamicmic, IchangedRl to 3k3 and replacedR2witha 220kO potso that Icouldsettheamplifiergainto the requiredlevel.
DESKMICROPHONE.Thismonth'sconstructionprojectisa desk microphonethat can be usedwith yourhomemadeoroommercialtransmitter.Thedesignis basedon myoriginaldeskmicrophonethat has,so far,givenmorethan 20 yearsofservice,
NegativelGround
PositivelOutput
Output
FIGURE 3: Typical electret mic connections:(left) three-terminal type (see Figure 4a) and(right) two-terminal type (see Figure 4b).
PHOTO 3: Detailed view of how the mic insert is mounted.
I RADCOM. MAY2010 . HOMEBREW]
althoughI havehadto replacethe electretinserttwiceduringthis period.Thismicrophonewasbuilt in an aluminiumbox,which givesgoodRFscreening.Theelectretinsertismountedon a flexiblemetal'gooseneck'removedfrom acheaptable lamp.Ashort lengthof plastictubingwas usedto securethe electretmic ontheendof the gooseneck.Thismicrophoneisshown in Photo2. TheTx/Rxswitchandmic gaincontrolsareon thetop of the box.Thesmallpushbuttonwas usedto operatea 1750Hz toneburstoscillatorforVHFrepeateroperation.
A similarmethodofconstructionwasusedfor the newmicrophone.An aluminiumbox(MaplinABI0 or similar)wasusedasthe base.Unfortunately,adesklampwitha suitablegooseneckwas not availableonthis occasion.Aftercheckingoutsomeofthe localelectricaland DIYstores,I foundthat the onlydesklampswith asuitablegooseneckactuallycostconsiderablymorethan a 'proper'microphonegooseneck.Therearea few possiblealternativesliketheflexiblemapreadinglightsavailablefromcaraccessoryshopsor flexiblelampgoosenecksthat aredesignedto plugdirectlyintoa USBsocket.OneverypracticalapproachasusedbyEI2EUBis to usea lengthof co-axcable(Westflex103 is ideal)asthe gooseneckwithstandardPL259and S0239 connectorsformountingit onthe base.I eventuallyboughta 6in gooseneck(MaplinYW72P),a DM-9dynamic insert(MaplinQN39N)and,tomakea reallyposhjob of it, a mic windshield(Maplin LB35Q).TheSMTmicrophoneamplifier/LPFPCBdescribedearlierwasusedto amplifythesignalfromthe dynamicmic upto the 50-1OOmVlevelrequiredbymytransceiver.ThePTTswitch (althoughpush-to-taIk isn'treaIlyan accuratedescriptionof thistypeof switch) isastandardDPDTsingleholemountingtype.
The DM-9 insertisa slightlyloosefiton the endofastandardmic gooseneck.I useda thin layerof PVCtapeto fill thegap.Photo3 showsclose-updetailsof howthemicrophoneinsertis mounted.It doesn'tlookverypretty,but the windshielddoesagoodjob ofconcealingtheuglybits. Photo4 showsthefinishedproject.
POWERSUPPLY.Theobviouschoiceof powersupplyfora microphoneamplifierisa small9Vbattery,but therearesomedisadvantagestothis approach.If youdodecideto usea battery,youwill needto fit aswitchsothat the amplifiercan beturnedoff whenthe mic is notin use.Itwould bepossibleto
useasparepoleonthe PTTswitchsothatthe amplifierisonlypoweredupwhileyou aretransmitting.Thisapproachcan beproblematicas it is likelyto leadto audible,popsandclickson thetransmittedsignal.Aseparatepoweron/offswitch is probablya betteroption,but if theswitch isaccidentallylefton, the batterywill runflat aftera coupleof days.Youcouldfit a LEDpowerindicatorbut, sinceitwould drawmorecurrentthan the mic amp, it mightdo moreharmthangood.In myexperience,the bestwayto providepowerforthe microphoneisto stealit fromthe radio.This isveryeasyto do if you usea home-maderigandyouhavea sparepinon themicconnector.Becarefulabouttaking powerfromthe micsocketofa commerciallymaderig.Manytransceivershavea DCsupplyof about8V PHOTO4: Thefinished microphone.
v+r ,I v+ I
:
1:
I II II II II II II I
:+ t-- :I IL J
External components
Outb)
Out
Gnd Gnd
FIGURE 4: Representativecircuit diagramsof the (a) three-and(b) two-terminal electret mic typesshown in Figure3.
+8-12V DC
U1A
2
1n
OV
FIGURE5: Thestandardmicrophonepreamplifiercircuit.Seetext for Cl, R1 and R2 valuesfor electretanddynamic mics.
r-
~
,I II Optional I
: 2k2 :I I--~------
Mic
+
10~
?1
100- =10k I 0P22
U1B
7
10k 10k 10k
R2"i 22n
100k
10k I -r 10n
[HOMEBREW MAY2010. RADCOM I
FIGURE6: Connection
diagramforthe Il)icrophoneamplifier(Figure5)and radio.
Amp/LPF
available from the mic socket. It is possibleto damage some transceivers by shorting
this DC output to ground. Accidental short
circuits can lead to blown voltage regulators
and damaged wiring. You have been warned!
Figure 6 shows the configuration of the
microphone, amp/LPF module and PHswitch. Note that I have included the mic
gain pot shown in the photographs.
I used a spare pin in the mic socket to
provide power for the microphone amplifier.
To eliminate the possibility of clicking/popping
when changingfrom receive to
transmit, the
amplifier is
1000 I always poweredI up, regardless of
whether I am
transmitting
or receiving.Ever' :n these
environmentally
aware times,the3-4mA
I draWl by theI w;ca"1p1ffier
car be regarded
J as ;':1Signfficant.: usedihe
microphone for a few days without telling
anyone that I had a new microphone. During
this period, nobody noticed al1y change in my
audio quality. Comparison tests 'la'..-eshown
that the dynamic mic compares quite wellwith myoid electret mic.
There is plenty of room in the base on the
mic for additional circuits, such as oscillators
for Morse code generation, two-tone testing,
subaudible tone, DTMF generators etc---Other possible additions include 'E\IeI
indicators, tone controls, equalisers,
v+
4k7 PTT
r!
AF
PTTswitch Gnd
~ ~ ~---_.
digital speech record/playback, AF or RF
clippers/compressors - but please don't
install a roger beep!
The microphone should be connected to
the rig using screened cable that is grounded
to the rig ground/chassis at one end and the
metal case of the mic at the other. Special
microphone cable has several wires with
separate screening for the audio wire only.
This guarantees good isolation between theaudio wire and the other wires used for PH
and power. As this type of cable is not readily
available, I use ordinary multi-core screened
cable which has a single screen enclosing all
wires in the cable. Old computer keyboard
cables and some types of network cables
are a good source of high quality screened
wire. I have been chopping up old AppleTalk
RS-422 cables to make my microphone
leads. RF signals on the PH and/or power
wires could easily leak into the audio wire
via capacitive coupling. To keep RF signals
out of the microphone cable, it is a good idea
to solder lOnF capacitors from the PH and
power pins directly to ground at the back of
the microphone socket of a home made rig.
Next month: A VHF DDS/PLL frequency
synthesiser.
REFERENCES:[1] HomebrewAugust2006, May2009, October2009.
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