Negative Impedance Preselector

8/13/2019 Negative Impedance Preselector

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Negative Resistance PreselectorAll easy way to lip l ip that old superhet

Parker A. Cope W2G Mf78 40 E Tranquil Blvd

Prescott Valley AZ 86 4

nterference . from whatever source, isthe bane of the radio receiver user.Some inte rference is on the same fre

ency as the desired signal, and theresor much that can be done about tha t.ul there s hope for eliminating or at

east reducing off-frequency inte rferenceith RF selectivity.Of f-frequency interference falls into

wo genera l catego ries: strong signalshat drive the recei ver into nonlinea rperation and prod uce intcrmod ul ationstortion or cross-modulatio n. and spuous receiver responses. RF se lec tiv ityn reduce the amplitude of the interfer

ng signal 10 tolerable levels. The rubmes in the selectivity thai can be

chieved with prac tical and economicalunable tuned circuits. Narrow handidths require hig h-Q tuned circuits, ande Q tops out at about 1 for prac tical

nductors sui table for HF receivers.All is nOI lost. though. The ci rcuit de ribed in the following pa ragraphs cancrea se the Qs o f a tuned circuit 10more

han 1 000 if need be . Th e theory andesign equ atio ns arc given to allowhanges to he made 10 accommodate articular si tua tion or the pa ris at hand .Rece ivers fall into two gene ra lasses: tu ned rad io frequenc y T RF )

superhe terod yne . The TRF is conptually simple and free o f spurious re

ponses. Unfortunately. it is not verylec t ive. A crystal sci is a TRF withoutF am plifier s) , although it may hav est-detection audio amplifiers .

The supe rhet receiver converts the input RF signal to a freque ncy that permitsgreater selec tivity. It offers many adv antages. but at a pr ice; it has spurious respo nses . The proc ess of converti ng thedes ired signa l to the IF prod uces twosi gnals tha i can be rec eived equa llywell. One is the IF above the local oscillator , and a second one is the IF belowthe local os cillator. These two responsesarc separated by tw ice the IF; one is thedesired signal and the othe r is the image.

Rejecting the image respon se is a primary co ncern in superhet receiver design . When the fF is low. the image andthe de s ired frequen cy are close togetherand it is mo re di ffi cu lt to suppres s theimage. There have been two primary solutions to reje cting the image. One is to uscone or more RF amplifiers in front of thefi rst mixer for imp roved RF selectivity;the o ther is 10 use two or more) IFs. In thedual conversion recei ver. the fi rst IF ismade high enough to case image rejection .and the second made low enough to makeselectivity manageable The multiple conversio n receiver is mor e complex , hutenjoys grea ter popula rity today than thesingle conversion types.

The sing le conversion rec eiver usuallyhas an IF in the range of 455kHz. withthe local oscillator operating above thedesired frequency, Thi s arrangementputs the image 9 10k ii i above the tunedfrequency. Tuning to 550kHz in thebroadcast band pu ts the: image at1 60kHz. which is also within the

broadcast band and makes image suppression critical.

The fi ve-tube tD receivers o f the4 s and 50s had on ly one tuned ci rcuitbetween the antenna and the fi rst mixer.In inexpe nsive receivers. a sing le conversion is used and image resp onse is often sacrificed on the altar o f cost. Someinexpensive shortwave receive rs alsoused the same design concept. Need lessto say, they received lo ts of sig na ls. b UIhalf of them were images.

Using RF amplifier stages betweenthe antenna a nd the converter is effect ivein suppress ing the image . Each RF stageincreases off-frequency su ppression by6dB. The number of sections in the tuning capaci tor indicat es how many RFstages there arc: one sec tio n thc smallest) for the osci lla tor; one section for theantenna: and one fo r each of the RFstages. A three-section capacitor indica tes one RF stage.

An image response down 30dB fromthe tuned frequency is pretty good . butnothi ng to brag about. A recei ver with asing le RF amplifier can provide fair image suppression for tuned frequencies upto 7MHz. Above 0m. the pe rformancefa ll s noticea bly, A shortwave receiverwithout an RF amp l ifier will have seriou s image responses at 40m and can bequest ionab le ev en in the broadcast band. only reliable , high-Q RF circuits wereav ailable ...

The response of one single-tunedstag e falls l B for every doubl ing of the Amateur Radio Tx ay Augu t 997 21


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022N3906

Rc OUT

+

ML....--IN

The ratio o f collect or current to basec urrc r n hie . o f Q2 lN3906) is as sumedto be 20 0 (specified as 100 minimum.3(x) max imu m at Ic = lOrnA). Si nce Ic isve ry much greater tha n ld- the current inR, is essentially Ic . To produce Vgs =2.66V I ld = O.05mA. Ie = lOrnA). Rsmu st be 266U. 27012. the nearest standard 5% va lue . is use d for Rs. The effective transconduct ance Grn is:

The forward tnmsconductancc of Q I.gfs. can be calculated with the expression:

Gill = gfshfe = O.l 5 x 10-4 x 200 =O.OSmhos

Us = 2IQ_ = Voff _Vgs)

2.5 x 10 4 (Id = O.05mA )

Th is ma y he rewrit t en to so lv e forVgs /V o ff

12N5457

Rs --::t270 n

which can be rewritten to solve for Voffor Vgs for the typical values of Vgs- id.idss:V rr = Vg, , 1 1 -v(_ld_)1 3.06VdssVgs = Voir [ 1 - (-- d_)1Idss

2.66V fur Id = 0.05rnA)

F ig Z A pr u ul noninverting amplifier

maxi mu m. Vorl mu st he either measuredor calculated for usc in ca lculating ldand gfs. Th e relation ship of ld to IdssVgs- and Vo ff is:

) I - Vgs -tI - dss VoifVgs _ J1 1 _ 1 - -VolT Idss

A practical noninvcrting RF amplifieris shoevn in FiR. 2. T he ga in o f thea mp l i fi e r is de te rmi ned by thetra nsconductance of the pair o f transi sto rs . Gm - Rs. and the ratio of to Rs.Th e g ai n m ay he expressed as :

T he N -c hannel JFET. QI. is a2N5457. 11 is s imilar to the M PFI02. buthas so me ty p ical va lues gi ven in the da tashee t whereas only maximum an d mini mums arc give n for th e M PFI02 . Ty p ica l values for the 2N t 7 arc : lJ =0 .1rnA for Vgs =2 .5V: ldss =3.0mA forVgs = OV. Voff. the gate 10 source vo ltag e that red uce s lJ to zero . is given inthe data shee t as O.5 V minimum 10 6V

Wh en the ju nction o f R and Rf is re moved from the am p lifier in Flg, 1 theopen ci rcuit voltage o f th e sourc e E ap pears at the input of the amplifier and itsou tp ut is AE. Si nce the sa me currentflows in R and Rf. when the vol tageacross R is E. the voltage across Rf mu stbe A E-E or E A - I Wh en the value ofRf is chosen so thai the voltage across Ris E, Rr/R = A - I . Wh en the vo ltageac ross R is E and the j unction of the re sis tors is reconnected 10 the inpu t, nocurrent flows from the sourc e int o th eresistors. Th e resistance loo king int o thej unct ion o f the resis tor s is infinite. andthe conduc tan ce looking into Rf is exactly equal 10 the negative conductanceof R.

VGsf is the vo ltage gai n o r the sourcefollower. The gai n of a source follower isoften assu med to he unity when. in fact . his a lways somewhat les s than that.

Th e gain of a so urce follower is :

Th e gain is nca r un ity onl y when theproduct Gm Rs is m uch greater than on e .Gm is th e change in the c urrent in Rs fo ra change in gate vol tage. Th e cur re nt inRs is essential ly Ic. Since ld = Ih' and Ie= Ihhfe, Gm can he expressed as :

I AEE rv IIR RfL

Fit: I A cone ptu l ~ t I \ resisI I l in Il erll/or

C = the circuit s resonatinglance in pF

R = the eq uivalent paralle l resistancein 12When the tuning capacitance is 20pE

a bandwidth of 28.5kH I. requires a parallel resistance o r ab out 2XOkf . Th is pa rall el resistance is much higher than thato f any prac tica l inductor. But . adding anegative resistance acr oss th e resonantci rcui t increases the ef fec tive resistanceas wel l as the Q.

1\ negative resis tance is not a co mpone nt yo u can bu y from your loca l electro nics supplier. bUI it is something yo uca n ge nera te wit h a simple ci rcuit. Aco nceptual negative resis tance genera toris sho wn in F ig . I R is the total resi stance appe ari ng 'II the in put of the am plifier. the ef fective pa rallel res istanceof the source and the input resistance o fthe amplifier. T he ga in o f the amplifie ris A and the output is in phase with theinput. Rr prov ide s feed back from theou tput ofthe amplifier b to R.22 73Amateur Radio Today August 1997

whereBW3 = the 3d B bandwidth in HI.fo = the center frequency in H I.

bandwidth, Therefore. when the response is down 301l B. the bandwidth is32 25) times the 3US bandwid th. Tohave a frequency (the image ) 910kHzoff-frequency be down 30dB requiresthe 3dB bandwidth to he about 28.5kHl .A tuned circuit with a Q of 60 at 7\Hb.has an equivalent parallel resistanceof a bo ut 6 9k and a ba ndwid th o f ab outJ I7kH /.. A bandwidth of 28.5kH1,requires a Q of ahou l 246.

T he b an dw idt h o f a parallel-tuned ci rcuit is related to the circuit Q as follows:


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The equation

..;.1......,= _1_ +_1_Rdesired Rtank Rn

The voltage ga in o f a source fol lowers calcula ted with eq uat ion 5 to Oc 0.93:

7 Amateur Radio Today August 1997 23

+ Q2L 2N3906 r-,- ,2N5457 I \- ~ Q so .

c i NaJ: Re zten; 1

Fig 3. A negative resistance can increaseR selecti v yincreased gain could exceed the maximumstab le gain of the RF amplifier. This cau-non is appropriate for hoth transistor- andtube-type amplifiers . When the Rf selcc-tivity is used in the antenna section beforethe signal experiences any amp lification ,strong off -frequency signals can be sup-pressed he fore they can drive a stage intoits nonlinear rcgrons and ge ne ra tec rossmodulation or intermodulation prod-ucts. Fig 4 shows the simplified antennasection o f a receiver. The negative resis-tance block is the circuit shown in . 'ig .T he negative resistance generator can

hring the o ld single co nvers ion receiversback to service for ju st a few dolla rs anda coup le of hours with a so lderi ng iron .Yard sales often have old shortwave ra-dies tha t can yield the bas ic stuff forturn ing a sow 's car into a silk purse . I fpush comes to shove, you could use anold shortwave receiver that otherw isemight not he worth reviving because itdoesn' t have tubes in the socket s (tubesare expensive these days. if you can findthem). I f it has the RF section reasonablyintact, coils. band-switching. and tuningcapacitor, it can be used to build anoffboard preselector. True, the trackingo f the prcsclcctor and receiver is a prob-lcrn. but the improvement in inter ferencerejection can be worthwhile-espcc iallyin the 40m band. where the high-poweredbroadcast s ignals ra ise Cain. faI 7 OOPS R -r-

NESISTANCECIC .. 15pF kl365pF

Fig 4. Pt rtial schema tic of the antentlasection ofa simple receive

very stab le ampli fier. I f Ie tried to r iseVgs would rise , Id would dec rease and Icwould decrease .

The amplifie r shown in 3 has Rccomposed of 30012 fixed and 500n variable. The gain varies from 2.0 to 3.8When Rf is 9 1k. Rn wi ll he va riablefrom 91k to 33k. While the negative resistance can be contro lled with e ithe r Aor Rf, it is preferable to have the gainlow becau se a lower Re resu lts in greaterbandwidth of the amplifier and smallerIX: voltage drop across Re , wh ich willreduce the DC power supp ly voltagerequirements .The power supply for the negative resis tance genera tor is nOI critical ; theonly requirement is that the voltage behigh enoug h to keep the 2N3906 out o fsaturation when Rc is maximum . TheDC voltage. co llector to gro und , can beas high as 14.4V with worst cusc component values. which transl ate s in to a su pply vo ltage of about 15.2V. Themaximum VOG (drai n to gutc) of the2N5457 is given as 25V. Therefore. thesupply vo ltage can be any thing from15.2V to 25V. The cu rre nt drawn isabout lOrnA I 2 mA wo rs t case) whichcan probably be sto len from thereceiver ' s supp ly without ill effect s.

The neg at ive resis tance gene ra tor ca nbe built on a one-inch-square pcrfboardthat fits nicely on the RF (antenna) section of the tun ing capacitor. Bandswitching usually enta ils switchingco ils , and the Qs may vary from hand toband and with frequency over a ba nd .Therefore . th e se lectivity contro l Reshould be co nvenient to adj ust whentun ing . For maxim um bandwid th, thelead s from Re should ru n din..rc tly to thecirc ui t board and not be dressed in tu aharness or agains t the chassis .

It is worth not ing that if the tota l resistance across the tank is nega tive. thecircuit will have infinite Q and willosci ll a te . He a thk it employed thi stechnique in one of the ir shortwave TCccivers years ago . They used it in the IFto ac t as a combined selec tivi ty contro land BFO. Need less to say, it wa s not avery satisfactory arrangement .Variable RF selecti-vity ca n be use

ful even in a receiver with an RF stage .The only caution is to usc the high se lec tivi ty in the ante nna section . If the highQ tuned circuit is the plate o r collector load. the gai n of the Rf amplifierwill in c re as e as the Q in cr ea se s . The

(G R + I ) = 0.93m sGsf =

__1_ = _1 +_1280k 6Yk RnI_ I 1 _1Rn 6 Yk - 280k - Ylk

RrRn - (A - I)hows that a 9 1k negative resistance cangenerated with Rf Of 9 1k and an amfier ga in of 2.0 . or when Rf is 150k an

mplifier ga in o f 2.6 is required . Givenhe uncertain ty o f the Q o f the tuned ci ru it. it seems prude nt to make the gainriable by varyi ng Re . Chang ing Rees not change the operating po in t o f

he amplifier as long as Q23 does nottu rate. Thai is . as long as Vee isreater tha n IV.Th e DC operating point o f the amplier is de termined as Rsle. Since Ic is inependent o f co llec to r voltage whileee is greater than Vccsat and I(( is indendent o f V lis while Vds is abovech-off. the operating po int is inde

nt of the supply vo ltage. The ope rting point is determined by Rs and le.he high negative feedback leads to a

Tbc negative resistance is con trolledA and Rf. If the resistance across aned ci rcuit is 69k and the resistanceeeds to be 280k for the des ired banddth. the negat ive resistance neededn be determined as fo llows:

the va lues of VG sf and Rs estabshed . Rearranging the eq uat ion to so lver Rc yields:

Rc= R(VG ; _ I)

(I- )A = VGsf RsThe ga in of the amp lifier can now bealculated with the eq uati on

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Negative Impedance Preselector