CA2199935A1

CA2199935A1 - Quadrature modulator with integrated distributed rc filters

Info

Publication number: CA2199935A1
Application number: CA002199935A
Authority: CA
Inventors: Paul W. Dent
Original assignee: Individual
Current assignee: Ericsson Inc
Priority date: 1994-09-14
Filing date: 1995-09-14
Publication date: 1996-03-21
Also published as: JPH10505979A; AU3554295A; MX9701906A; EP0781467A2; SG70628A1; CN1156963C; FI971053A0; KR100384230B1; WO1996008865A3; BR9508952A; FI971053A; US5629655A; NZ293170A; CN1290065A; CN1063300C; CN1290064A; KR100385761B1; CN1162373A; US5530722A; FI20040794A

Abstract

A quadrature modulator for generation of complex modulated signals includes a unique pair of balanced, low-pass filters which, through a switching arrangement for switching into and out of the circuit distributed RC line sections and corresponding ground resistances are able to redress problems associated with manufacturing tolerances.

Description

wos6/oss6s 2 ~ ~ 9 9 3 5 Pcr/usss/ll745 QUADRATURE MODULATOR WITH
INl~E{;~ATED DISTRIBUTED RC FILT~S

Baclc~,ulld 1) Field of the I~ u.
The i~e~Lio~ relates to m.othoflc and a~ s for the re~li7~tion of ~ u.~ band-stop, band-pass and low pass filters as ;..~ i circuit el~ on a silicon chip for use in a yl~z~ mnd~ tor which can 10 gell~,ase cQmrlPY m~nl~t~d radio Ci~ lC

2) D;~ -u~;o~ of Related Art.
The prior art for the co~ ion of ~.~u.~ selec~ve filters inrllldes (1) passive i -z ;In r flters; (2) passive r~,~iator or filters; (3) active ~C filters; (4) ~ v~l ~ RC filt :rs; (5) gyrator~ o filters; (6) IIA~ -;C5;1-n line or wa~uide filters; (7) ~wilched C~1 ~C;IO~
filters; and (8) digital filters, each of which is ~lic~u~ed below.
The consl,uc~n of l ~ or~l)A -;lor filters on a silicon chup is conC~inP~i by the very srnall ;~ J~ e values that can be achie~d with spiral m.ot~nt7~tion p~ rnc within a frequency range above about 2 GHz.
Passive .~i~or~p~;tor filters can only ~ .f~ a limited subset of the possible ~qu.~y responses that might be ~ ed, and at low fie~uencies are limited by the available RC E)r~ that ~~n be made while also being limited at high fre~u~nries by stray (~ lA~ ) c~ra~itAnre and , A n~

W096/08865 2 1 ~ ~9S~ P~ 5,ll745 Active RC filters can provide useful perfu..nAllr~ up to a few m~g~hPrtz, but are limited by the pe,ru~ n~e and bandwi~llh of amplifiers as well as the ça~ effects "-r~ "P~l above. Ullfo~lu~ly, the amplifiers co..~....lP power and limit the ~l~c ~ange.
D;~ll;b-~ RC filters are, on the other hand, i~h~.c,lLly based on the ~,A~ ;c cap~ e aDd .~ e F~ f t~, such as describe~ in "Ti~1c~ ,a ~ 7~cs Filten CMOS", by Katarina ~n~son and Mats k~ n LIJ1~XI(TEIE-70Z9)1 pp. 1-26 (1987).
Gyrator~ ;ror filters use an active i .,pc~ L~g circuit to 10 make a c~l ;lor function as an ~ ic~o~, so that LC equivalent filters may - be built. These circuits are useable for bAn~lp-Acs filters up to a few mPg~h~m. The Gyrator~p~;l~- filter can be classed as a form of ac~ive RC filter.
T~ ;Qn line or waveguide filters require el~ ....~t~. that are 15 typically a quarter w~ Ih long so their co~hu~;~ion on a chip is limited to the micro-wa~e~ above 2 GHz.
SwiLched cap~;lor filters operate accor~ing to a llu~ber of dirr~re.
rs, but all require ~ lor awil~heS to operate at a very much higher frequency than the O~ali~g rl~que~,y range of the filter. This 20 l~i.hi~-~ their use to a few hu~d kilohertz. Moreover, the dy~ic range of swiL~hed~rA~a~;'or filters is limited by their high noise levels.
Digital filters are very flexible in the frequency ~ se l~elluiht -- - they can realize, and have the advantage of no toleranres On the other hand, the signal to be filtered must first exist in digital form and the required 25 analog-to~igital CO~ Lûla restrict both the dy~ic range and speed.
Digital logic power co~ul~pLion is also a factor which le~ll;c~ such filters to the 300 kHz region or below in practical applirAtions The fi~lue~ ra~ge upon which the present invention focllc~s is the 0.3 M~Iz to 300 MHz region. This is above the range of most of the 30 ~Cl~n;~lues m~rlt;0~1 while being below the range for I~An~ ;On line ~ i ~q~s5 solt~tinnc. Hithcrto there hds been no p.a. 1;.'~l silIcon~ r~ ble solnnon for ~se ~e tl~ 1~5 of L~ ue.,~, vhich el~o...l~Acs virn~ally the en~re radio c~ ;t~ c Lc.lu~ cy ~ LL~. Acco~ ly, the ~ L ~
was co~e;~ to addres~s this ill~yOlL~L ~ange of rl~lv~ ~;es The prcsent S i~ LiOll makes use of cQn~et!ri of the t1i~. ;b! ,~-1 RC r~ t h.. ;.ll~c5 ~ ;oll ~i above.

Suu~
The presen~ invention r~lates to m~thot1s and ap~ald~s for the i7~tit n of r~ue~ band-s~op. band-pass and low pass filters as 0 ;. ,h l At~ ~1 Ci~ Uit Plt ~ '; on a silicon ch~p.
The iu~ e mPthnd allows the m~nllfat~ re of co--~ u~-time, analog filters in r~ .c.lLy ranges not cou~ ~ly cu~ by other, known .,es. Suchfiltmare~pically~ u~,dinthe g~n ~a~ n of c~..pl~~
radio signals with the aid of digital signal ploces~ol~ and 5 ~l..a.~ ,, m~ tnr~. The i,~ . filters are aimed to be suitable for CO~hu~ iO~ as part of i~t~r~ f d circuits for analog or mixed analog/digital radio co.. ~n;- ~;onc signal plOC~ applif~l;. nc In the prese~ ~...~i~n, new d~hi~u~d RC filter ~hu.;~.S and appli~ io.~C ~e ~lic~losed aud in particular, means to o~ o~e the 20 plùble us created by, ,; - -r~- ,--- ;--~ tole.~s in the ~SiaL~ve and ~lir~ iC
layer ~u~ ies. The ~~ RC filter ah~lfL~u-, include means for s~le~ ely awitChil~g in and out of the circuit I~Clc ~ RC li~es and for s.l~li~.ly swi~l~ g in and out of the circuit iu~....- ..~i.l nulling ~e~ia~ù~a. Several em~o~ are ~icrlûse~

W096108865 2~ 35 P~ 9~ 745 Brief De~ ion of the D~AwinPs The ~ve. Lion wi71 now be d~ ;be~i with ,cf~.c.~ce to the ~f o'~ all)~ g d~d.. i~ga in which:
Figure la ia a partially s~h II-l;f ~;Z~IAIII of the a~ e of a S ~7;~l l ib ~ RC line in aC :o~e with the prescnt ~iol,;
Figure lb is the cirwit symbol for a di~ u~d RC line filter auch aa shown in Figure la;
Figare 2 is a ~fh- ~I;f ~ lAIII of a prior art t~ ,;hl.~.7 RC
device;
Figure 3 ia a s~ ;t t7i~m of a prior art ~n~17~h7re m~llztor ~~ t for ~ a7l aLl,i~ mf~ t~ signal;
Figure 4 is a s~ ~II-l;r Ai~am of a ~ Al~ c m~x~ tt r in a~co~ e with ~he present i~ . sioll;
Figure 5 is a s~h~ ;f tlia~am S~w~g prior art Gilbert mixe ~ as b~l~n--~Y7 m~ t7111~t~
Figuse 6 is a srh. Il~l;C Ai~Tam of a section of a b~l~nred filter in a~u~ce with the present i~ion;
Figure 7 is a s~ ;t~ Ai~m of a complete b~l~nred filtes in - aCCCll~lllC with the present i l-ellLio~;
Figure 8 is a srh. .I~;t~ Ai~m of a a~wiae-a~ijlia~le RC line in acco~ ce wi~ the prcaent hl u~ion;
Figure 9 is a sr~ ;r t7i~g~m of a ~wi~hcd n~lling les~lur combinable with the ~wil~d RC line of Figure 8 in acco~nce with the present i~ Liùn;
Figure 10 is a sr~ m sl~w~~g the use of an adjustable notch filter accu,-ling to the prese~t invesltion in a feedb~ loop for o~ an ~ t~hl~ c ~mI~lifi~r ,~ae;
Figure 11 ia a "h. 1l~ 1 " 1 of a plef~ d ~ I.. ~1l of a awi~h~d-~nable RCNUIL device in acco,d~nce with the present i"~.~Lio~; -q3S

WO96108865 .~ 3S/li745 Figure 12 is one possible a~ise-adjustable mllIing resistor for use with the awi~ched-~nabk RCNUIl device shown in Finure 11 in accG~ e with the present i~ Li~;
Figure 13 is a srh ~ l;r (~i~m of a p,efi .~c~ lge 1~ ~.r of a - S a~e ~ hle mllling resistor for use with the swiL~he~-tunable RCN~
de~,nce sho~,vn in Figure 11 in acco~d~ with the present invention; an~d Figure 14 ia a gsaph of the ~ u~ ~yo~ae of the filter shown m Figure 7.

De~ailed Des~ ion of ~e ~f~ d Embo~
The il~e.~Li~ filter COnaL~u~LiOn in~ s a dis~buted RC Iine as shown in Figure la which utilizes the sh~t-le~iaLiYiL~ p~pc.hes of deposit d co--~h~ films such as a polysilicon film ~iali~ cr 10, and the c~ ..re per-unit-area y,opc,~ies ~L~ I the ~ c filter 10 and a cQ~ lJ~ r., plate 14 (con~d to a cu.. by a co.. ,r~-~;on point 14a) with a thin ~irl~-~l;r layer 12 i~,~o~ ~-.~Q the layers 10 and 14. The , filter 10 ;..rl~ s an input con~.r~l;o" point lOa and an output Con,~ ;on point lOb.
In order of the layers' a~ e from ~ r~ to top level, the filter is composed of a ~nl~ 13 c~ , e.g, silicon, ~1nmin~
~pllinm ~. ~. ni-lr, sapphil., or pol,y~ide, an inml~tin~ film 11 c~
silicon ~lio~ , alllmilla ~llnlm ~ , 5~)phi~c, polyamide, etc., a CO~'J'-I;~e plate 14 of heavily doped polysilicon, ~ l" gold or ~e like, - -a thin fi~ . ;r hyer 12, and a lC~ , filter 10 co~posed of polysilicon or the like.
Rr~ ol~ formed by the polysilicon film 10 are ~eated as ~i.ctnh over and in~ At~ &om a c~ l plate 14 and, thus, as a distributed RC
line that may be ~ ;hed by the L.~ -A~te per unit le~h, c~p~c;l;~ e per unit length, and kng~h.
lhe circuit symbol for a ~;~l- ib~ RC line is shown in Figure lb.

tq35 WO 96t08865 1~ /U~9SI11745 Such RC lines have an i~,~ low-pass type of freque~cy ~es~ol se that i~'t ~ 5 higher r~. nr;rS, but the alt-off is rather geD~le. Sharper cut-off low-pass filters gen~ y achieve their Ch~ '5 With the aid of n-Jl hr5 ill the stop band.
A notch in the ~ u~ J~u~e may be formed using a di~uibu~d RC line by co~ g its r~p~ or plate 14' t~in~ to ground ~ through a r ~i~l 21 of s~ Yalue, such as sho~n in Figure 2. For l~llifUll~ RC
lines, the ~tch is cQ...pl~ h~ when the resistor C~ f ~ to ground has the appl~ t~ value 0.056 ~mes the total ~ u~-f~ e Rtot of the l~ , filter 10', and the notch L~lueh~ is app~ tl l~ 11.2/RC
radians per second where Rtot is the total ~u~-~ r~ll-p and C is the total Once a co..~pl~rt or par~al notch can be formed, other C~ U~
Q~.~S ca~lbe ay~ .f~ 1 suchas b~ l~, orb~ ss, t~e latter by inrhl~i~ the notch de~ ice in the Ç~ba~ loop of an ~ pl;l~P~ such as shown in Figure 10, ~ s~d below.
Accol~g to a first aspect of the in~ ~io4 ~-~;3h lu'~, b~l~nre~l, low-pass filters are plo~ided in colij~llll l;~ln with a so called ~ mot~ tor for the ~u~o~s of ~ p an ;~bih~jly m~ll~t~d radio r~ue~
signal.
Acco~ling to a second aspe t of the ~e~ion, means tO o~,~ullle the high pro~3octinn ylucess spreads (i.e., der~Liull from ideal values on iVe arld ~1irl~ layer plo~lLies) are ~.ovi~l. In some processes, typical spreads on the sheet l~ia~viLy and c~ --fe per unit area ~J~IA~ i can be up to 15% on c~r~cit~nre and as much as 100% maxlmin ratio on sheet ~ia~i~ily. Without the i~ means, the notch fi~u~lc~
give~ by the RC ~l~lucl could not be x~ to within an octave. The means c~n be used to bnng the notch ~ within a desired toleranc~ when such ~l~esses are used. Ihe present i~ ion achi~ s this W096108865 2 1 Y 7 ~5 1~1~u53slll745 by eLr~li~ely ~,uvi~ g a ~h~wise~ able li~e leng~h tha~ can be o~JIA~ lf~ in-circuit to set the filter ~ ue~ to a desired value.
Figure 3 shows a pr.ior art All~llg~ of a .l~JAA, ~ , mod~ or for hr,;,;..~ an a~ .~i3y modlli~3 sig~al. A digital signal p~UCCSaOf S (I)SP) 30 r~ r~s ~me-spaced ~-~"1~k5 of the real a~d ;---ap;n~-~ parts of a desired complex m~hll~tinn The real part is give~ by the desired amplin~de ~mes t~e cosine of the desired phase angle, while the im~gjnqry part is given by the amplitude ti~nes the sine of dle p~se angle. In this way both Ampli~de Mo~lllqt~ (AM) signals or Phase Mo~ (PM) sign-q-ls can be 10 ge - ~tt'~, or sigDals c~ both, the result of which is g~on~AIly known as complex m~lll~t~ cignq1C. The 1~ I SA~)1~S c~q~ d by the DSP 30 are ~ l; . ~ to a pair of Digital-to-Analog (D-to-A) C~ a 31 that C~ .L each ~ l sample pair into a pair of analog ~roltages lcnown as I an-p~ase) and Q (Q-.~-~l. Al~e) signalC. A s~u~ ~e of such .. - ;r~l s~mrl~s t,~ ~ ~ 5 I and Q wa~rlJ~s but in a a~wiSe f~hi~m The steps in the w~_fOlula callsc lm~lecir~hle spec~r~l components that would i~.f~.e with l~jaC~I radio ch~nn~lc unless ~u~ . Some g .~hn--~ 5 for ~to-A Cù~ -aiull ~u~i~C imespolation h~ n saL~pl~
giving sloping wa~fO~ 7~ sample ~ralues, which .e~l~ces but does not ,--rr;- ;- .~lly el.~ .Ah tl~e ~,,,dcsu.d cu~u~O~c~i. ~onCpq~t~nt I a~d Q smoo~ filters 32 are nf~e~,-- y. Th~e are low-pass filters that pass all m~lllAtion spec~l Cu~u~ La of in~erest but ~u~p~ss the higher çl~que~ Cu~uu~ a of the -~ C~ th the stepwise or ~ic~"..... .....se linear I, Q wa~efo~s rom the D-to-A CO~ ula 31.
The a~oull~ed I, Q wa~;rullus are applied to a pair of b~l~nre~l mo~ torc 33 lo~ r with cosine and sine carrier r,~ C;~A1C~ this &~ ac -- ~~ being known as a ~ mod~ tor~ The ~ c~nt d~scr hecl so far and i~ 1 in Figurc 3 belongs to the well-hlown prior ar~ ~ ~

21~i935 It is ~ )O~ L for ~ signal y~ lr"";o" that (1~ the two b~
mixers are ~ --, r~iy ~ ( hA1 (2) the levels of the I and Q sigIlals are ~ controlled relative to each other, and (3) the b~l~n~ed mixers have low carrier lealcage or offset, that is, the ou~ut sig~al of a b~l~nred 5 m~~ tor should be zero whcn its ,~eeh~. I or Q mod~ ti~ signal iS
zero.
Since the I and Q signals ~ary from pOaiLi~, to ~Li~'~, if a circuit is l~.luh~d to operate only from a single pOaiLi~_ supply, then the zero point of an I or Q wa-.fOl~ cannot be ~fin~ tO be zero voltage, but must be 10 defined to be some pOai~., r.,f ~.,~ voltage such as half the supply voltage.Then when an I or Q wa~,_fo,~ swings below this l.fe.e~ce voltage it will be i ~ let~ as ~ , and ~I:iaili~, when it swings above.
U~lfulLu~l~, it is ~1;rr;-~ to g ~ a l~f. .e~ce voltage from the DSP 30 that is e~actly equal to the voltage the D-to-A cou~ s supply 1~ with an in~ut "..."~ al value of zero. This p.~blem is o~ .-;u~uc in the invention by use of the b~t~n~ed co..r;~ Ati~ shown in Figure 4, which uses special D-to-A co~ i~ t~hn~ es to g~! uLA,r I and Q sign~l~ as well as their co,-,pl~ "rr~ I and Q.
In ac~ ce with the present invention as shown in Figure 4, the 20 r~ t I and Q signals fmrn DSP 30' are l.,~ r. l~d to a delta-sigma (~-~) convertor 41. This device is built a~o.~hl~ to known art to ~l.,.dtC a high bitrate st~eam of bina~y '1's and 'O's having a short-term ave~ge value p~u~,lional to the ""~ input value. With a m~iml-m possible uu~.ical input value the bit stream produced would be 11111 ... (the 25 voltage of a '1' con~lition being e~ual to the chosen supply voltage) while the ",;";"""" "".", ;~l input value will g~ Ate the bit pattern 00000 .... A
half-scale uu u~ical input will ~nluce the bit s~n 1010101010 ... ha~ing an average voltage e~ual to half the supply voltage. Accold~ to an aspect - of the present .~ , ema iu~.~r gates 42 are provided at the output of 30 each delta-sigma cou~.~Lur 41 to ~~ tion~lly ge~ ~d~ the a~mplP.~

~1 q~935 wo 96/08865 ~ ,ss/l1~4s bilah~s. That me~ns when delta-sigma CO~ a 41 p~luce a bil s~ream 100100100100 ... havi~g a mean of 1/3 the supply voltage, the c~....p!~ .. ,r~. y bit s~eam wiIl be 011011011011 ... having a mean of 2/3rds the supply voltagc. -The dirr~ ce ~I-._cn these two is 1/3-213 = -1/3 of the supply voltage. If the C~ V~ I~CS 111011101110 .. having a mean of +314 of the supply voltage then tbe cc....pl~ signal OOOlOOOlûOOl ... will have the mean 1/4, so that the dirr.,~ce is 314-1/4 =
+1/2 supply. ~OI~ce~uf' l~ by using the diLr~ e ~ n the col,~,.tor output signal and its cc ~ ,~ to ~ L an I or Q signal, the value 10 .e~l~d can be pOaiLi~. or ~g~ even w,ith a single pOailiV~ voltage supply, and no 1~,f~ ce voltage necd be g~ h ~ The b-q-lq-nred mi~crs 43a and 43b are tLe,efo. ,y~o~ided with ~ql~nred, two-~vire inputs rather than singlc ~ mputs, that are l~U"si~. to the diLL.~n~e in the signals on the two wires and ~espollaiv~: to the ~hsollltf~ or c~.. -n-mode voltage (sum of the voltages) on the t~vo wires.
High bitrate delta-sigma mo~ ti-m bibl.~s are simply converted to the analog voltage they ~ by fol~ng the IIWVi~g a~.~a~,c voltage over a large ..~..k. of bits. This rnay be done using a continnonC-time, low-pass filter having a ba~ Iwi~llh which is a small f., ~;o.. of the bitrate, 20 but still ~ e~t to pass all desired mod~ ti~nn cw~ . For the b~l~n~ed sig~lal confi?J.~d~ n developed in this ~e.lLiùll, b~nre~l filters 44 are i~,~oscd b~ the delta-sigma col~ r outputs and the I, Q
b~l~nred m~ tors 43.
The b~l~nred mo~ tors 43 may include so~alled Gilbert ixers 43a 25 and 43b such as shown in Figure 5. As shown in Figure 5, the b~l~nred I
or Q inputs 50a and 50b of the Gilbert mLl~ers is applied to the b~s of two ~AIlc;~tu~ 51a and 51b. The P ~ .s of the two l.~r,~ ol~ 51a and 51b are c~..."". nly COI...f~ through l. ~e.,L.~ Ola 52a and 52b to a cu."".on - bias curre~t source 53. Each of the coll~ctors to the two 1.,~ .5 Sla and Slb are l~ ly CQn~ tO a pair of comm~ nly COIIIIf~t~ of 2 1 ~t~
WO96108865 P~ ,9S/1174S

~ - 10-~o pairs of !~ IO-~ 54a, ~4b a~ 5~a, 5~b. I~e base of o~e r,~
54a, 5~b from cach of ~e 1 ~ lo~ pairs 54 and 55 are commonly c~ to one side of a cosine or sine signal ~ .~ or 56, with the other base of one t~AII~ S4b, 55a of each of the ~ r pairs 54 and 5S
5 being c~ ronn~t~ ~1 to the-other side of the cosine or sine ge ~ t,r 56. The CQllPCtOrS of one ~ ror of each of the ~o !~A..~ ..r pairs 54a and ~5a ar~ co...~ ly cn~ i to one ou~put line 57a, with the other cQllPrtr~s of onc FET of each of the two 1~ pairs 54b a~d 55b being co... -ly co~ r~i to the o~er output line. These ~ nr~ m~ tor~
10 can be formed in the same ~ 1-AI-~ as the b~l-q-nred low pass filters.
Tbe ou~ of the ~ q-nred m~~ tors 43a and 43b of Figure 4 are added ~ r by an adder 43c, to result in a c~ m~lllqt~-l radio signal.
qlqnred I or Q input s;ignals swing around a m~ voltage of 15 half the supply (e.g., around 2.S volts for a 5 volt supply) but ~e qmplin of the peak-t~pea~ voltage e~ 7m should be so~.~ lcss, for c~l,k, ~/-250mV. The delta-sigma co~e.~or output, hu.._~ l, can at its c~h~CS swing ~.~n O and 5 volts on each output line or its ccmrl~m~nt and tL.~,fu~ a-1120 ~ of the b-q-l-qnred mode signal is calle~ for 20 while no ~ nn~ n of the C~ mode voltage (2-5 volts) is this e~llbo~t;....o..t A basic filter secdon 44 acco~i~g to the inven~ion which ~rcompli~hPs the a~ n--~l jon of the ~qlqnred mode signal is s~own in Figure 6. Two i~ntir~l RCNULL devices 61, 62 provide a low-pass 25 filtrrin~ action to both bql-q-nred (push-pull) and cQmmon mode si~qlC~ with a notch in the fi~ o~e.
The b~l~nr~d filter inr~ ps two input ~rminql~ for I, I or Q, Q and two output b ".;..~tc SOa and 50b, and a co....n~n tP~minql c~.n~ct~ to ground. A series resistor Rs is conn~b ~ n in~ut t~ IlllnAl I or Q
30 and an output ~ ...;..~l 50a, and . n i~Pn*rq~l resistor Rs is c~n..~r~l ~t~

2 i q'l93~

a seeond input ~ Q or Q and a se~ond OUtpUt f~-in~-~ 50b. Each of the ~ a Rs can be formed by de~GsiLL~lg a pattem of l~,siaLive ...~t. ~
over ~,~c~ c~ plates with an ~.~c~g ~;c~ ie layer, such as shown in Figure 1, to p,uvide t~ r;hv~l d ~ .re be.~.. cn the ~ iaLivc pattern and the cQ~ J~ ., pla~e.
A rc~iaLù~ or lc~iaLu~a RNU~. are co~n each of the cor~h~ platestotheco~ t~ ...;n~l, or~ nthe CQ~ plates, or both. A shunt resistor Rp iâ co...~t~d ~~ n the output ~ ",;..;~ of at least one of the filter ;~ ,,c (when ~as ~l;c~ e~ below with .ef~ .c~ce to Figurc 7)~
This filter 44 has a c~ mode ~n of unity at direct current and low ~i. yv 1~ ;f s, as there is no r~ n~e to ground. In other words, the pair of bql-qnr~ ldw~ass filters ~to a dirr~,~ degr~c, or not at all, the c~ mode signal defined as ~e sum of the voltages on 15 the two input line;s or two output lin~es, cu l ~)a ~,d tO the b-l~n~--ed mode def~ed as the dirr."e~;e of the voltages o~ the two input or output lines. In the b-q~ ed mode, the ~ Jdl;.~n is Rp/(2Rs+R~) due to the lesi~lo~ R~
Conn~ the output t~ in~l~, This may be se~ to 1/20 or other desired value less than unity by choice of t~e shunt resistor R~ relative to 20 ~,Ye filter Rs. The desired value is defined as the dirr~,~ in voltages h~e~ the two i~put lines or the two output lines.
Another effe~--t of the shunt resistor Rp is to e ..l.h~;,. the high U,~lCy respo~se in the bql-qnre~l mode relative to the low ~ u,"~;y 1~J~JU~ as the high ~ U~ ;on tends to unity. This has the 25 desirable effect of sl~c~g up the rate of cutoff. The rate of cutoff may be further ~ ~d by e,.~al ~ of the RC line.
A comple~e filter design cor.~ of a c-qsr~ of such b; lqnrc-l sec~ion~- is shown in Figure 7. A series of bql~qnred RCNU~l devices 70, 71, 72, 73, each rk~ -t, ;~i by a stamng line width, an ~A~n- ~t ~ f ;~
30 factor (MAXI~N width ratio), a total le.;~ e Rtot and a total c~r~ e WO 96108865 , _ P~ /11745 ~ ~ t i'~3~

Ctot are r~ra~ co~ I by de~o~ g flicl,;h.J~ RC lines having ~ia~ AIl~ llC d~pos ~ l over a co~ e pla~e over an iJ~lL~
. ;r film on a ~ lb~llA~ such as show~ i~ Figure 1. Shum l~la Rpl, Rp2, Rp3, Rp4 are con~ the ou~ut ~rminAls of each section to provide ~ lAt~-l A/~ A~ The total Alr~ AIion in the b~l~nr~ mode is set to the desired value by choice of these shu~t l~ r~
but there is a c~ -- of ways to do the ~a-,~ om all Al~ llAl;on ill the first s~on Rpl to all All'~ I~IA~ the last sec~ion 1~4. An o~
;(m of the A~ ;o~ b.--.~n the s~l;ol~c can be fou~d by ~ial and 10 crror using CQ...~ cimll1q*nn vhich gives the sl~e~l rate of cutoK
Li~ewise, an u~ set of line widtbs and ta~ers can be fouDd within C~ A;~ on .~ line width and ~I-A~ allo ved filter area that gives the sl~L ~ate of cutoff. The values of a near ~u! ;l~ .l design for a cutoff ~equency of 150 ~Hz are shown in Table 1 a~d its re~l*~
~ u~ o~ is shown inFigure 14.

W096/08865 ~ i 13 ~ r~~ 9S/ll745 W~ LINE WIDTH AT THICK END= 20.00000 MIC.RONS
TAPERING FACTOR= 20.00000 R~. TOTAL RESISTANCE= 118.83687 kQ
C~. TOTAL CAPACITANCE= 47.53474 DF
RNUUn NULLING Rk~ OR= 3.16380 kS2 ATrENUATION FACTOR= 1.41410 R~. SHUNT ATIENUATION Rl~ I OR= 286.97623 kQ

71 W~ LINE WIDTH AT THICK END= 1.01000 M~CRONS
TAPERING FACTOR= 1.010000 R~ TOTAL RESISTANCE= 271.01572 kQ
C~ TOTAL CAPACITANCE= 5.47452 pF
RNwn NULLING Rk~ lOR= 15.22687 Ic~2 Al I k~UATION FACTOR= 2.82820 R~ S~IUNT Al-rENUATION Rk~l~ l OR= 194.20894 kSl 72 WMAX73 LINE WIDTH AT THICK END= 1.01000 MICRONS
TAPERING FACTOR= 1.01000 R.~. TOTAL RESISTANCE= 171.4054~ kQ
C~ TOT~T CAPACITANCE= 3.46239 pF
RNUW3 NULLING Rk:i~lOR= 9.63032 IcS2 ATl~UATION FACTOR= 4.00000 R~m SHUNT ATTENUATION R alalOR= 98.98187 kQ

73 WMAX74 LINE WIDTH AT THICK END= 1.0100~ MICRONS
TAPERING FACTOR= 1.01000 R.r~ . TOTAL RESISTANCE= 98.96096 kQ
C~. TOTAL CAPACI~ANCE= 1.99901 pF
RNULL74 NULLING RESISTOR= 5.56007 IcQ
ATTENUATION FACTOR= 1.25000 Rp74 SHUNT ATTENUATION RkalalOR= 692.78949 kQ

WO~G,'O~Y~S 2 ~ ~9935 P~~ gS/ll745 A p~ problem is how to control in mass p~ lu ;lion the iviL~ of the d~pOait~d films to be equal to the target value ~ ..Pd in the de~sign. If the l~ia~i~iLy varies, the whole fi~lu.~y ~w~d~e scale~s ~.o~.~ionally. For P~ k, double the r.~ iLy would halve the cutoff 5 and n~ll f~ s while half the ~wiali~iLr would double all frcq~lerlri.~s In the case where ~ n ~ .Ai~r~5 are too wide to permit the u~ s~n~ to be held within de~sired }imits, the second aspect of the iu~.~on may be applied to adjust the L~ u.~ ~e~ ce to be within limi~
after l"G...,r;~ . This ia done by meaDs of an iu~ e mea~s for 0 ab_~lWiSe ~A~ ;A~ n of the line length.
A firat co..~ A~ n a~Ul~liUg to this aspect the iu~..lLi)ll is shown in Figure 8. k is to be ~ oo~ that the cir~uit of Figure 8 can replace any Of l~ia~ s of the b~1qn~ed mlll devices 70, 71, 72, 73 sho~n in Figure 7.
The ste~wise-a~ C~hl~ line or notch ~c.lu.~ filter in~ es at least one i~ut 70a, at least one ou~ut 70b and a common t~min~l 70c. A
~IUIU~ of 1~ia~ P ~ i 80, 81, 82 and 83 may be formed as thin films aih;1 ovcr a cc~ r~ r of cQr~h~ plates with an .~g ~;elr~ layer, aUCh as show~inFigure 1. The ~ e 20 c~ are c~.... P ~d in series bet.._.~ the in~ut 70a and output 70b ~rmin~lc, A ..~ ~r of a~i~ ~ 85~ 87~ 89 are ~ ~ to be able to ~lr~ .ly bypass or shol~ul a l~ Li~e I~J~ C~ A
CGll~ol~ding ~------h-'~ of s~it~es 84~ 86~ 88 are ~IAngc~ to be able to a~1e~ C~ F~ f~J~ CO~ plates with I~Ji~e ~ 1~ .. .,1~ which have not been b~a~1 and he~ ll~uuph a l~ AI~e COI~ tO CO~ t~min~l 70C. The value of the series conn~
lc~;~lAn~5 is cl~l acco~ g to which of ~e ~Jial.i't~; el~ is 1.,~.

wos6/oss6~ 3 $ P~ JS9SI1174S

An RC liIle 80 of a no~n~1i7f-fl length of olle UDit is p, ",~ "1y i c~it, while other RC linr 81, 82, 83 of l~h~, for ~rle 1/2, 1/4, 1/8, etc., unitâ may be awiL~hed in or out of circl~it by awiL~ J s~l~c~ ~
paria of awit~h~s 84, 85; 86, 87; and 88, 89. The crr~ line leng~h may tlms be swi~hcd ~t~ the values 1, 1.125, 1.25, 1.375, 1.5, 1.625, 1.75, and 1.875 in this ~ lr Since i~leaai~g the length i~c~e~ses both the tOtal Ih~clUg~ A~e aDd CA~ 'G, the RC l ~odu~t follows the squa~e of these values, and thus is con~olled over almost a 4:1 ~ange.
If it is only desired to va~ the RC 1 lO~ over a 2~ ,e, the ~ line length need only be root(2)--1.414 times the 111~ 11111111 line length and this is acl~,~bk with swiD~h~d 5~l;0.lc of length 0.207, 0.1035, 0.052 units, etc. With only three such awi~l~d a~/;~ C, S~o liIlc-s~s c~l~,yQn~i~ to 10% Lc~ steps are achi~ablc, and if the nearest LY~.1UC~CY ste~ to a desired value is sel~ct~l the e~or is o~ly i59~.
To create a tu~able notch filter with the above ~ r the l~ia~l ~om the r~ ~ l~.r plate to ground ia alaO varied to m~inr~in a c~i~ r.~. l;. ., (e.g., 0.056) of the thrwgh-~c~ .fe Thus, a swi~Lhed resistor to grou~d is also used, -uch as shown in Figure 9 for e~mple It is to be lm~ ~ that tbe awiL~hl~ nulling resistor circtlit of Figure 9 can replac~ one or more of the ~ RNWtl ~Lo~ of Figure 7.
~s shown in Figare 9, th~e ~wiQl~ nulling ~wialur usable in con; ~n. ~;on with the circuit of Figure 8 inrln~es a fir.st, non-switc~able null - resistor 90 with a relative value RNULL, to which is con..~ d one end of three seriws co--nf~d null ~ s 91, 92, and 93. T~e thre: null ,. ~;~rAnr~5 91, 92, 93 having relative valuw of, e.g., 1/2 RN~., 1/4 RNU~.
and 1/8 R~ULL, ~w~ .ly. The three ~ rAnres 91, 92 93 are sele.~veIy swi~h~ble into and out of the circuit by parallel conn~ awic~hes 94, 95 and 96.
While thç cir~uits shown in Figure 8 and Figure 9 may be l~ Able using field effe~t IIA~ 1 awi~Lhes, there can be problefns W096108865 2 1 q 9 ~ 35 PCI'IUS95111745 with the ~Al~r;rA~e and ~ n~e of the swiL~hcs, as well as l;.~ nc on the ~ic range of signal voltage swing through the filter i~osed by the s~ritch ! ~ A ~ 11 A ~ ' S .
l~e p~f~ d ;-~lr~ A~ n of a tunable notch filter in au~o~ ce 5 with the prcsen~ ~ioQt such as shown in Pigurcs 1-1 and 13t largely e~ ,;n-~ s these p1ubl~s and gives a notch filter that can operate with a ail sig~nal ~g.
It will be ~p~t~ that once a notch filter can be formed on a desired fre~ency, low-pass filters can be co~hue~d by the r~cr~e 10 co.~ - of such device~s to po~ihon ~tc~es in the stop band so that all f~e~uc ~ri~s abovc a cc~ain ~ngc are au~ ~S~ d to a desi~d extent. Such filtes may not have the same ~ ~ of altoff as, for c~lc, LC filtes, but the p~ lt ~,~iUIl does allow y~ and useful filters to be rnade in the ~e~ range 0.3 to 300 MHz, a~d such a filter has been ~ cec~rlllly 15 fa~.;~tl d that passes f~e~l.J~n~ rs up to about 3 MHz with ~ittle Al~ A~;OI1but has high d~ ;on at 12 MHz and above by the pOsil;o.~ o of ~ chcs at 12.5 MHz, 35 MHz, 52 MHz and 300 ~Iz. ne~se t~ An~5 on the higher ~ n.,trl~5 fi~ther away from the p~c-sL ~n~ have little eKeet on the ~JA~ n~ it was ~ t~ m this ;~ An~ they did not need to be - 20 tlm~Ahlt~, and that only the filter having its notch L~u.~;y near~st the ~,a~hA~ had to be tunable to ranove the effect of process spreads.
Figure 10 shows how an adjustable notch filter can be used to obtain a hA~ As-c Amrlifi~r ~ n~ SA~1;tAhIf~ for r~ U.~y-selec~ive, i-ltt ~IIlrA/liAAt,~
LC~lu~;r a~ -AI;on in radio ~y~tL~s of ap~o~-iate ba~lwidth. A
25 tuDable notch device 102 acco.di~lg to the iu~e~Lidn is co~ c~ ~ as the fe~dh;~ path around an amplifier 101 such that the gain is ~upp~ssed outside the notch L~lu.~r whcn the filter 102 allows a s~ong, negative f~h~c~ signal through. while the gain is high around the notch fre~uency when~e nc~a~ f~elhA~l effect is ~ luced. A cascade of such n~nable 30 sele~ Amplifi~-s can be used to fo~m an ;~ rAtf d c~r~it ;~-t . ~ Atr 2 1 ~ 5 strip for amall, portable radio fCiX;V~a. The "tuning bits" shown in Figure 10 refer to con~rol signals which operate awi~ches 125-132 of Figure 11 and 14~143 of Figure 13.
A p~ d ~ for an ~ hl~ ~tch filter that does not S suffer loss of ~u~ic range due to the t."~ awil~ S iS ~scnkd belou.
Adj~ of the notch r.~ is provided by means of a at~w~ adJustable line leng~ using an a.l~
confi ~ This is used with a ,~ aLpwi~ adjus~le l~iak~ to form the ~ hle notch device.
A ~l~,fe.~d ;"~ of the aljua~ble RC line is shown in Figure 11. A main, ~.".a,\. .~l~ in-ci~uit line section 110 is c~cr~
co~ with switrh~ble s~';....c 111, 112, 113, 114 on either side. Two a-witchable s~c~na 111, 112 on the left hand side as shown in Figure 11 15 have l;.~fl~.~ that are a first f.,~ .. dL of the main liIle length L. The two switrh~bk s~l;~a~c 113, 114 on the right hand side have fr~rtirn~l lengths 3dL. Thus, various err~ line lengths can be acL~-~ by Swi~ g by cu~ onli~g awit~hcs 115, 116, 117, 118, 119, 130, 131, 132 the switrh~ ;- -\c in or out of circuit in the following com~;nA~;n..c 111 112 113 114 Effective line leng~
out out out out L
out in out out L+dL
in in out out L+2dL
OUt out in out L+3dL
out in in out L+4dL
in in in out L+5dL
out out in in L+6dL
out in in in L+7dL
in in in in L+8dL

An il l~o~t feature achieved by the above ~g~ L is that the line s~;o.~c awit~h~ into circuit are always co..~ s, i.e., no w0 s6/oss6s 2 1 ~ 9 q 3 5 pCTlUS95/11745 co~uon of lines such as "in out in" is lLed. Ihis er~ables simpli~ "~
of the swi~hil~g so that the c~ ,rol plates only of the lines need to be ~wi~hcd~ In other word, the ~wi~ is thereby simplified as it iS only n~sS~. ~ to switch the c~p~ plate te~Tnin~lc of the line s~ ;n~ and not S the sesies ~ . To switch a line secuon dL or 3dL to add to the main line length, ItS r~r~rit~r plate i co~ to the c~r~;~ol plate of the main line (e.g., by a~witch 115). To p~e.lL the line section adding to the main line kngth, its c~ o~ plate is either left ~ .",.r~ tl ~ or co.~ ~r~ to ground (e.g., by switch 119). The awileL~l out sectinn~ fole appear as 10 acp~ P7 short RC lines or series ~ia~ula that are in cqcrad~ with the device and not additi e to the c~r~ main line length T, Thus, when the main line ~ Al is Co~ ctJ.'~ to grou~d via the mllling resistor of Figure 13, for e, the ~ to the Dull in the li. .lu~ ce so created is not atr ~-~ by the awi~d-0ut ~c~ c To ~o~idc a ~-1~1l k;~,, atL~wi~ c~ble null~ng le~ the ~. ~a..~, ..P .I of Figure 12 could in p~ be used. This has a main resistor RNU~ 123 of n( m~ql value 0.056 of the ~ e of the main RC
line total " ~ we. Tvo aw;~h3hl~ a~ C (126, 127~ of r.~,;.,., dL/L
times the main mllling ~ia~r RNU~1. and tu o s~itc~ble sf ~ c (124, 125) of value 3dL/L are ~.~,vid~d, en~hli~ the same con~oi signals that select the line s~-l;onc to be used to aelect col~ ond~g swl~3hle parts of the nulling le~ia~r of Figure 12.
A disaclv~e of the ~ g. ~ of Figure 12 is that the ~ e of the switch l ~ s that can be falJli~kd on a silicon chip is app.~ciable co~d to the awi~hed re~;c~ e. Th~,efol~, the i~r~ed ~g.~.lt of Figure 13 is ~ 5~.l In Figure 13, ad~Y~ nt of the total crL~ e l~ n~e R is ~cco...l~lished by awilched shunt l~JiaLo~a of high ~alue i~stead of awilchcd series lC~iatOla of low value. The main resistor value R in Figure 12 ia now 30 shown in Figure 13 divided into a fraction aR a~d a f~rion (a-1)R In WO 96/0886~ 2 1 9 ? ~ 3 5 Pcr~?sgs/ll74~

parallel with the firs~ Ction aR are conn~t?~d two ~ a Rl and R2 switchable into and out of the circuit by two ~ n~;c~ s 135, 136. Swi~cl~
in Rl will reduce the ~rr..~.~e value aR to aR dR where dR is e?~ual to (aR)2/(aR+Rl), while awi~liillg in both R1 and R2 vill reduce the err. .Li~,., S value aR to a~-2dR. ~ ikewise, the two ~A?Aition~ R3 and R4 connr~ i in p~ l to (l-a)R and switcha~le into and out of the circuit by two ~itin~l ~n~ictQl~ 137, 138 allow the ~ e (a-1)R to be lel~
to (a-1)R-3dR or (a-l)R~dR. T~us, all values of total . ~ e ~om R to R~dR in st~s of dR can be achi~
Since the adj~c~ t of R is in the duw~wc~ld direction~ the value of R must initially be set to 8dR ohms higher than in Figure 13, and the swiochi,.g I~A~ OI~ 135-138 must be o~c,~ by inversc control sig~als to those of the swilLhcs 115 to 118 of Figure 11. The value of the f~rtion "a"
may be chosen so that ~nAllPst of thc four switchable resia~ R1, R2, R3 15 and R4 is as grcat as possible in order to ...;.-....;,~ the ;..n.~ h~e of series switch .~ e. If "a" is too smalI, then R1 and R2 will be ~ eecsA. ;ly small while R3 and R4 are large, and vice versa if "a" is too large.
Th~cfc,,c, an o~ .. e~ists that can be fou~d by c~
The coaaLIu~ of the notch filters and a~r~s~hie notch filters and 20 their a~lir~t;~ c has been flf''G ' ~hed here under the ~s~ ;n~ that i~g~Lion on a silicon i~ d Ci~ Uil iS the aim, but one skilled in the art can readily adapt the invention to other forms of fabrirAtion or appli~A~
such ~lAl.lA-I;ons nevertheless being considered to be within the scope of the invendon as set for~ in the claims. The above ~lisrllssion of the ~Y~npl~r 25 embo~ h~ iS for 1~U~O~, of Ç~1~An~At;On and not limit-Ation The scope of ~e i~ nLi~n should be ~et~ ;n~ by ,e~re~ce to the a~e~ claims.

Claims

I claim:

1. A quadrature modulator for the generation of complex modulated signals, comprising:
a digital sample generator producing a sequence of numerical value pairs respectively representing a real and imaginary modulating waveform;
a delta-sigma convertor circuit converting said sequence of pairs of numerical values to a high bit rate stream I representing said real waveform and a high bit rate stream Q representing said imaginary waveform, and additionally a bitstream I being the complement of I and a bitstream Q being the complement of Q;
a pair of balanced, low-pass filters each having two inputs accepting said bitstreams I, I and Q, Q respectively and each having two output lines providing balanced, filtered I and Q signals;
a first balanced modulator having a first, balanced input including two lines connected to said balanced, filtered I signals, and a second input connected to a cosine carrier frequency signal generator;
a second balanced modulator having a first, balanced input including two lines connected to said balanced, filtered Q signals, and a second input connected to a sine carrier frequency generator; and an adder for adding said first and second balanced modulator outputs together.

2. A quadrature modulator according to claim 1 wherein each of said first and second balanced modulators is responsive to a difference in voltages represented to the two lines of its said balanced input and unresponsive to change in the sum of said voltages.

3. A quadrature modulator according to claim 1 wherein said pair of balanced, low-pass filters attenuate to a desired level a balanced mode signal, being defined as the difference in voltages between said two input lines or two output lines, and attenuate to a different degree or not at all a common-mode signal, being defined as the sum of the voltages on said two input lines or two output lines.

4. A quadrature modulator according to claim 3 wherein said balanced, low-pass filters are comprised essentially of respective and capacitive elements.

5. A quadrature modulator according to claim 3 wherein said pair of balanced, low-pass filters include resistive and capacitive elements formed by depositing resistive and dielectric films on a substrate.

6. A quadrature modulator according to claim 5 wherein said substrate is formed of a material selected from the group consisting of silicon, alumina, gallium arsenide, sapphire, and polyamide.

7. A quadrature modulator according to claim 5 wherein said substrate is a semiconductor material and said balanced modulators are constructed on said substrate using transistors formed in said semiconductor material.

8. A quadrature modulator according to claim 3 wherein said balanced, low-pass filters are constructed by use of distributed RC lines having a resistive pattern deposited over a conducting plate with an intervening dielectric layer.

9. A quadrature modulator according to claim 8 wherein said conducting plate is connected to ground through a resistance so as to form a notch in the filter frequency response.

10. A quadrature modulator according to claim 8 wherein said conducting plate of one RC line is connected through a resistance to said conducting plate of another RC line.

11. A quadrature modulator according to claim 9 wherein a number of said frequency notches are formed by a corresponding number of RC lines having resistances connected between ground and respective conducting plates.

12. A quadrature modulator according to claim 8 wherein an effective product of a total resistance with a total distributed capacitance of at least one of said distributed RC lines can be adjusted to a desired value after formation of said distributed RC lines.

13. A quadrature modulator according to claim 9 wherein an effective product of a total resistance with a total distributed capacitance of at least one of said distributed RC lines can be adjusted to set a notch frequency to a desired value after formation of said distributed RC lines.

14. A quadrature modulator according to claim 8 wherein at least one of said distributed RC lines is tapered.

15. A quadrature modulator according to claim 14 wherein said taper is exponential.

16. A low-pass filter suitable for construction on an integrated circuit, comprising:
a number of filter sections each having two input terminals and two output terminals and a common terminal, each filter section comprising:
a series resistor connected between a first input terminal and a first output terminal of said filter section and a substantially identical resistor connected between a second input terminal and a second output terminal of said filter section, each of said resistors being formed by depositing a patternof resistive material over respective conducting plates with an intervening dielectric layer to provide distributed capacitance between said resistive pattern and said conductive plate;
a resistive means connected between each of said conducting plates to said common terminal, or connected between said conducting plates, or both;
a resistor connected between the output terminals of at least one of said filter sections, said filter being cascade-connected such that the output terminals of one section are connected to the input terminals of the next.

17. A low-pass filter according to claim 16 wherein at least one of said resistive patterns is tapered.

18. A device according to claim 17 wherein said taper is exponential.

19. A device according to claim 16 wherein a product of a value of said series resistor with a total of said distributed capacitance of at least one of said sections can be adjusted to a desired value in order to determine a notch frequency after formation of said low pass filter.

20. A step-wise-adjustable notch frequency filter comprising:
at least one input terminal and at least one output terminal and a ground terminal;
a number of resistive elements constructed as thin films deposited over a corresponding number of conducting plates with an intervening dielectric layer, said resistive elements being connected in series between said at least one input and said at least one output terminal of said filter;
a number of switches arranged to be able to selectively bypass certain of said resistive elements;
a corresponding number of switches arranged to be able to selectively connect together certain of said conducting plates associated with unbypassed resistive elements and to a common terminal which is connected through a resistance to said ground terminal; and means to change the value of said resistance depending on which of said resistive elements are switch-bypassed.

21. A at stepwise-adjustable notch frequency filter comprising:
at least one input terminal and at least one output terminal and a common terminal;
a first number of resistive elements constructed as thin films deposited over a corresponding number of conducting plates with an intervening dielectric layer, said resistive elements being connected in series between an input and an output terminal of said filter;
a second number of switches arranged to selectively connect certain of said conducting plates together to a resistance connected to said common terminal; and means to change the value of said resistance depending on which of said conducting plates are switch-connected to said resistance.

22. A device according to claim 21 wherein said switches can be operated to connect selected conducting plates alternatively to said common terminal or through said resistance to said common terminal.

23. A device according to claim 21 wherein said switches can be operated independently to select each conducting plate to be connected either to said common terminal through a separate resistance for each plate or to be connected to a common resistance connected to said common terminal.

24. A device according to claim 21 wherein said switches are operated only to select conducting plates associated with sequentially contiguous resistive elements to be connected together.

25. A device according to claim 22 wherein said switches are operated only to select conducting plates associated with sequentially contiguous resistive elements to be connected together.

26. A device according to claim 23 wherein said switches are operated only to select conducting plates associated with sequentially contiguous resistive elements to be connected together.

27. A device according to claim 20 wherein the lengths of said switch-selectable resistive elements are in the binary ratios 1: 1/2: 1/4 ...1/2n, where n is equal to the number of resistive minus one.

28. A device according to claim 20 wherein one of said resistive elements is permanently in circuit and the remainder of said resistive elements are switch selectable and have length ratios in a binary progression.

29. A device according to claim 24 wherein one of said conductor plates is permanently connected to permanently to said common terminal and the remainder of said conducting plates can be selectively switch-connected to said common terminal.

30. A device according to claim 29 wherein said remainder of conducting plates being switchable are disposed on each side of said permanently connected plate as dispersed on a semiconductor substrate.

31. A device according to claim 30 wherein said switchable plates disposed on one side of said permanently connected line have associated resistive elements of length one unit increment and the switchable plates disposed on the other side have associated resistive lengths equal to N+1 unit increments where N is the number of switchable plates disposed on said first side.