Note: Descriptions are shown in the official language in which they were submitted.
WO 92/1~299 PC~/US~2/01017
~1~0~7
AM-FM COMBINED Sl EREO F~ECEIVER
Technical Field
This application relates to AM/FM receivers.
Background ot the Inv~ntion
This invention r01at~s to ths fi~ld of AM and FM recaiv~rs and
15 more partioularly to those recoN~rs which ars capable of rec~iving both
AM st~reo and FM stsreo. The pr~sent inv~ntion addr~sses a m~thod of
minimizing th~ cemponants us~d to cr~at~ this system and th~refore the
cost and size of implementin~ said system. This minimization of size and
cost is nec~ssary for AM S~ereo to be snjoyed by the maximum arnount
20 of radio list~ners.
In MarBh of 1982 the FCC adopt~d its unique Report and Order
authorizing the so called ~market-placo approaohR to the sslection of a
standard for br~adcast of st0roophonio program mat~rial in th~ AM band.
At thl3 beginnin~ ot the mark~tplaea comp3tition th~ra we!e five
25 c~mpeting syst0m~. Ov~r ths past ~ight yaars th~ number of ~mpetitors
has been redu~d to two. Of these ~h~ C-Quam~9 syst~m snjoys a
preponderant advantags in th~ numbar of stations and has r~csntly been
dedared the~De Facto standa~l by the FCC. Over the ~sriod of this
c~mpetitlon thcre has been signifioant research and development in
30 attemp~ing to build AM Stereo dsc~d~rs that were capable of rec~gnizing
the diffsr~nt st~rso broadcasting systems and decoding them. SeYeral of
these approaches were supplied by IC manufactures to various rec~iv~r
manufactures irl hopes ~hat they would uss them in their radios.
Howev~r overthe resulting year very few receivers have used these
WO 92/14299 PCT/US92/01017
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- 2 -
IC's. The perceived major cause for the ve~y limited usage of these
circuits is their expensive cost and relatively poor pe~ormance when
compared to the single system IC desi~ned for the Motorola system. The
development of this single syst~m IC was completed by Motorola in 1982
and ov~r 20 million parts wcre sold by th~ 4th quart~r of 1990. Howcv~r,
it has b~n o~s~rved by th~ inv~ntor and oth~rs that this part is uscd
almost totally by the automotive companies in th~ir pr~mium radio lin~s.
This deprived th~ averag~ list0ner of th~ pleasur~ and distant coverag6
benefits of AM Stereo.
The inventor determined that the b4st way to allow the ~enefits of
AM Stereo to be enjoyed by a greater num~er of people was to reduce
the c~st without si~nificantly low~rins th~ performance of tha rec0iv0r.
Sinc0 th~ basic environmsnt of ths AM st2r~0 r~iv0r generally includes
th~ FM star~o r~iver, it was concluded that the best way to perform this
15 goal was to d~si~n the AM st~r~o systsm to reuse as mu~h of th~ FM
ster~o ~irci~itry as possibl~. The pr~s~nt invention covcrs the switching
and partitionin~ of cirouitry to accomplish this ~I.
Summary of thc Inv~ntion
It is an object of the invention to provide an improved, that is a
lower cost, Ah~FM c~mbined stereo recciver. Ac~ordin~ly, an improvod
AM-FM c~mbined storeo rec~iver is provided havin~ a single processor
for removing, in the FM mode, ar~tang0n~ distortion, and in th~ AM mode,
25 to proc~ss th~ phass term from aretangent {1 +; + P~ l
~rrnine {1 + L + R } Fulther, the radio inc~udes a single
discriminator circuit wherain, in the FM mode, the discriminator tank
circuit is used to convert the frequency deviation to basaband audio and,
in the AM mode, th~ discriminator tank circuit is used as the frequency-
30 determinin~ element for the voltag~ controlled osc llator (VC0), th~ VC0being used to translato th~ intermediate froquoncy (IF) signal to
baseband. Further still, a single pilot ton~ circuit is shown which detects
WO 92/14299 P~/VS92/OtO17
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0ither tha AM pilot ton~ or FM pilot ton~ and in th~ FM cas~ suppli0s a
raference signal to tha FM dscoder to translat~ th~ L - R tO bas~band.
Brief Descnption of the Drawings
Fig. 1 is a block diagram that shows a first embodiment of an AM-
FM combinsd stereo receiver 100, acoording to tha invention.
Fig. 2A is a first spectrum diagram.
Fig. 2B is a second spectrum dia~ram.
Fig. 3 is a block dia~ram for a quadrature nehvork acc:ording to the
invention.
Fig. 4 is an ~xpan~ block dia~ram of th~ pilot ton~ VCO and
quadrature neh~ork.
Fl~. 5 is an expanded biock dia~ram ot th~ pilot tone VCO.
Fi~. 6 is a block diaQram ~ th~ piiot ton~ thr~shold circuit.
Detailed Descriptian of the Invention
Fi~. 1 is a block dia~ram that shows a first embodiment of an AM-
FM combine&i s~ereo r~eiver 100, accordin~ to the invention.
R~ferring now to Fig. 1, there is shown th~ oir~ tlow struc~ur~
from th~ ant~nna input 101 to tha L + R and L - R outputs. Much of ~his
circuit~y is described in scme d0tail in othcr patent applications that will
ba cited. Th~ RF ~ront end, which indudes the prssel~tQr, mix~r and
local ~cillator (L0) is similar to most low voltage c~mbined AM/FM
ste-eo IC'5 (s~8, for instanc~, Bl~k diagram TA8122).
For AM Starso the expected input signal is of ths form A ~ cos (WCt
+ 0), where A = (1 + LeR + Ri~ht) and o = tan~1 [(L - R)/(1 + L + R)]. This
signal is propa~ated in the AM broadcast band which spectrally extends
from 520 kHz to t710 kHz. The antenna 101 recsives th~
electromagnstic wava and c~nverts it into an AC voltage and curr~nt.
This curr0nt is applied to the input of the praselector 103. The
pres~lector is a radio frequancy fil~ring circuit which partially removes
signals other than the desired station (A cos (WCt + 0)). This circuit is
W O 92/14299 PC~r/US92/01017
21~QI47 i ~ -4 -
typically compossd o~ one or two r~sonant circuits, which are constructad
of inductors and tunable capacitors or varactors. In a home or portable
receiver, the AM and FM antennas ar~ often separat0. In these r~ceivers
the AM antenna is gener~ly a c~il with a large fsrrite or powdered iron
core though it and is tuned with the variabl0 capacitor. This ant0nna,
rsferred to as a 'loop stick', performs the function of antenna and
praselector in the same circuit. The p~selected signal is then supplied
to the mix~r circuit 105. The mixer can be described as a circuit that
multiplias the presel~t~d F F with a ref~r~nce si~nal supplied by the
10 local oscillator 111. Mixing in the tim0 dornain is equivalent to adding
and subtracting in the fraquancy domain. Thsretor~ th~ output fr0qusncy
from the mixer 105 is the absolute value of the prcs~lected RF signal
minus the local oscillator frequency. This is generally written as I~Carrier
Frequency - Local Oscilla~or trequency}l = Intennadiate Frequency (~.
g., IF). 11l0 local oscillator 111 i~ desi~ned SL~:h that when it is at the
desired frequancy, which whsn mixed with the pressle~ed RF produces
the desired IF, that the RF carrier is at the cent0r trequency ot the
preselsctor fll~ring. This is generally dofined in th~ industry as baing
'tracked'. Th~ above approach is d~scribed as a supsrheterodyne
20 rac~iv~r. The IF ~i~nal is then furth0r fil~ered to eliminate other interfering
signals by th~ IF filt~r 107.
For FM steroo th~ sxpectad input is aiso of th0 form A ~ cos (WCt +
o). Howev~r in this cas~ tha c t~rm is a frsquency d~viation of the
bassban~ t~rm shown in Fi~. 2A.
2~ Referrin~ now to Fi~. 2A, this baseband signal has the Left + Right
(L I R) si~nal described as normal monophonic audio. The Left-Right
signai is a double sideband suppressed carri0r si~nal (DSBSC) whera
the suppr~ssed carrier is at 38 kHz. A pilot tone is add~d at 19 kHz, that
is phass Gohsr~nt with the suppr~ d carrier (38 kHz) terrn, such that it
30 can bo used to derive a reference signal in the FM S~0reo decoder. This
signal will be necessary to translat~ the L - R signal to 0 Hz (D. C.) which
is nec~ssary to dev~lop the correct st~reo output. The baseband FM
Sterao signal modulates the carri~r such that the paak carrier dsviation is
75 kHz.
W O 92/14299 PC~r/US92/01017
5 2~00~7
The FM antenna is generatiy a whip (e. ~., straight piece of wire) or
a home roof mounted antanna. This circuit as with the AM rec~iver
converts the ~lectroma~netic wave into a volta~e and cl~rr0nt. The signal
is supplied to the preselector 113, which as described for the AM receiver
is mad~ of inductors and capacitors, and partially removes signals other
than th3 dosired carrier. This presal0c:tHd si~nal is th~n conv~r~sd in the
mixer 115 to the d~sired IF frequency. A~ with th~ AM saction the signal
is ~urther filterad in th~ IF filt~r 119 to r~mova int~rf~rin~ signals. This is
dsfined as a superhetrodyne r0caiv~r and is well know in ~h~ indust~.
Therefora, both ~he AM and FM signals ~o thou~h relatively the
same process to be conver~ed to an IF signal 179. The ma~or differcnce
is that the AM RF si~nal is at 520 kHz to 1710 kHz and is c~nverted to an
IF of 4~0 kHz, whereas the FM si~nai is at ~ MHz to 108 MHz and is
converted into a 10.7 MHz IF si3nal.
The output of tha IF amplitisr 109 is oonn~ctsd into tha envslope
dotector circuit 141 alon~ with oth~r arwits. This block supplies both
amplification and the narrow band AGC action for the radio. Ths ~ain is
nec0ssary to supply ths d~dor with lar~e ~nou~h si~nal lev01s that ~he
decodin~ is accurat~. Th~ AGC func~ion is n~ y to hold ~ho output
si~nal ~179) a~ the dl3sired l~v~l. Th~ ~nvelope detector 141 p~rforms
the function of multiplyin~ the IF si~nal 179, A cos (wjt + c) by the limited
form of A ~ c~s (w t + o) or sgn {cos (w~t + ~)}, wher~ s~n is the signum
function. This multiplication can be wrmen as A c~s (wt + ~) ~ sgn ~
(Wjt I B)}, which results in (A/2) ~ (A c~s (2(~t + 0))/2~. Any scaling of
2~ the A term in the demodula~ion proc~ss is ignored for conveniencs. The
(AJ2) ~ c~s (2(Hfit + 0)) term is removed by an audio lowpas~ filter and
only th~ A~2 t~rm is lef~. In the AM mode th~ A terrn is 1 + Left + Right. In
the FM mode 'A' is the snvelope multip~th term and ct~uld, in high
performance receive~s, be used as an indication of tho signal quality.
As shown in Fig. 1, th~ first ~mbodimant in~ludes the limiter 143,
multipliar 153 and resonant circuit 137, 139 which form a discriminator
circuit for th~ FM mod~. In the AM mode the voltagc controllad oscillator
(V(::O) 161 is add~ along with a divid0r 163 to form a phas~ lockod loop
(PLL). Both of these circuits are us~d eo extract the ~ t0rm from th~ IF
WO 92t14299 PCr/US92/01017
2~ 4 47:
-6-
si~nal 179 (A t COS (WCt + ~)~. Th~ r~ason two difforent circuit
configurations ar0 used is that when 0 is in the form of phase modulation
as for AM ster~o, a PLL is ~en3rally th~ desirsd decoding approach,
however if the ~ t~m is in the torm of a frequancy deviation as with FM
stereo, a limiter discriminator approach may bo more dasirable. Other
Gonfigurations may have a PLL detection for both AM and FM or a
discriminator detecting both AM and Fhl.
For the FM ~nfi~uration, switches 129, 131, 133 and 135 ar~ both
all set to th0 position designated F. To achieve the AM configurationt ~h~
same switches are set to the position d~si~nated A.
In the AM configuration, A ~ cos (wjt ~ ~) is supplisd to th~ input ot
the limiter 143. The limiter as oxplained previously r~moves the
amplitud0 modulation (A). This circuit is sirnilar in function ~o the ~sneric
part MC1355. Th~r~fore the outp~n trom the limiter 143, supplied to th~
multiplier 153, is sgn {c~s (~t + a)~, wh~re sgn is tha sisnum function.
This multiplier circuit 153 is similar to th~ ~enerio part MC1596.
The voltage controlled oscillator 161 operat0e at a o0ntsr
frequency par~ially controlled by tha rasonant frequency ot the inductor
and capacitor 137, 139. The VCO steenrg voltage 17~ is supplied by the
outpu~ of the PLL filter 165. This volta~e 175 causes a small Yariation in
the operating frequency o~ the oscillator, which ideally is linearly
proportional to the differQnc4 bet~H~an the nominal refarenc~ volta~e
(VCO c~ntrol volta~ at c~nt~r frequ~ncy) minus the PLL tilt~r voltage.
This VCO is similar to the FM oscillator of the oscillator / modulator
section of the Motorola part MC1376. Writing the frequency to voltage
r~lationship ~or ths id~al VCO r~sults in an equation ot the form of Fo~
{(Vref - Y~l) k} ~ Fr~, wh~re k = VCO gain per vott, Vref is ths ~ntrol
volta~e of ths VCO wh~n it is dsfined as freo running and Vpil is the
voltage at 175. The outpu~ 177 ot the VCO 161, which is pressnt on the
resonant circuit 137, 139, is supplied to the di~ital divider 163.
For the ~onvenisnce of using the same resonant circuit 137, 139
with th~ FM dot~ct4r, a r0sonant frequ~ncy of 10.8 MHz is chosen for AM.
Th0 divid~r 163 is se~ te divide the frequency ot the VCO signal 177 by
24. Therefore the incoming signal, which is at 10.8 MHz, is divided by 24
WO 92/14299 Pcr/ US92/01017
7 2100447~
and the outgoing signal is at 450 kHz, and wvco will be defined as 2
Fvco(450kHz). Both the limiter 143 o~tput s~n {cos (wjt ~ ~)} and the
divider 163 ou~put s~n {cos (wvcO)} are supplied to the multipliar 153.
Th~ outpu~ of ths multiplier 153 is th~rEfore {(wjt + 0) + (WVC~t)~/2 ~ {~wjt
~ 0) - (WVC~t)~/2. This output signal i5 th~n filt~red in th0 capacitor 159
shown attach~d to th~ multiplier 153. Since wvCO and wl are significantly
higher in frequency than audio and approxima~ely the same frequency,
the filter 159 can ~asily r~move th~ (wjt + wyc5~t) t~rrn. This results in
{(wjt + ~) - (Wvcot)} being the output ot th~ multipli~r 153. Circuitry in the
tan~ant ~enerator 155 and multiplier 157, which will be dascnbed later,
convarts (wjt + 0 - wVcot) into (A) ~ tan {(wjt + ~) - (WVcot)} This signal is
supplied io ~hn PLL filtar 165 to control ~he frequency of the VC0 161.
Undor norrnal operatin~ conditions the PLL is defin~d as locked.
In this oase wjt - wVcut and th~ output of mul~iplisr (153) reduces to o.
The action of the PLL is describ~ in d~tail by Vitarbi and others in well-
known r~f~r~nces. Ths panic~lar action of th~ tangQnt PLL is descnbed
in Lawrencs M. Eddund, "Automatic IF Tan~ant Look Control CirclJit, lJ.
S. P~t~nt 4,872,207, issued ~tober 3, 1 98g, which patent is heroby
incorporated by rafe~nc~.
Assuming now that switches 129, 131, 133 and 135 are se~ to
position designsted F, ~hc circ~Jit is set to the FM confi~uration. For the
FM stsr~o transmission syst~m the si~nal 179 supplied to th~ limiter 143
can be d~fin~d as A ~ cos~w;t + 2~ m(t)d(t)}, whcr~ m(t) i5 tho
modul~tion and fd is the frequency deviation oonstant. The ~xact details
of FM demodulation in a limiter discriminated approach as shown above
is w~ll dsscribed in c~mmunication t~xts such as the 11~ R~f~r0nce Data
handbook for Radio En~in~ers or Principles ot Communications by
Ziemer and Tranter. This circuit can ba implemented usir~ a CA3089.
Heuristioally the FM modulation oan ba dascribed as moving tha
centar fr~qu~ncy of the carrior by ths amplitude of ~ho modulation.
Therefore, for FM stereo, th~ signal can be d~scrib~d as th3 baseband
signal shown in Fig. 2A. Included is ths L + R signal in the 0 to 15 kHz
r~gion and th~ L - R signal which is a doublo ~idoband suppress~d
carri~r signal (DS~SC) wh~r~ th~ suppr~ssed carnsr is at 38 kHz, whil~
WO 92/14299 P~/US92/0~17
2100447 -8-
the pilot tone is at 19 kHz. This baseb.ind signal is used to change th~
frequency of the carrier, where each in~stantanaous amplitude of the
baseband can map into an instantanaous deviation of the earrier
fr~quancy.
Tha limitar 143 r0mov~s any AM modulation which may hava
b~en accid~ntly induced in th3 propasation channel. The limit~r 143
also turns the carrier sine wave into a squar~ wav~, 0, 9., s~n.(cos (wjt +
~)). This signal is supplied to both th~ r~sonant circuit 137, 139 and th~
multiplier 153. The phase r~sponse YS. carrier frequ~ney charact~ristic of
th~ rasonant circuit 137, 139 is tan-1 ((fr*q(instantaneous) - freq.
(averag~)) /fraq. (bandwidth/2)) + 90. The resultant multiplication in
multipli~r (153) has the limited inGoming IF signal s~n. ~c~s (w~t + 0)}
sgn. ~sin (wjt + ~ + (01))}, whieh r~sults in o 1 aft~r filt0rin~, whars ~1 is
ths tan-1 (freq (instantane~us) - tr~. (av~ra~ / freq. (ba~dwidth/2)).
1~ This recovered si~nal is tan~ K1 ~ ths transmitted bas~band si~nal),
wher~ K1 is th0 discriminator conv~rsion gain. As lon~ as the bandwidth
of the r0sonant circuit 137, 139 is wid~ and the corr~sponding phase
daviation due to the r~sonant circuit 137, 139 is l~w then tan-1 (K1 ~' O)
can be described as K1 0-
To ac~amplish the transition between the AM(o) detector and tha
FM(~) detector switches 129, 131, 133 and 135, VCO 161 and dividar
163 ar0 provided. Switch 133 conn~s to th~ rasonant circuit 137, 139
and conn~ts ~ith~r the VCO 161 to drivs this ciraJit for th~ PLL
approach or it connects the limit~r 143 ~hrou~h a pl~e shifting capacitor
151 to driv4 the discriminator circ~it. Sw~ch 135 c~nnects th~ multiplier
153 altcrnat~ port to aither th0 divid~r 163 for th~ AM PLL or directiy to
the resonant n~twork 137, 139 for th~ limit~r discriminator FM approach.
By switching the discriminator couplin~ capacitor 151 and ~horefore
removing it from the circuit for th~ PLL moda, whera th~ oscillator
operatss at 10.8 MHz, and switching it into the circuit for the 1û.7 MHz
operation, of the limiter discriminator used in FM, the shilting of the
rssonant circuit 137, 139 to 100 kHz low~r in froquency for FM can by be
easily accomplished by the spacific choice of capacitor valuo. It is
conceivabl3 that in some applications a PLL dec~der would be desirad
WO 92/14299 PCI/US9~/01017
~004~7
., ~
for FM alon~ with AM. In this situation switch 133 would not be n~ed0d.
Therefore in this case swiSch 135 would be selecting whether th~ PLL
would be operatin~ at 10.7 MHz or 450 kHz.
The tan~ent function bloc~ 155 is necessary to corr~ctly d~code
5 both the AM and FM signals. As discuss~d above the output of the FM
disoriminator is the tan-1{K1 ~ th~ transmittecl basaband signal}.
Th~r~fora, to psrfectly recover the {K1 ~ transmitted basaband signal} the
tan-1 function must be romoved. This c~n ~asily be don~ by passing th~
signaJ throuyh a tangant function block 1~5. Math~mati~ally this is
10 describ~d as tan ~tan 1 (IC1 ~ transmi~ed baseband si~nal)} = ~K1
transmitted bas~band sisnal}.
For AM as d~scribed earli~r the o term is s tan-1 ((L ~ R)/(1 + L +
R). To remove the tan~ nction it is necassary to pass ~he AM 0 term
through the tan~snt hJnc~on 155 resulting in ~(L - R)/(1 + L + R)~. This
15 approach is doscrib~d in Norman W. Part~sr, et ai., ~AM Stereophonic
Receiver,~ U. S. Patent 4,172,966, issued Octob~r 30, 1979, which patent
is hereby incorporated by r0ferenc~.
The tan~ent function ~qnerator 155 can be built usin~ many
diff~rant approach~s. Or~e approach usin~ a preci~ion piecewise linear
20 c~nstruction is described in Charles J. Marik, ~Tan~snt Function
Generator For AM stel~on, U. S. Patent 4,278,839, issued July 14, 1 g81,
which patent is her0by incorporated by referen~a.
At the output of ths tan~ent fun~ion bJock 15~ the FM signal
should be an axa~ replica of the transmitter bas~band si~nal as shown
25 inF~.2A.
P~ferrin~ now to Fig. 2A, this signal has the L + R audio in ths
corr~:t form, in other words th0 L + P~ DC term is at O Hz. Therefore, this
signa~ can be used directly to supply th~ L + R terrn for-ha matrix. To
derive the L - R compon0nt for the matrix the double sideband
30 suppressed carri0r signal must be converted around DC. Conceptually,
and ignoring any scalin~ fac~ors, this requires multiplyin~ the signal ~(L -
R) 7 a~s (wt)}, where w = 2~ 33 kHz, by cos (wt). Th~refore, {(L - R) ~ cos
(~IVt) ~ c~s (wt)~ = {(I - R)/2} ~ cos (0~ ~ {(L - R)/2} ~ cos (2wt). Since th~
2wjt tenn is at 76kl lz, it is g~nerally not a ~ncarn. Th~ 38 kHz
WO 92/14299 PCl/US92/01017
; 2100~7 -`
mu~iplying signal is derived in the pilot tone detector and will be
discLssed later. This signal is supplied to the decoder multiplier (1~7)
along with the linearizad basaband signal. The resulting spectrum out of
(1~7) is shown in Fig. 2B.
Reforring still to Fis. 2B, it will be appreeiated that, as axp~t~d,
~he L - R tcrm is raf~r~nced at DC and oan thsrsfore be used to supply
lhe audio matrix (not show). Th~ 19 kHz pilog ton~ is multipli~d by 38 kHz
produ~in~ two nsw si~nals on~ at 19 kH2 and th~ oth~r at 57 kHz. Tha L
+ R si~nal which was at baseband has ~ean multiplied by the 38 kHz
c~rrier and beeomes a 38 kHz double sideband suppressed earrier
sl~nal.
In th~ AM mode tha multiplier 157 is deliveraJ (L - R) / (1 ~ L + R)
from th~ tan~nt block 15~. To conv~rt this signal to L - R the term must
b~ multiplied by 1 + L ~ R, which is the ern~elope det~tor output in the
AM mod4. Th~rsfor~, tor AM Si~r~o th~ switched port of th~ de~der
multiplier (157) is set to rec~ive tha snv~lope output. The rasultin~
calclllation is {(L- R)/(1 + L + ~)} (1 + L + R) Y (L - R). This L- R can
then be supplied to th0 natrix for c~nvsrsion into ster~o. The L + R term
to the matrix (not shown) is supplied from tha ~nvalope d~t~ctor 141.
Whsr~as tho previous multipli~rs used in the radio could be of the
MC1596 chopper type, since an IF si~nal was bein~ multiplied a~ainst
an IF si~nal, in the pr 3sent multiplication it is two baseband audio
signals that aro bein~ multiplied. Thsr0tore ths mu~i~iar 157 must ba of
a type ~ibly ~imilar to a MC159~.
The pilot ton~ dete~or for both the AM and FM si~nals consists of,
the I multipli~r (145), th~ a mul~ipli~r (147), the quadratur~ genora~or
(127), the VCO (125), 1 filter capacitor (149), loop filter capacitor (123)
and threshold dat~ctor 502. This is shown in hg. 1. The pilot tone
detector stnwtu~e is similar to that of a LM567C tone decodar or the
dec~der desoribed in Lawrance M. Ecklund, "Tone Detector with Pseudo
Phase Locked Loop,~ U. S. Patent 4,618,981, issu~ October 21, 1986,
which patant is heraby incorporated by referen~. The structure of the
tone decoder can be ba~ioally divided into two parts. Th0 first part
includes the Q de~ector (147), VC0 (12~), loop filter (123) and
WO 92/14299 PCl'/US92/01017
-11- 2ioo~47
quadrature generator (1?7). This section is used to deterrnine the exact
phase of the inc~min~ pilot sign~ and in the FM mode it also develops a
38kHz refer~nce signal that is phasa cohoren~ with the suppressed
38kHz referenc~ signal. Th~ second part of tha detector includ~s the I
multipli~r (145), I filt~r (149), quadratura network (127), VCO (125) and
thr~shold d~t~ctor 502. Wh~n a pilot ton~ is pres~nt and th0 d~od~r is
~locked~, tha I d~tector davelops a DC volta~a which corrosponds to the
amplitude o~ th~ pilot tons. Whan this volta~ raach~s a pr0s~t l~val it
tris~srs the thr~shold det~or (502) which indicat~s to th~ ussr, through
some interface devica, th~ the s~ion is in stereo.
In ths particular c~e of AM s~oreo ths pilot tone is ~t 25Hz and i
pres~nt in the L - R si~nal at the output of th~ deoodar multiplier (157).
This pilot ton~ is prssent with all the othor L - R audio information and is
at a valua of 5% with r~spe~ to the maximum L - R information. The
1~ si~nai is supplied to ons por~ of multipli~r (147) and multiplier ~145).
Wh~n th0 d~odsr is 'locked' tha oth~r port in th0 Q multiplisr is supplied
by sin (2~ ~ 25t). Ther~fore the resultin~ output ot the multiplier is [{L - R}
+ .05 ~ cos (2J~ 25t + o)] ~ sin (2~ ~ 25t) ~ (L~ sin (2~^ 25t + o) +
.02~ ~ sin (0 ~ 0) +.025 ~ slin (2 ~ (21~ ~ 25t ~ o)). It is assumed that th~
first and the third terms h~o only high fr0qu~ncy ~n~r~y æ ~ompared
with the desired second terrn and if filtered by the capac`itor (123~ can be
ignored. . Ths second tenn is 0 in th~ locked state sinc~ 3 is asslJmed to
bs equal to 0. This loop perfor~ as a PLL, thc behavior of, which is well
~ow in the indust~. -
Tha YCO frequency for tha pilot tone VCO (12~) is deflned the
same way as thfl AM dec~der VCO (161 ) e.~. Fout, {(Vref - Vp~ k} +
Fraf, wh~rs k = VCO gain per volt, Vref is ~h~ tuning volta~e at th~
oscillator fr~e running frequency and Vpll is the volta~e on capacitor
(123). The VCO is ~o be used for FM stereo whers the pilot frequ~ncy is
1 9kHz and a phas~ accurate 38kHz signal mus~ bs g~nsrat~d. Further
the same VCO is ussd for AM Stsreo wher0 tha frequenoy of the pilot is
25Hz. On0 relativ~ly 0asy way to ac~ommodate this d~lta in fraquency is
to place a digital divider circuit between the VCO (125) and tha
Quadrature g3nerator (127). this is shown in attached Fig. 4. Typically
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the VCO (125) fr~quency for FM is 76kHz and the quadratura ~en0rator
(127) is a + 4 circuit. The quadrature nehvork is formed of thre~ divid~ by
2 ~rcl~its shown in Fig. 3 and will be diso,ussed in the FM section later.
Raturning to Fi~. 4 in the FM m~ the VCO t125) is ~nne~ed directly
5 throu~h the switch 3 to the quadrature generator (127) and ~he output
frequency to th~ multipliers is 76k~1z+4 or 1 9kHz. For ~he AM mode the
frequ0ncy into the quadrature generator is 1 OOHz to supply the 2~Hz
r~f~rance signal to tha multipli9rs. Th9ïefOr~, th~ divide numb~r for
divid~r 2 is 760. This divider is similar to MC14024, but r~uiring 1 1
10 divider sta~es to gonerat~ division by 760.
A second form ot the VCO which is mor~ practical for some
applications is shown in Fi~. 5. In this case a dual PLL i5 used and tha
VCO's could have an operation similar to a LM565. Th~ main reason for
this approaoh is to avoid the extemal r~sonant elemsnt ~121 ) shown in
15 Fg. 1, dev~lop a DC voltag0 that corrasponds to a ~iv0n VCO frequency
and also r~dudng any spurious signals that th~ resonant el~msnt may
produce in the broadc ast band. in this approach th~ first PLL 402 is used
to ~erive the correct nominal Vpll voltaQ~ for VCO ~125). The resistiva
and capaativ3 ~lem~nt~ to ~orm 406 and 121 are c~nstructed on the
20 decoder IC. In Inte~ratad Circ~Jits the absolut0 valu~ of compon~nts is
poor, but th~ matchin~ betwa~n similar eomponsnts is very good.
Thercfore, tho volta~e 'Vpll' 403 which will ~ive th~ VCO in ~LL (402) of
Fg. ~ a fr~quency F, will also ~iv~ th0 VCO (125) fr~uency F, provided
that tha resonator eiemsnts (121 ) are cunstructed on ths sam~ die with
2~ that of 4~6. Giv~n this phonomena, and that 402 frequancy locks and
cor~tions (rQmov~s ths modulation) of tho output of 404, the YC:O (125)
has the same free running frequency as ~he output of 404. Ther~fore,
this processin~ c~uld use the output ot the lirn~er (143), rasonant
element (137, 139) or tha decoder divider (163) divided by 404 to supply
30 the desir~d VCO (125) nominal oporating fr0quency.
A possible variation of this approach is to scale the rssistive and
capacitive olem~nts of (121 ) and 406 that sa~ the c0nter frequency of th~
two VCO's. Sinca, the ratio of th~ IC elam~nts is good, the fr~qusncy
setting resistor and capacitor (121 ) of th~ soc~nd VCO can be made
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-13 - 2~ 004 ~ 7
larser than the first VCO thus lowerin~ the frequency of the second VCO,
minimizing the numb~r of divider sections needed in 404
Fig. 6 shows a detailed picture of the pilot indic~or circuit. Actual
indication of th~ pilot tone comes from the I detector (145). Th~ circuit
has a cos (2~T I 25t) r~f~r~ncs si~nal (17~) suppli~d trom th~ quadrature
g~n~rator of ths pilot ton~ PLL. Th~ oth~r signal suppliad ~o th~ I
d~tector is L - R, which as previously st~ed contains (.05) ~ (21~ ~ 25t)
for AM stareo. Th~ r~sulting output of tha mu~tipliar is ,025 ' cos ~0) =
.025 and the high frequency terrns. This multiplier ou~put is ~Itered by the
capaator (149) and c~mpared ~nth a ref~renc~ thr~shold 'VBB' as
shown. If the size ot the si~nal is large enou~h ~o axc~ed the piloS tone
level ref~renc~ leval \tBB' thsn th~ output of the threshold d0tector goes
high indicatin~ a st~r~o station is pr~s~nt.
Ratuming to Fi~. 1 and tho FM pilot ton~ d~tsctor. Th~ 19 kHz
Gomposit~ bas~band si~nal including ths FM piiot tone is 8uppli~d ts th~
pilot ton~ I and Q multipliers 145, 147 from thQ output of the tang~nt
linearizin~ circuit 1~5. A~ with th~ AM approach ~he pilot tono d~tector
can bs s~parated into two major compon~nts. Al~ befors thsso
~mponents are th~ PLL pilot ton~ axtraction system and the actual
detection of ~hs pilot ton~ lov~l. Th~r~ aro som0 ~ain and level chang0s
that are necessary in the PLL dependin~ upon tho K1 term in ths
discriminator. Assumin~ tha~ tha o~put ~ain of ~hc tan~ent function 155
is switched so tha~ 100% AM modulation is the samc as 100% FM
moduiation. Then tha only chan~s n~ssary ar~ to switch out tha
divider tB3, as shown in Fl~. 4 and chan~ the valua of VB~ to 9/5
hiyh~r to account for th~ 9% FM pilot l~val v~rsus th~ 5% pilot loval for
AM stsr~o.
The stnK:tura of the quadratur~ ~enerator 127, which is similar to
that of th~ MC1310, is shown in Fig. 3. As shown ther~in, ths 76 kHz
input 167 is suppliod to the input of th~ first divider 301 and is divid~d by
2. This divid~r has two outputs 207, 309 at 38 kHz which ara 1 80 out of
phase with each other. Th3 0 output 307 is supplied as the (3~ kH~)
drive signal 169 to the FM sterso d~c~dsr. This output 307 is also
supplied to anothsr ~ 2 circuit 303 whose output supplios the I pilot tone
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2~0~7 - 14-
multiplior 145. Th~ -180 output supplies another+ 2 circuit 305 whose
output suppli~s th~ r~t6rence signal 173 for the Q pilot tone multiplier
147. This ~echniquo is w611 known.
While various ~mbodimsnts of an AM-FM Combin~d St3rl~0
5 Receiver, acc~rdin~ to the present invention, have be~n describ~
herein above, ths scope of the invention is definec by the following
claims.
