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DS8908BN
DS8908B AM/FM Digital Phase-Locked Loop Frequency Synthesizer
TL/F/5111
DS8908B
AM/FM
Digital
Phase-Locked
Loop
Frequency
Synthesizer
June 1990
DS8908B AM/FM Digital Phase-Locked
Loop Frequency Synthesizer
General Description
The DS8908BisaPLL synthesizer designed specificallyfor
usein AM/FM radios.It containsthe reference oscillator,a
phase comparator,a charge pump,an operational amplifier, 120 MHz ECL/I2L dual modulus programmable divider,
anda 19-bit shift register/latchfor serial data entry. The
deviceis designedto operate witha serial data controller
generatingthe necesary division codesfor each frequency,
and logic state informationfor radio functioninputs/outputs. 3.96 MHz pierce oscillatorand divider chain generatea
1.98 MHz externalcontroller clock,a20 kHz,10 kHz,9 kHz,
anda1 kHz reference signals, anda50Hz time-of-day
signal. The oscillatorand divider chainare sourcedbythe
VCCMpin thus providingalow power controller clock drive
and time-of-day indication whenthe balanceofthe PLLis
powered down.
The 21-bit serial data steramis transferred betweenthefre-
quency synthesizerandthe controllerviaa 3-wirebussys-
tem comprisedofa data line,a clock line, andan enable
line.
Thefirst2bitsinthe serial data stream addressthe synthe-
sizerthuspermitting other devices suchas display driversto
sharethe same bus. The next14bitsare usedforthe
PLL(Na1) divide code. The 15thbitis used internallyto
selecttheAMorFM local oscillator input.A high levelon
thisbit enablestheFM input anda lowlevel enablestheAM
input.The 16thand 17thbitsare usedto selectoneof the4
reference frequencies. The 18thand 19thbitsare connect-via latchesto open collector outputs. These outputscan usedto drive radio functions suchas gain, mute, AM,
FM,or charge pump current source levels.
The PLL consistsofa 14-bit programmableI2L divider,an
ECL phase comparator,an ECL dualmodulus (p/p a1)pre-
scaler,a high speed charge pump,andan operationalam-
plifier.The programmable divider dividesby (Na1),N being
the number loaded intothe shift register. Theprogrammable
divideris clocked througha d-/8 prescalerbytheAM input througha d$*/64 prescalerbytheFM input.TheAM input
will workat frequenciesupto15 MHz, whiletheFM input
worksupto 120 MHz. The VCOcanbe tuned withafre-
quency resolutionof either1 kHz,9 kHz,10 kHz,or20 kHz.
The bufferedAMandFM inputsare self-biasedandcanbe
driven directlybythe VCO througha capacitor. The ECL
phase comparator produces very accurate resolutionofthe
phase difference betweenthe input signalandthe reference
oscillator. The high speed charge pump consistsofa
switchable constant current source and sink. The charge
pumpcanbe programmedto deliver from75mAto750mA constant currentby connectionofan external resistor
frompin RPROGRAMto groundorthe open collectorbitout-
puts. Connectionofprogramming resistorstothe bitoutputs
enablesthe controllerto adjustthe loop gainforthe particu-
lar reference frequency selected. The charge pumpwill
source currentifthe VCO frequencyis high and sink
currentifthe VCO frequencyis low. Thelow noise opera-
tional amplifier providedhasa high impedance JFET input
anda large output voltage range. Theop amp’s negative
inputis common withthe charge pump outputandits posi-
tive inputis internally biased.
Features Uses inexpensive 3.96 MHz reference crystal FIN capability greater than 120 MHz allows direct syn-
thesisatFM frequenciesFM resolutionof either10 kHzor20kHz allows usage 10.7 MHz ceramic filter distribution Serial data entryfor simplified control50Hz outputfor time-of-day reference driven from sep-
aratelow power VCCM2 open collector buffered outputsfor controlling various
radio functionsor loop gain SeparateAMandFM inputs;AM inputhas15mV (typi-
cal) hysteresis Programmable charge pump current sources enablead-
justmentof system loop gain Operational amplifier provides high impedance loadto
charge pump output anda wide voltage rangeforthe
VCO input
Connection Diagram
Dual-In-LinePackage
TL/F/5111–1TopView
Order Number DS8908BN
SeeNS Package Number N20A
TRI-STATEÉ isaregistered trademarkof National SemiconductorCorp.
C1995National SemiconductorCorporation RRD-B30M105/PrintedinU.S.A.
