LMX3305 National Semiconductor, LMX3305 Datasheet - Page 7

• Manufacturer Part Number: LMX3305
• Description: Triple Phase Locked Loop for RF Personal Communications
• Manufacturer: National Semiconductor

1.0 Functional Description
The LMX3305 phase-lock-loop (PLL) system configuration
consists of a high-stability crystal reference oscillator, three
frequency synthesizers, three voltage controlled oscillators
(VCO), and three passive loop filters. Each of the frequency
synthesizers includes a phase detector, a current mode
charge pump, as well as programmable reference [R] and
feedback [N] frequency dividers. The VCO frequency is es-
tablished by dividing the crystal reference signal down via
the R-counter to obtain a comparison reference frequency.
This reference signal (f
phase/frequency detector and compared with the feedback
signal (f
down by way of the N-counter, and fractional circuitry. The
phase/frequency detector’s current source output pumps
charge into the loop filter, which then converts the charge
into the VCO’s control voltage. The function of phase/
frequency comparator is to adjust the voltage presented to
the VCO until the feedback signal frequency and phase
match that of the reference signal. When the RF PLL is in a
“Phase-Locked” condition, the RF VCO frequency will be (N
+ F) times that of the comparison frequency, where N is the
integer divide ratio, and F is the fractional component. The
fractional synthesis allows the phase detector frequency to
be increased while maintaining the same frequency step
size for channel selection. The divider ratio N is thereby re-
duced giving a lower phase noise referred to the phase de-
tector input, and the comparison frequency is increased al-
lowing faster switching time.
1.1 REFERENCE OSCILLATOR INPUTS
The reference oscillator frequency for the RF and IF PLLs
are provided from the external references through the OSC
pin. OSC
tivity of 0.5 V
R-counters. OSC
can be driven from an external CMOS or TTL logic gate.
Typically, the OSC
oscillator.
1.2 REFERENCE DIVIDERS (R-COUNTERS)
The RF, Rx and Tx R-counters are clocked through the oscil-
lator block. The maximum frequency is 25 MHz. All RF, Rx
and Tx R-counters are CMOS design. The RF R-counter is
8-bit in length with programmable divider ratio from 2 to 255.
The Rx and Tx R-counters are 10-bit in length with program-
mable divider ratio from 2 to 1023.
1.3 PRESCALERS
The LMX3305 has a 16/17/20/21 quadruple modulus pres-
caler for the PCS application and a 8/9/12/13 quadruple
modulus prescaler for the cellular application. The Rx and Tx
prescalers are dual modulus with 8/9 modulus ratio. Both
RF/IF prescalers’ outputs drive the subsequent CMOS flip-
flop chain comprising the programmable N feedback
counters.
1.4 FEEDBACK DIVIDERS (N-COUNTERS)
The RF, Rx and Tx N-counters are clocked by the output of
RF, Rx and Tx prescalers respectively. The RF N-counter is
composed of two parts: the 15 MSB bits comprise the integer
portion and the 4 LSB bits comprise the fractional portion.
The RF fractional N divider is fully programmable from 80 to
32767 over the frequency range from 1200 MHz-2300 MHz
for PCS application and 40 to 16383 over the frequency
range from 800 MHz-1400 MHz for cellular application. The
N
IN
), which is obtained by dividing the VCO frequency
input can operate up to 25 MHz with input sensi-
PP
IN
minimum and it drives RF, Rx and Tx
IN
input has a V
is connected to the output of a crystal
R
) is then presented to the input of a
CC
/2 input threshold that
IN
7
4-bit fractional portion of the RF counter represents the frac-
tion’s numerator. The fraction’s denominator base is deter-
mined by the four FRAC_D register bits.
The Rx and Tx N-counters are each a 13-bit integer divisor,
fully programmable from 56 to 8,191 over the frequency
range from 45 MHz–600 MHz. The Rx and Tx N-counters do
not include fractional compensation.
1.5 FRACTIONAL COMPENSATION
The fractional compensation circuitry of the LMX3305 RF di-
vider allows the user to adjust the VCO tuning resolution in
1/2 through 1/16th increments of the phase detector com-
parison frequency. A 4-bit denominator register (FRAC_D)
selects the fractional modulo base. The integer averaging is
accomplished by using a 4-bit accumulator. A variable phase
delay stage compensates for the accumulated integer phase
error, minimizes the charge pump duty cycle and reduces
the spurious levels. This technique eliminates the need for
compensation current injection into the loop filter. An over-
flow signal generated by the accumulator is equivalent to
one full RF VCO cycle, and results in a pulse swallow.
1.6 PHASE/FREQUENCY DETECTORS
The RF and IF phase/frequency detectors are driven from
their respective N- and R-counter outputs. The maximum fre-
quency at the phase detector inputs is 10 MHz unless limited
by the minimum continuous divide ratio of the multi-modulus
prescaler. The phase detector output controls the charge
pump. The polarity of the pump-up or pump-down control is
programmed
Tx_PD_POL depending on whether RF or IF VCO charac-
teristics are positive or negative. The phase detector also re-
ceives a feedback signal from the charge pump in order to
eliminate dead zones.
1.7 CHARGE PUMPS
The phase detector’s current source output pumps charge
into an external loop filter, which then converts it into the
VCO’s control voltage. The charge pump steers the charge
pump output CP
down). When locked, CP
with small corrections. The IF charge pump output current
magnitudes are nominally 100 µA. The RF charge pump out-
put currents can be programmed by the RF_Icpo bits at
100 µA, 400 µA, 700 µA, or 900 µA.
1.8 VOLTAGE DOUBLER (V
The V
over a range of V
charge pump circuit. An internal voltage doubler circuit con-
nected between the V
lows V
cuit at close to twice the V
voltage doubler mode is enabled by setting the V2X bit to a
HIGH level. The voltage doubler’s charge pump driver origi-
nates from the oscillator input. The device will not totally
powerdown until the V2X bit is programmed LOW. The aver-
age delivery current of the doubler is less than the instanta-
neous current demand of the RF charge pump when active
and is thus not capable of sustaining a continuous out of lock
condition. A large external capacitor connected to V
(=0.1 µF) is needed to control power supply droop when
changing frequencies.
P
CC
pin is normally driven from an external power supply
= 3V (
using
±
10%) users to run the RF charge pump cir-
o
CC
to V
to 5.5V to provide current for the RF
CC
RF_PD_POL,
CC
o
and V
is primarily in a TRI-STATE mode
CC
(pump-up) or Ground (pump-
P
)
P
power supply voltage. The
supply pins alternately al-
Rx_PD_POL,
www.national.com
or
P