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LMX2323 Datasheet(PDF) 6 Page - National Semiconductor (TI)

[Old version datasheet] Texas Instruments acquired National semiconductor.
Part No. LMX2323
Description  PLLatinum 2.0 GHz Frequency Synthesizer for RF Personal Communications
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Manufacturer  NSC [National Semiconductor (TI)]
Direct Link  http://www.national.com
Logo NSC - National Semiconductor (TI)

LMX2323 Datasheet(HTML) 6 Page - National Semiconductor (TI)

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1.0 Functional Description
The basic phase-lock-loop (PLL) configuration consists of a
high-stability crystal reference oscillator, a frequency synthe-
sizer such as the National Semiconductor LMX2323, a volt-
age controlled oscillator (VCO), and a passive loop filter. The
frequency synthesizer includes a phase detector, current
mode charge pump, as well as programmable reference [R]
and feedback [N] frequency dividers. The VCO frequency is
established by dividing the crystal reference signal down via
the R counter to obtain a frequency that sets the comparison
frequency. This reference signal, f
r, is then presented to the
input of a phase/frequency detector and compared with an-
other signal, f
p, the feedback signal, which was obtained by
dividing the VCO frequency down by way of the N counter.
The phase/frequency detector’s current source outputs
pump charge into the loop filter, which then converts the
charge into the VCO’s control voltage. The phase/frequency
comparator’s function is to adjust the voltage presented to
the VCO until the feedback signal’s frequency (and phase)
match that of the reference signal. When this “phase-locked”
condition exists, the RF VCO’s frequency will be N times that
of the comparison frequency, where N is the divider ratio.
The reference oscillator frequency for the PLL is provided by
an external reference TCXO through the OSC
block can operate to 40 MHz. The inputs have a V
CC/2 input
threshold and can be driven from an external CMOS or TTL
logic gate.
The R Counter is clocked through the oscillator block. The
maximum input frequency is 40 MHz and the maximum
output frequency is 10 MHz. The R Counter is a 10-bit
CMOS binary counter with a divide range from 2 to 1,023.
See programming description 2.2.1.
The N counter is clocked by the small signal f
IN input. The
LMX2323 RF N counter is a 15-bit integer divider. The N
counter is configured as a 5-bit A Counter and a 10-bit B
Counter, offering a continuous integer divide range from 992
to 32,767. The LMX2323 is capable of operating from
100 MHz to 2.0 GHz with a 32/33 prescaler.
1.3.1 Prescaler
The RF inputs to the prescaler consist of the f
IN and fINB pins
which are the complimentary inputs of a differential pair
amplifier. The differential f
IN configuration can operate to
2 GHz. The input buffer drives the N counter’s ECL D-type
flip-flops in a dual modulus configuration. The LMX2323 has
a 32/33 prescaler ratio. The prescaler clocks the subsequent
CMOS flip-flop chain comprising the fully programmable A
and B counters.
The phase/frequency detector is driven from the N and R
counter outputs. The maximum frequency at the phase de-
tector inputs is 10 MHz. The phase detector outputs control
the charge pumps. The polarity of the pump-up or pump-
down control is programmed using PD_POL, depending on
whether RF VCO characteristics are positive or negative
(see programming description 2.2.2). The phase detector
also receives a feedback signal from the charge pump, in
order to eliminate dead zone.
The phase detector’s current source output pumps charge
into an external loop filter, which then converts the charge
into the VCO’s control voltage. The charge pumps steer the
charge pump output, CP
(pump-up) or Ground
(pump-down). When locked, CP
o is primarily in a TRI-STATE
mode with small corrections. The RF charge pump output
current magnitude is set to 4.0 mA. The charge pump output
can also be used to output divider signals as detailed in
section 2.2.3.
The programmable functions are accessed through the
MICROWIRE serial interface. The interface is made of three
functions: clock, data and latch enable (LE). Serial data for
the various counters is clocked in from data on the rising
edge of clock, into the 18-bit shift register. Data is entered
MSB first. The last bit decodes the internal register address.
On the rising edge of LE, data stored in the shift register is
loaded into one of the two appropriate latches (selected by
address bits). A complete programming description is in-
cluded in the following sections.
A digital filtered lock detect function is included through an
internal digital filter to produce a CMOS logic output avail-
able on the LD output pin if selected. The lock detect output
is high when the error between the phase detector inputs is
less than 15 ns for five consecutive comparison cycles. The
lock detect output is low when the error between the phase
detector input is more than 30 ns for one comparison cycle.
An open drain, analog lock detect status generated from the
phase detector is also available on the LD output pin, if
selected. The analog lock detect output goes high when the
charge pump is inactive. It goes low when the charge pump
is active during a comparison cycle. When the PLL is in
power down mode, the LD output is always high.
The PLL can be power controlled in two ways. The first
method is by setting the CE pin LOW. This asynchronously
powers down the PLL and TRI-STATEs the charge pump
output, regardless of the PWDN bit status. The second
method is by programming through MICROWIRE, while
keeping the CE HIGH. Programming the PWDN bit in the N
register HIGH (CE = HIGH) will disable the N counter and
de-bias the f
IN input (to a high impedance state). The R
counter functionality also becomes disabled. The reference
oscillator block powers down when the power down bit is
asserted. The OSC
IN pin reverts to a high impedance state
when this condition exists. Power down forces the charge
pump and phase comparator logic to a TRI-STATE condition.
A power down counter reset function resets both N and R
counters. Upon powering up the N counter resumes count-
ing in “close” alignment with the R counter (the maximum
error is one prescaler cycle). The MICROWIRE control reg-
ister remains active and capable of loading and latching in
data during all of the power down modes.

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