LMX2331UEVAL National Semiconductor, LMX2331UEVAL Datasheet - Page 37

• Manufacturer Part Number: LMX2331UEVAL
• Manufacturer: National Semiconductor

Specifications of LMX2331UEVAL

• Lead Free Status / Rohs Status: Not Compliant

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 LMX233xU, a
voltage controlled oscillator (VCO), and a passive loop filter.
The frequency synthesizer includes a phase detector, cur-
rent mode charge pump, programmable reference R and
feedback N frequency dividers. The VCO frequency is es-
tablished by dividing the crystal reference signal down via
the reference divider to obtain a comparison reference fre-
quency. This reference signal, F
input of a phase/frequency detector and compared with the
feedback signal, F
frequency down by way of the feedback divider. The phase/
frequency detector measures the phase error between the F
and F
proportional to the phase error. The charge pump then
pumps charge into or out of the loop filter based on the
magnitude and direction of the phase error. The loop filter
converts the charge into a stable control voltage for the
VCO. The phase/frequency detector’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 VCO fre-
quency will be N times that of the comparison frequency,
where N is the feedback divider ratio.
1.1 REFERENCE OSCILLATOR INPUT
The reference oscillator frequency for both the RF and IF
PLLs is provided from an external reference via the OSC
pin. The reference buffer circuit supports input frequencies
from 5 to 40 MHz with a minimum input sensitivity of 0.5 V
The reference buffer circuit has an approximate V
threshold and can be driven from an external CMOS or TTL
logic gate. Typically, the OSC
of a crystal oscillator.
1.2 REFERENCE DIVIDERS (R COUNTERS)
The reference dividers divide the reference input signal,
OSC
circuits feeds the reference input of the phase detector. This
reference input to the phase detector is often referred to as
the comparison frequency. The divide ratio should be chosen
such that the maximum phase comparison frequency (F
or F
The RF and IF reference dividers are each comprised of
15-bit CMOS binary counters that support a continuous in-
teger divide ratio from 3 to 32767. The RF and IF reference
divider circuits are clocked by the output of the reference
buffer circuit which is common to both.
1.3 PRESCALERS
The f
of a bipolar, differential-pair amplifier. The output of the bi-
polar, differential-pair amplifier drives a chain of ECL D-type
flip-flops in a dual modulus configuration. The output of the
prescaler is used to clock the subsequent feedback dividers.
The RF and IF PLL complementary inputs can be driven
differentially, or the negative input can be AC coupled to
ground through an external capacitor for single ended con-
figuration. A 32/33 or a 64/65 prescale ratio can be selected
for the 2.5 GHz LMX2330U RF synthesizer. A 64/65 or a
128/129 prescale ratio can be selected for both the
φIF
in
IN
p
, by a factor of R. The output of the reference divider
) of 10 MHz is not exceeded.
RF (f
signals and outputs control signals that are directly
IN
IF) and f
p
, which was obtained by dividing the VCO
IN
RF (f
in
IN
pin is connected to the output
IF) input pins drive the input
r
, is then presented to the
CC
/2 input
φRF
PP
in
r
.
37
LMX2331U and LMX2332U RF synthesizers. The IF circuitry
contains an 8/9 or a 16/17 prescaler.
1.4 PROGRAMMABLE FEEDBACK DIVIDERS (N
COUNTERS)
The programmable feedback dividers operate in concert with
the prescalers to divide the input signal, f
The output of the programmable reference divider is pro-
vided to the feedback input of the phase detector circuit. The
divide ratio should be chosen such that the maximum phase
comparison frequency (F
ceeded.
The programmable feedback divider circuit is comprised of
an A counter (swallow counter) and a B counter (program-
mble binary counter). The RF N_CNTRA counter is a 7-bit
CMOS swallow counter, programmable from 0 to 127. The IF
N_CNTRA counter is also a 7-bit CMOS swallow counter,
but programmable from 0 to 15. The three most significant
bits are ’don’t cares’ in this case. The RF N_CNTRB and IF
N_CNTRB counters are both 11-bit CMOS binary counters,
programmable from 3 to 2047. A continuous integer divide
ratio is achieved if N ≥ P
prescaler selected. Divide ratios less than the minimum con-
tinuous divide ratio are achievable as long as the binary
programmable counter value is greater than the swallow
counter value (N_CNTRB ≥ N_CNTRA). Refer to Sections
2.5.1, 2.5.2, 2.7.1 and 2.7.2 for details on how to program
the N_CNTRA and N_CNTRB counters. The following equa-
tions are useful in determining and programming a particular
value of N:
N = (P x N_CNTRB) + N_CNTRA
f
Definitions:
1.5 PHASE/FREQUENCY DETECTORS
The RF and IF phase/frequency detectors are driven from
their respective N and R counter outputs. The maximum
frequency for both the RF and IF phase detector inputs is 10
MHz. The phase/frequency detector outputs control the re-
spective charge pumps. The polarity of the pump-up or
pump-down control signals are programmed using the PD-
_POL RF or PD_POL IF control bits, depending on whether
the RF or IF VCO characteristics are positive or negative.
Refer to Sections 2.4.2 and 2.6.2 for more details. The
phase/frequency detectors have a detection range of −2π to
+2π. The phase/frequency detectors also receive a feedback
signal from the charge pump in order to eliminate dead zone.
IN
F
f
N_CNTRA: RF or IF A counter value
N_CNTRB: RF or IF B counter value
P:
IN
= N x F
φ
:
:
φ
RF
frequency
RF or IF input frequency
Preset
prescaler
LMX2330U RF synthesizer: P = 32 or 64
LMX2331U RF synthesizer: P = 64 or 128
LMX2332U RF synthesizer: P = 64 or 128
LMX233xU IF synthesizer: P = 8 or 16
or
modulus
IF
*
φRF
(P−1), where P is the value of the
phase
or F
φIF
of
) of 10 MHz is not ex-
detector
the
IN
, by a factor of N.
dual
comparison
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