LMX2355SLB National Semiconductor, LMX2355SLB Datasheet - Page 10

• Manufacturer Part Number: LMX2355SLB
• Description: PLLatinum Fractional N RF/ Integer N IF Dual Low Power Frequency Synthesizer
• Manufacturer: National Semiconductor

www.national.com
32,767
48–79
1–47
1056
Functional Description
1.0 GENERAL
The basic phase-lock-loop (PLL) configuration consists of a
high-stability crystal reference oscillator, a frequency synthe-
sizer such as the National Semiconductor LMX2354, 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
input of a phase/frequency detector and compared with an-
other signal, f
dividing the VCO frequency down by way of the N counter
and fractional circuitry. The phase/frequency detector’s cur-
rent 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 volt-
age presented to the VCO until the feedback signal’s fre-
quency (and phase) match that of the reference signal.
When this ‘phase-locked’ condition exists, the RF VCO’s
frequency will be N+F times that of the comparison fre-
quency, 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
division value N is thereby reduced giving a lower phase
noise referred to the phase detector input, and the compari-
son frequency is increased allowing faster switching times.
1.1 REFERENCE OSCILLATOR INPUTS
The reference oscillator frequency for the RF and IF PLLs is
provided by an external reference through the OSC
OSC
an input sensitivity of 0.5 Vpp. The OSC bit (see program-
ming description 4.1.1), selects whether the oscillator input
pins OSC
separately or by a common input signal path. When an
external TCXO is connected only at the OSC
not at the OSC
*
80*
81
...
...
Minimum continuous divide ratio is P • [MAX{A,B}+2]
N
RF
pin. OSC
19
0
0
0
1
.
IF
and OSC
p
, the feedback signal, which was obtained by
18
0
0
0
1
RF
.
IF
/OSC
pin, the TCXO drives both IF R counter
Divide ratios less than 48 are impossible since it is required that C 3
17
0
0
0
1
.
RF
LMX2354 RF N Counter Register in Fractional Mode with P = 16/17/20/21:
RF
Some of these values are legal divide ratios, some are not
drive the IF and RF R counters
16
block can operate 50 MHz with
0
0
0
1
.
15
r
0
0
1
1
.
, is then presented to the
C Word
14
0
0
0
1
.
IF
13
input pin and
0
0
0
1
.
IF
pin and
12
0
0
0
1
.
11
1
1
0
1
.
10
10
0
0
1
1
.
and RF R counter. When configured as separate inputs, the
OSC
the RF R counter. The inputs have a V
and can be driven from an external CMOS or TTL logic gate.
1.2 REFERENCE DIVIDERS (R COUNTERS)
The RF and IF R Counters are clocked through the oscillator
block either separately or in common. The maximum fre-
quency is 50 MHz. Both R Counters are 15-bit CMOS
counters with a divide range from 3 to 32,767. (See program-
ming description 4.1.3.)
1.3 PROGRAMMABLE DIVIDERS (N COUNTERS)
The RF and IF N Counters are clocked by the small signal fin
RF and fin IF input pins respectively. The RF N Counter can
be configured as a fractional or fully integer counter. The
LMX2354 RF N counter is 19 bits with 15 bits integer divide
and 4 bits fractional. The integer part is configured as a 2-bit
A Counter, a 2-bit B Counter and a 11-bit C Counter. The
LMX2354 is capable of operating from 500 MHz to 1.2 GHz
with the 8/9/12/13 prescaler offering a continuous integer
divide range from 40 to 16,383 in fractional mode and 24 to
262143 in full integer mode. The LMX2354 is capable of
operating from 1.2 GHz to 2.5 GHz with the 16/17/20/21
prescaler offering a continuous integer divide range from 80
to 32,767 in fractional mode and 48 to 52,4287 in full integer
mode. The RF counters for the LMX2354 also contain frac-
tional compensation, programmable in either 1/15 or 1/16
modes. The LMX2354 IF N counter is 15-bit integer divider
configured with a 3-bit A Counter and a 12-bit B Counter
offering a continuous integer divide range from 56 to 32,767
over the frequency range of 10 MHz to 550 MHz. The IF N
counter does not include fractional compensation. The
tables below show the differences between the LMX2354 in
integer mode and in quadruple modulus prescaler with P =
16/17/20/21. Also, the tables show that the bit used for the
lower modulus prescaler values is different between the
LMX2350 and the LMX2354. For the LMX2350 bit N
(MSB of the A Word) is used for the 16/17 modulus and for
the LMX2354 bit N
lus. So if the LMX2354 is replacing a LMX2350 then bits
N
<
8
IF
>
9
1
1
0
1
.
pin drives the IF R counter while the OSC
and N
8
0
0
0
1
B Word
.
<
9
>
need to be swapped.
<
7
0
0
0
1
.
8
>
=0 is used for the 8/9/12/13 modu-
0
0
0
1
6
A Word
.
5
0
1
0
1
.
the fractional word when
These bits are used for
the part is operated in
4
CC
Fractional Word
fractional mode
/2 input threshold
3
RF
<
2
drives
9
>
=0
1