LMX2350 National Semiconductor Corporation, LMX2350 Datasheet - Page 10

• Manufacturer Part Number: LMX2350
• Description: Pllatinum Tm Fractional N Rf / Integer N If Dual Low Power Frequency Synthesizer
• Manufacturer: National Semiconductor Corporation

www.national.com
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 LMX2350/52, a
voltage controlled oscillator (VCO), and a passive loop filter.
The frequency synthesizer includes a phase detector, cur-
rent 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 com-
parison frequency. This reference signal, fr, is then pre-
sented to the input of a phase/frequency detector and com-
pared with another signal, fp, the feedback signal, which was
obtained by dividing the VCO frequency down by way of the
N counter and fractional circuitry. The phase/frequency de-
tector’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+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
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 PLL’s is
provided by either an external reference through the OSCin
pin and OSCx pin, or an external crystal resonator across
the OSCin and OSCx pins. OSCin/OSCx block can operate
to 50MHz with an input sensitivity of 0.5Vpp. The OSC bit
(see programming description 3.1.1), selects whether the
oscillator input pins OSCin and OSCx drive the IF and RF R
counters separately (Low) or by a common input signal path
(Hi). The common OSC mode allows the user to form a local
crystal oscillator circuit or drive the OSCin pin from an ex-
ternal signal source. When a crystal resonator is connected
between OSCin and OSCx along with 2 external capacitors
to form a crystal oscillator both reference chains are driven
simultaneously. When a TCXO is connected only at the
OSCin input pin and not at the OSCx pin, the TCXO drives
both IF R counter and RF R counter. When configured as
separate inputs, the OSCin pin drives the IF R counter while
the OSCx drives the RF R counter. The inputs have a Vcc/2
input threshold 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 50MHz. Both R Counters are 15 bit CMOS
counters with a divide range from 3 to 32,767. (See program-
ming description 3.1.3.)
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 LMX2350 RF N
counter is 19 bits with 15 bits integer divide and 4 bits
fractional. The integer part is configured as a 5 bit A Counter
10
and a 10 bit B Counter. The LMX2350 is capable of operat-
ing from 500 MHz to 1.2 GHz with the 16/17 prescaler
offering a continuous integer divide range from 272 to
16399, and 1.2 GHz to 2.5 GHz with the 32/33 prescaler
offering a continuous integer divide range from 1056 to
32767. The LMX2352 RF N counter is 18 bits with 14 bits
integer divide and 4 bits fractional. The integer part is con-
figured as a 4 bit A Counter and a 10 bit B Counter. The
LMX2352 is capable of operating from 250 MHz to 500 MHz
with the 8/9 prescaler offering a continuous integer divide
range from 72 to 8199, and 500MHz to 1.2 GHz with 16/17
prescaler offering a continuous integer divide range from 272
to 16383. The RF counters for the LMX2350 family also
contain fractional compensation, programmable in either
1/15 or 1/16 modes. Both LMX2350 and LMX2352 IF N
counters are 15 bit integer dividers 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 counters do not include
fractional compensation.
1.3.1 Prescaler
The RF and IF inputs to the prescaler consist of fin and /fin;
which are complimentary inputs to differential pair amplifiers.
The complimentary inputs are internally coupled to ground
with a 10 pF capacitor. These inputs are typically AC coupled
to ground through external capacitors as well. The input
buffer drives the A counter’s ECL D-type flip flops in a dual
modulus configuration. A 16/17 or 32/33 prescale ratio can
be selected for the LMX2350, and the lower frequency
LMX2352 provides 8/9 or 16/17 prescale ratios. The IF
circuitry for both the LMX2350 and LMX2352 contain an 8/9
prescaler. The prescaler clocks the subsequent CMOS flip-
flop chain comprising the fully programmable A and B
counters.
1.3.2 Fractional Compensation
The fractional compensation circuitry of the LMX2350 and
LMX2352 RF dividers allow the user to adjust the VCO’s
tuning resolution in 1/16 or 1/15 increments of the phase
detector comparison frequency. A 4 bit register is pro-
grammed with the fractions desired numerator, while another
bit selects between fractional 15 and 16 modulo base de-
nominator (see programming description 4.2.4). An integer
average is accomplished by using a 4 bit accumulator. A
variable phase delay stage compensates for the accumu-
lated integer phase error, minimizing the charge pump duty
cycle, and reducing spurious levels. This technique elimi-
nates the need for compensation current injection in to the
loop filter. Overflow signals generated by the accumulator
are equivalent to 1 full VCO cycle, and result in a pulse
swallow.
1.4 Phase/Frequency Detector
The RF and IF phase(/frequency) detectors are driven from
their respective N and R counter outputs. The maximum
frequency at the phase detector inputs is about 10 MHz for
some high frequency VCO due to the minimum continuous
divide ratio of the dual modulus prescaler. For example if the
phase detector frequency exceeds 2.37 MHz, there are
higher chances of running into illegal divide ratios, because
the mimimum continuous divide ratio of the LMX2350 with
32/33 prescaler is 1056. The phase detector outputs control
the charge pumps. The polarity of the pump-up or pump-
down control is programmed using RF_PD_POL or IF_PD-
_POL depending on whether RF/IF VCO characteristics are
positive or negative (see programming descriptions 3.1.4