LMX2471 National Semiconductor, LMX2471 Datasheet - Page 18

• Manufacturer Part Number: LMX2471
• Description: 3.6 GHz Delta-Sigma Fractional-N PLL with 1.7 GHz Integer-N PLL
• Manufacturer: National Semiconductor

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 LMX2471, 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 comparison frequency, f
phase/frequency detector and compared with another signal,
f
VCO frequency down by way of the N counter and fractional
circuitry. The phase/frequency detector’s current source out-
puts a charge into the loop filter, which is then converted into
the VCO’s control voltage. The function of the phase/
frequency comparator is to adjust the voltage presented to
the VCO until the frequency and phase of the feedback
signal match that of the reference signal. When this ‘phase-
locked’ condition exists, the VCO frequency will be N+F
times that of the comparison frequency, where N is the
integer component of the divide ratio and F is the fractional
component. Fractional synthesis allows the phase detector
frequency to be increased while maintaining the same fre-
quency 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 comparison frequency is in-
creased allowing faster switching times.
1.1 PHASE DETECTOR OPERATING FREQUENCY
The maximum phase detector operating frequency for the
LMX2471 is 50 MHz. However, this is not possible in all
circumstances due to illegal divide ratios of the N counter.
The crystal reference frequency also limits the phase detec-
tor frequency. There are trade-offs in choosing what phase
detector frequency to operate at. If this frequency is run
higher, then phase noise will be lower, but lock time may be
increased due to cycle slipping. After this phase detector
frequency gets sufficiently high, then there are diminishing
returns for phase noise, and raising the charge pump current
has a greater impact on phase noise. This phase detector
frequency also has an impact on fractional spurs. In general,
the spur performance is better at higher phase detector
N
, the feedback signal, which was obtained by dividing the
EN Pin
High
High
High
High
High
Low
ATPU[1:0] Bits Enabled +
RF N Counter Written To
Yes
No
No
No
No
X
COMP
RF_PD Bit
, is input of a
X
X
0
0
1
1
RF_CPT Bit
18
X
X
0
1
0
1
frequencies, although this is application specific. The current
consumption may also slightly increase with higher phase
detector frequencies.
1.2 OSCILLATOR
The LMX2471 provides maximum flexibility for choosing an
oscillator reference. One possible method is to use a single-
ended TCXO to drive the OSCin pin. The part can also be
configured to be driven differentially using the OSCin and
OSCout* pins. Note that the OSCin and OSCout* pins can
not be used as an inverter for a crystal. Selection between
these two modes does have a noticeable impact on phase
noise and sub-fractional spurs. Regardless of which mode is
used, the performance is generally best for higher oscillator
power levels.
1.3 POWER DOWN AND POWER UP MODES
The power down state of the LMX2471 is controlled by many
factors. The one factor that overrides all other factors is the
EN pin. If this pin is low, this guarantees the part will be
powered down. Asserting a high logic level on EN is neces-
sary to power up the chip, however, there are other bits in the
programming registers that can override this and put the PLL
back in a power down state. Provided that the voltage on the
EN pin is high, programming the RF_PD and IF_PD bits to
zero guarantees that the part will be powered up. Program-
ming either one of these bits to one will power down the
appropriate section of the synthesizer, provided that the
ATPU[1:0] ( Auto Power Up ) bits do not override this.
There are many different ways to power down this chip and
many different things that can be powered down. This sec-
tion discusses how to power down the PLLs on the chip.
There are two terms that need to be defined first: synchro-
nous power down and asynchronous power down. In the
case of synchronous power down, the PLL chip powers
down after the charge pump turns off. This is best to prevent
unwanted frequency glitches upon power up. However, in
certain cases where the charge pump is stuck on, such as
the case when there is no VCO signal applied, this type of
power down will not reliably work and asynchronous power
down is necessary. In the case of asynchronous power
down, the PLL powers down regardless of the status of the
charge pump. There are 4 factors that affect the power down
state of the chip: the EN pin, the power down bit, the TRI-
STATE bit, and writing to the RF N counter with the
RF_ATPU[1:0] bits enabled
PLL is active with charge pump in the active state.
PLL is active with charge pump in the active state.
PLL is active, but charge pump is TRI-STATE.
Asynchronous Power Down
Asynchronous Power Down
Synchronous Power Down
PLL State