LMX2571_16 Datasheet, PDF (12/62 Pages) Texas Instruments – LMX2571 Low-Power, High-Performance PLLatinum™ RF Synthesizer

LMX2571
SNAS654A – MARCH 2015 – REVISED JULY 2016
Feature Description (continued)
Int. charge
pump
CPout
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100pF
50pF
Figure 15. Integrated Loop Filter
7.3.6 Low-Noise, Fully Integrated VCO
The LMX2571 includes a fully integrated VCO. The VCO generates a frequency which varies with the tuning
voltage from the loop filter. Output of the VCO is fed to a prescaler before going to the N-divider. The prescaler
value is selectable between 2 and 4. In general, prescaler equals 2 will result in better phase noise especially
when the PLL is operated in fractional-N mode. If the prescaler equals 4, however, the device will consume less
current. The VCO frequency is related to the other frequencies and Prescaler as follows:
f_VCO = f_PD * N-divider * Prescaler
(2)
In order to reduce the VCO tuning gain, thus improving the VCO phase noise performance, the VCO frequency
range is divided into several different frequency bands. This creates the need for frequency calibration in order to
determine the correct frequency band given a desired output frequency. The VCO is also calibrated for amplitude
to optimize phase noise. These calibration routines are activated any time that the R0 register is programmed
with the FCAL_EN bit equals one. It is important that a valid OSCin signal must present before VCO calibration
begins.
This device will support a full sweep of the valid temperature range of 125°C (–40°C to 85°C) without having to
re-calibrate the VCO. This is important for continuous operation of the synthesizer under the most extreme
temperature variation.
7.3.7 External VCO Support
The LMX2571 supports an external VCO in PLL mode. In PLL mode, the internal VCO and its associated charge
pump are powered down, and a 5-V charge pump is switched in to support external VCO. No extra external low
noise op-amp is required to support 5-V tuning range VCO. The external VCO output can be obtained directly
from the VCO or from the device’s RF output buffer.
7.3.8 Programmable RF Output Divider
The internal VCO RF output divider consists of two sub-dividers; the total division value is equal to the
multiplication of them. As a result, the minimum division is 4 while the maximum division is 448.
Int.
VCO
CHDIV1
4,5,6,7
CHDIV2
1,2,4,8,16,32,64
OP MUX
Ext.
VCO
CHDIV3
1,2,3,Y,9,10
OP MUX
Figure 16. VCO Output Divider
There is only one output divider when external VCO is being used. This divider supports even and odd division,
and its values are programmable between 1 and 10.
7.3.9 Programmable RF Output Buffer
The RF output buffer type is selectable between push-pull and open drain. If open drain buffer is selected,
external pullup to VccIO is required. Regardless of output type, output power can be programmed to various
levels. The RF output buffer can be disabled while still keeping the PLL in lock. See RF Output Buffer Type for
details.
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