LLCC68 datasheet(24/106 Pages) SEMTECH

24 / 106 page

LLCC68

Data Sheet

Rev. 1.0

DS.LLCC68.W.APP

July 2019

24 of 106

Semtech

www.semtech.com

4.1.4 TCXO Control Block

Under certain circumstances, typically small form factor designs with reduced heat dissipation or environments with

extreme temperature variation, it may be required to use a TCXO (Temperature Compensated Crystal Oscillator) to achieve

better frequency accuracy. This depends on the complete system, transmitter and receiver. The specification FERR_L in

Table 3-8: "Receive Mode Specifications" on page 19 provides information on the maximum tolerated frequency offset for

optimal receiver performance.

Figure 4-2: TCXO Control Block

When a TCXO is used, it should be connected to pin 3 XTA, through a 220

 resistor and a10 pF DC-cut capacitor. Pin 4 XTB

should be left not connected. Pin 6 DIO3 can be used to provide a regulated DC voltage to power the TCXO, programmable

from 1.6 to 3.3 V. VBAT should always be 200 mV higher than the programmed voltage to ensure proper operation.

The nominal current drain is 1.5 mA, but the regulator can support up to 4 mA of load. Clipped-sine output TCXO are

required, with the output amplitude not exceeding 1.2 V peak-to-peak. The commands to enable TCXO mode are described

in Section 13.3.6 "SetDIO3AsTCXOCtrl" on page 76, and that includes DC voltage and timing information.

Note:

A complete Reset of the chip as described in Section 8.1 "Reset" on page 45 is required to get back to normal XOSC

operation, after the chip has been set to TCXO mode with the command SetDIO3AsTCXOCtrl.

4.2 Phase-Locked Loop (PLL)

A fractional-N third order sigma-delta PLL acts as the frequency synthesizer for the LO of both receiver and transmitter

chains. LLCC68 is able to cover continuously all the sub-GHz frequency range 150-960 MHz. The PLL is capable of

auto-calibration and has low switching-on or hopping times. Frequency modulation is performed inside the PLL

bandwidth. The PLL frequency is derived from the crystal oscillator circuit which uses an external 32 MHz crystal reference.

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