AN4494

XTAL centering test

5

XTAL centering test

The BlueNRG integrates a low-speed frequency oscillator (LSOSC) and a high-speed (16

MHz or 32 MHz) frequency oscillator (HSOSC).

The low frequency clock is used in low power mode and can be supplied either by a 32.7

kHz oscillator that uses an external crystal or by a ring oscillator with maximum ±500 ppm

frequency tolerance, which does not require any external components.

The primary high frequency clock is a 16 MHz or 32 MHz crystal oscillator.

The frequency tolerance of the high-speed crystal oscillator must be below ±50 ppm.

The BlueNRG device, as with all RF systems, is highly dependent on accurate clocks for

correct operation. A deviation in clock frequency is directly reflected as a deviation in radio

frequency, and this can degrade RF performance, violate legal requirements or in the worst

case lead to a non-functioning system.

For these reasons the crystal frequency must be centered, and the easiest way to find the

optimum load capacitor values for a given circuit and layout is through experimentation.

5.1

Test case specification identifier

XTAL_center_TEST

5.2

Test prerequisite

For this test, the UFL connector (see Table 1: "Test points") is not mandatory.

5.3

Test description

For the reasons previously explained, the crystal frequency must be centered, and the

easiest way to find the optimum load capacitor values for a given circuit and layout is

through experimentation. The radio can be set to put out a constant carrier at a given

frequency.

By measuring the output frequency with a spectrum analyzer, the offset can easily be

found.

5.4

Test setup

5.4.1

Hardware

A spectrum analyzer is required for this test.

5.4.2

Software

ST BlueNRG GUI, to emit a frequency tone.

5.5

Test procedure

The following procedure is valid for the high-speed oscillator (16 MHz or 32 MHz):

Connect the BlueNRG board to the spectrum analyzer through an RF cable if it is equipped

with an UFL connector, otherwise plug a 2.4 GHz antenna into the input port of the

instrument.

Power up the BlueNRG platform.

DocID026361 Rev 3

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