CC400-RTR2 datasheet(17/31 Pages) TI

17 / 31 page

CC400

SWRS050

Page 17 of 32

20.

Optional LC Filter

An optional LC filter may be added

between the antenna and the matching

network in certain applications. The filter

will reduce the emission of harmonics and

increase the receiver selectivity.

The filter for use at 433.92 MHz is shown

in Figure 10. The component values may

have to be optimised to include layout

parasitics. The filter is designed for 50

Ω

terminations.

Figure 10: LC filter.

21.

Crystal oscillator

An external clock signal or the internal

crystal oscillator can be used as main

frequency reference. An external clock

signal should be connected to XOSC_Q1,

while XOSC_Q2 should be left open. The

crystal frequency must be in the range 4 -

13 MHz.

Using the internal crystal oscillator, the

crystal must be connected between

XOSC_Q1 and XOSC_Q2. The oscillator

is designed for parallel mode operation of

the crystal. In addition loading capacitors

(C151 and C161) for the crystal are

required. The loading capacitor values

depend on the total load capacitance, CL,

specified for the crystal. The total load

capacitance seen between the crystal

terminals should equal CL for the crystal to

oscillate at the specified frequency.

parasitic

161

151

The parasitic capacitance is constituted by

the pins input capacitance and PCB stray

capacitance. Typically the total parasitic

capacitance is 4.5 pF. A trimming

capacitor may be placed across C151 for

initial tuning if necessary.

The crystal oscillator circuit is shown in

Figure 11. Typical component values for

different values of CL are given in Table

14.

The initial tolerance, temperature drift,

ageing and load pulling should be carefully

specified in order to meet the required

frequency

accuracy

certain

application.

specifying

the

total

expected frequency accuracy in SmartRF

Studio together with data rate and

frequency separation, the software will

calculate the total bandwidth and compare

to the available IF bandwidth. The

software will report any contradictions and

accurate

crystal

will

recommended if required.

C161

C151

XTAL

XOSC_Q1

XOSC_Q2

Figure 11: Crystal oscillator circuit.

L52

C53

f = 433.92 MHz

C52=15 pF

C53=22 pF

L52=8.2 nH

C52

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