MAQRF112 datasheet(9/15 Pages) MICREL

9 / 15 page

Micrel, Inc.

MAQRF112

February 7, 2013

Revision 1.0

FSK Operation

Figure

shows

the

reference

oscillator

circuit

configuration

for

FSK

operation.

operate

the

MAQRF112

device

FSK

mode,

one

additional

capacitor, CFSK, is needed between the XTLOUT pin and

the

XTAL_MOD

pin.

Crystal parameters

for FSK

operation are the same as for ASK operation except:

•

When the ESR of the crystal is at 20

Ω,

CFSK + CLOAD must not exceed 70pF.

•

When the ESR of the crystal is at 300

Ω,

CFSK + CLOAD must not exceed 30pF.

Figure 3. Reference Oscillator FSK Operation

CL1, CL2 and CFSK Calculation

Referring to Figure 3, CL1, CL2, and CFSK values depend

on CLOAD. Capacitor calculation is similar to ASK

operation, with the addition of a shunt capacitor, CFSK.

Selection of CL1, CL2, and CFSK determine RF center

frequency, high-side, and low-side RF frequency. See

Figure 4.

Use Equation 3 to calculate the FSK high-side frequency

calculation.

STRAY

LOAD

Eq. 3

Use Equation 4 to calculate the FSK low-side frequency

calculation.

STRAY

FSK

LOAD

Eq. 4

Figure 4. MAQRF112 Frequency Deviation

FSK modulation is achieved by pulling the crystal at the

desired data rate. Pulling the crystal frequency is done

by shunting CFSK with CL2. A logic-level LOW on the FSK

data input opens the internal switch, removing CFSK,

which pulls the reference oscillator high. A logic-level

HIGH closes the switch, paralleling the crystal CFSK with

CL2, which pulls the reference frequency low. See

Figure 4. RS is used to minimize crystal RF spurs when

using a low-quality crystal. A typical value of RS is 0

Ω to

1000

Ω and depends on the crystal chosen. Board-level

optimization is required to optimize for frequency

deviation and occupied bandwidth requirements.

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