MICRF112_13 datasheet(12/21 Pages) MICREL

12 / 21 page

Micrel, Inc.

MICRF112

February 2013

M9999-020713

Crystal Oscillator for FSK Operation

Figure

shows

the

reference

oscillator

circuit

onﬁguration for FSK operation. To operate the

MICRF112 in FSK mode, one additional capacitor is

needed between XTLOUT pin and the XTAL_MOD pin.

Crystal parameters for FSK operation are the same as

for ASK operation except:

•

When the ESR of crystal is at 20

Ω,

CFSK + CLOAD must not exceed 70pF.

•

When the ESR of crystal is at 300

Ω,

CFSK + CLOAD must not exceed 30pF.

Figure 3. Reference Oscillator FSK Operation

PLL×32

The function of PLL×32 is to provide a stable carrier

frequency for transmission. It is a “divided by 32” phase-

locked oscillator.

Power Amplifier

The power amplifier serves two purposes: to buffer the

VCO from external elements and to amplify the phase-

locked signal. The power amplifier can produce +10dBm

at 3V (typical).

Enable Control

The enable control gates the ASK data. It only allows

transmission when Lock, Amplitude, and Undervoltage

Detect conditions are valid.

Undervoltage Detect

The undervoltage detect block senses operating voltage.

the

operating

voltage

falls

below

1.6V,

the

undervoltage detect block sends a signal to the enable

control block to disable the PA.

Open-Drain Switch

The open-drain switch is used for FSK operation. FSK

data is fed into the FSK pin. The FSK pin is connected to

the gate of the open-drain switch. The open collector is

connected to the XTAL_MOD pin. In Figure 3, a

capacitor is shown connected from the XTAL_MOD pin

to XTLOUT. When the FSK pin goes high, the capacitor

between XTAL_MOD and XTLOUT pulls the frequency

of REFOSC low.

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