MICRF112 datasheet(13/20 Pages) MICREL

13 / 20 page

Micrel, Inc.

MICRF112

December 2011

M9999-120911

(408) 944-0800

Crystal Oscillator for FSK Operation

Figure 5 shows reference oscillator circuit conﬁguration

for FSK operation. To operate the MICRF112 in FSK

mode, one additional capacitor is needed between

XTALOUT pin and XTALMOD pin. Crystal parameters

for FSK operation are the same as ASK operation

except:



When the ESR of crystal is at 20Ω, CFSK + CLOAD

not to exceed 70pF.



When the ESR of crystal is at 300Ω, CFSK +

CLOAD not to exceed 30pF

Figure 5. 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: 1) to buffer

the VCO from external elements and 2) to amplify the

phase locked signal. The power amplifier can produce

+10dBm at 3V (typical).

Enable Control

Enable control gates the ASK data. It only allows

transmission when Lock, Amplitude and Under Voltage

Detect conditions are valid.

Under Voltage Detect

“Under voltage detect” block senses operating voltage.

If the operating voltage falls below 1.6V, “under voltage

detect” block will send a signal to “enable control” block

to disable the PA.

Open Drain Switch

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 XTALMOD pin. In Figure 4, a capacitor

is shown connected from XTALMOD pin to XTALOUT.

When FSK pin goes high, the capacitor between

XTALMOD and XTALOUT pulls the frequency of

REFOSC low

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