6 Functional Description

The following figure illustrates a functional block diagram of the MICRF112 transmitter. The MICRF112 can be best described as a phase-locked transmitter. The system can be partitioned into six functional blocks: crystal oscillator, PLL×32, power amplifier, enable control, undervoltage detect and open-drain switch for FSK operation.

Figure 6-1. Functional Block Diagram MICRF112 10 Pin ASK/FSK Version

Crystal Oscillator

The reference oscillator is a crystal-based pierce configuration. It is designed to accept crystals with frequencies from 9.375 MHz to 14.0625 MHz.

Crystal Oscillator Parameters for ASK Operation

The following figure illustrates a reference oscillator circuit configuration for ASK operation. The reference oscillator can drive crystals with an ESR range from 20Ω to 300Ω.

Figure 6-2. Reference Oscillator ASK Operation

When the ESR of the crystal is at 20Ω, the crystal parameter limits are:


ESR	20Ω
C_PAR	2 to 10 pF
C_MO	10 to 40 fF

When the ESR of the crystal is at 300Ω, the crystal parameter limits are:


ESR	300Ω
C_PAR	2 to 5 pF
C_MO	10 to 40 fF
C_LOAD	10 to 30 pF

Crystal Oscillator for FSK Operation

The following figure illustrates the reference oscillator circuit configuration 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 20Ω, C_FSK+ C_LOAD must not exceed 70 pF.
When the ESR of crystal is at 300Ω, C_FSK + C_LOAD must not exceed 30 pF.

Figure 6-3. 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 +10 dBm 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. If 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 6-3, 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.