RECEIVER FUNCTIONAL DESCRIPTION

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MC33592 Technical Data

MOTOROLA

RECEIVER FUNCTIONAL DESCRIPTION

The basic functionality of the ROMEO2 receiver may be seen by reference to the accompanying block

diagram (see figure 1). It is fully compatible with the TANGO3 transmitter.

The RF section comprises a mixer with image cancelling, followed by an IF band-pass filter at 660kHz, an

AGC controlled gain stage and an OOKdemodulator. The data output from the circuit may either be the data

comparator output, or, if Data Manager is enabled, the SPI port.

The local oscillator is controlled with a PLL referenced to the crystal oscillator. The received channel is defined

by the choice of the crystal frequency.

An SPI bus permits programming the data rate, UHF frequency, ID word etc., though to accomodate

applications where no bus interface is available the circuit defaults at power-on to a standard operating mode.

Depending upon the configuration, the circuit can be either externally strobed by the STROBE input or

internally wait-and-sleep cycled to reduce the power consumption. At any time, a high level on STROBE

overrides the internal timer output and wakes up ROMEO2. When the circuit is switched into sleep mode its

current consumption is approximately 100µA. The circuit configuration which has previously been programmed is

retained.

THE LOCAL OSCILLATOR PLL

The PLL is tuned by comparing the local oscillator frequency, after suitable division, with that of the crystal

oscillator reference. The loop filter has been integrated in the IC. Practical limits upon the values of components

which may be integrated mean that the local oscillator performance may be slightly improved by using an

external PFD filter, shown in Figure 2. In this way the user may choose to have optimum performance with the

addition of external filter components. The PLL gain may be programmed by bit PG: it is recommended that this

bit be set to 1, corresponding to low loop gain.

Figure 2 : External loop filter

C1=4.7nF, C2=390pF, R=1k

W

Freescale Semiconductor, Inc.

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