XE1218AS000BF データシートの表示（PDF） - Unspecified

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XE1218AS000BF

Low power single die FM receiver

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Datasheet XE1218A: Low

Power single die FM receiver

1. Functional description

1.1 General description

The XE1218A is an integrated FM receiver including all the necessary functions to process a modulated RF

signal at its input and deliver a “speech quality” audio signal at its output. The XE1218A was designed for

maximum system simplicity thus it requires only 12 external components to offer antenna in, audio out

functionality. The XE1218A also offers design flexibility. A direct connection to an earphone is possible through

the pin EAR, this takes advantage of the internal audio power amplifier or an external audio amplifier, or an

audio processing device can be connected via the OUT (line out) pin.

1.2 RF front end

The FM signal received through the antenna is delivered to an integrated LNA via an external matching circuit.

A low IF architecture is implemented to down-convert the incoming signal to an intermediate frequency (IF) of

25kHz. This IF signal is then filtered to remove any DC components and equalized to offer a clean digital signal

to the demodulator. The FM demodulator uses digital delay line architecture to convert the signal to a pulse

width modulated signal at twice the IF frequency. The XE1218A has an integrated AFC which is used for the

demodulator to have its clock frequency track the IF frequency.

1.2.1 LNA

The noise figure (NF) for this block is less than 6dB, for an antenna impedance in the 1 to 2 kΩ range. Out of

the 6dB, 3dB is due to the noise of the image frequency that is not rejected. The sensitivity is –100dBm

(SINAD=10 dB, fdev = 3kHz).

1.2.2 Mixer, down-conversion

The down-conversion is implemented as a Gilbert cell. This implementation uses low IF and does not have

provisions for integrated image rejection. The incoming audio channel will be demodulated at Flo = FIN – 25kHz.

This is to say that the image frequency will be located at FIN – 50kHz and will not be attenuated by the

XE1218A’s RF receiving circuitry. For multi-channel operation within reception range of several XE1218A

receivers, it is recommended that the various carrier frequencies be spaced by at least 100kHz, better yet

200kHz.

1.2.3 DC offset cancellation, IF amplifier chain

DC components are attenuated in an off-set cancellation block. An on-chip low pass filter provides attenuation

greater than 65dBm measured FIF + 100kHz

1.2.4 Demodulation and AFC circuitry

The demodulator converts the signal from the IF amplifier into a pulse width modulated signal at twice the IF

frequency. A follow-on block generates the AFC to track the IF frequency. Proper tracking of the IF frequency is

necessary for the stability of the RF link as the XE1218A operates in narrow band mode.

1.2.5 Crystal and local oscillator

The local oscillator provides the XE1218A’s down-converter mixer with the proper frequency: Flo. It is

implemented with a crystal oscillator working on the external crystal’s overtone harmonic.

1.3 Audio stage

1.3.1 Audio processing

The XE1218A integrated audio processing circuitry filters the input pulse width modulated (PWM) signal from

the demodulator and provides for two different output feeds. All necessary bias currents and voltages for this

analog block are on-chip, including a voltage multiplier circuit. When the incoming RF signal becomes low, the

signal to noise ratio at the audio output decreases, which drastically reduces the sound quality and intelligibility.

For user convenience, a mute function is implemented in the audio section. When the signal to noise ratio at the

output is too low (typ. 10 to 15 dB) the audio output is shut down.

3

D0204-157

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