XE1218 Просмотр технического описания (PDF) - Unspecified

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производитель

XE1218

integrated UHF transceiver

Unspecified 

XE1218

Ultra Low Power VHF Receiver

Overall functional description

The circuit is an FM receiver. It is composed of the following main functional blocks:

- the receiver chain including the audio output stage

- the local oscillator (LO),

- the control and service blocks,

The receiver is implemented with the following blocks:

- a low-noise amplifier (LNA),

- a mixer as a down-converter, whose function is to transfer the signal from VHF band to an intermediate

frequency (IF) of 25 kHz,

- an offset cancellation block, whose function is to attenuate the DC and quasi-DC signals,

- an IF filter chain, whose function is to amplify and filter the IF signal,

- a limiter, providing the demodulator with fully digital signals,

- a digital delay line FM demodulator, which converts the signal from the limiter into a pulse width modulated

signal at twice the IF frequency; this block also generates AFC and MUTE signals; the AFC signal is used

inside the demodulator to have its clock frequency track the IF frequency,

- an audio stage, whose function is to filter the input PWM signal and to provide volume control and output

buffering; the MUTE signal generated by the demodulator is used by this block to switch on/off the output buffer

driving the EAR output.

LNA : NF is less than 6 dB, for an antenna impedance in the 1 to 2 kOhm range. Out of the 6 dB, 3dB are due to the

noise at the image frequency, which is not rejected. The sensitivity is –107 dBm (SINAD=10 dB, fdev = 3kHz)

Mixers : This is implemented as a Gilbert cell. CAUTION : DUE TO INITIAL DESIGN CONSTRAINTS, CHOICE OF

ARCHITECTURE, NO IMAGE REJECTION FEATURES IS PROVIDED ON CHIP. FOR EXAMPLE IF Flo=F-

25kHz, THEN THE IMAGE FREQUENCY AT AT F - 50 kHz IS NOT ATTENUATED. IN CASE OF

MULTICHANNEL OPERATION, THE CARRIER SHOULD BE SPACED BY 100 OR 200 KHZ

Offset cancelation : This attenuate DC signal before IF chain

IF amplifier chain : This block provides amplification and low pass filtering (>65 dB attenuation 100 kHz away from

the 25 kHz IF frequency.

Limiter / Demodulator which converts the signal from the IF amplifier into a pulse width modulated signal at twice

the IF frequency; this block also generates AFC and MUTE signals; the AFC signal is used inside the demodulator

to have its clock frequency track the IF frequency. The MUTE circuit in the FM demodulator detects when the

received signal is too weak. When that is the case, the audio outputs will be muted so that the user doesn’t hear all

the noise.

The local oscillator provides the mixer of the receiver with the proper LO. It is implemented with a quartz oscillator

working on an overtone of the quartz.

The service blocks provide the necessary bias currents and voltages. This includes a voltage doubler circuit.

7

D0010-80

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