PCF7991AT datasheet(7/19 Pages) ETC2

7 / 19 page

printed 1998 Apr 20

Philips Semiconductors

Product Speciﬁcation (Rev. 1998 Apr 20)

Advanced Basestation IC

PCF7991AT

FUNCTIONAL DESCRIPTION

8.1

Power Supply

The PCF7991AT operates from an external 5 V power

supply. For optimum performance a stabilized supply

voltage should be applied.

8.2

Antenna driver

The antenna drivers are configured as a full bridge capable

to deliver a square wave shaped voltage to the series

resonant antenna circuit, which is connected between TX1

and TX2. The full bridge drivers are characterized by a low

output impedance featuring a large drive voltage to the

resonant antenna circuit. The antenna carrier frequency is

125 kHz typically.

8.3

Modulator

The modulator enables ASK (Amplitude Shift Keying)

modulation of the antenna RF signal after switching the

device into transparent mode (WRITE_TAG mode) by a

WRITE_TAG or WRITE_TAG_N command (see Table 2).

ASK modulation is achieved by blanking the antenna drive

signal under control of the data input (DIN). The modulator

features a timer circuitry that supports carrier blanking with

a programmable duration (see Table 2).

8.4

Oscillator

The on-chip oscillator operates either with a crystal or

ceramic resonator connected to XTAL1/2. Alternatively, an

external clock source (CMOS compatible) may be applied

at XTAL1. The oscillator frequency feeds a programmable

divider in order to derive the system clock and the antenna

carrier frequency of 125 kHz. The programmable divider

supports an oscillator frequency of 4, 8, 12 and 16 MHz

(see Table 11).

8.5

Receiver

The receiver senses and demodulates the absorption

modulation applied by a transponder that is inside the

antenna RF field. The demodulated and digitized signal is

available at the data output (DOUT) after switching the

device into transparent mode (READ_TAG mode) by a

READ_TAG command (see Table 2).

The receiver features a high sensitivity and an extended

input voltage range to ensure a large receiver dynamic

range. The antenna tap signal is fed to the receiver input

(RX) after attenuation by means of an external series

resistor (RV) and the receiver input impedance (RIRX) in

order to match the receiver input voltage specification. The

receive signal passes an on-chip second order low pass

filter and is further attenuated before it is fed to the

synchron demodulator and phase measurement circuitry.

8.5.1

SYNCHRON DEMODULATOR

The antenna current and therefore the tap voltage is

modulated by the transponder in amplitude and/or phase

depending on various system parameters. By employing a

unique Adaptive Sampling Time (AST) demodulation

technique, amplitude and phase modulation of the receive

signal is detected featuring an extended system operation

range. The receive sampling time is set by the

SET_SAMPLING_TIME command (see Table 2). The

appropriate sampling time can be derived from an on-chip

phase measurement and an offset that accounts for the

external antenna interface component values.

Receive signal sampling is inhibited when a WRITE_TAG

or WRITE_TAG_N command is issued in order to avoid

that write pulses de-sensitize the amplifier and digitizer

circuitry. Signal sampling is resumed when the

WRITE_TAG mode is terminated. For better receiver

setting a short delay after the last write pulse has to be

provided before the WRITE_TAG mode is terminated.

8.5.2

BANDPASS FILTER,AMPLIFIER AND DIGITIZER

After demodulation the receive signal passes a baseband

filter and amplifier prior to digitization. The amplifier gain

and bandpass filter cutoff frequencies are adjustable by

the SET_CONFIG_PAGE 0 command, in order to adapt

the receiver path to the system coupling factor and

transponder data rate.

For fast receiver settling after device power-up, sampling

time shift or when switching from WRITE_TAG mode to

READ_TAG mode the bandpass filter, amplifier and

digitizer circuit biasing condition can be initialized and

restored by a set of control bits accessible via the

SET_CONFIG_PAGE commands.

8.5.3

PHASE MEASUREMENT

The optimum receive signal sampling time depends on the

actual tuning condition of the resonant antenna circuitry.

The actual tuning condition of the resonance antenna is

determined by measuring the phase relationship between

the exciting signal at the antenna driver output and the

antenna tap voltage applied to the receiver input. In case

of perfect tuning, the phase should be 90 degree plus an

offset that accounts for the receiver input attenuation and

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