AMS Radon AS3993, Примечание к применению

Страница: 13 / 18

AS3993 – AN13 – Received Signal Strength Indicator (RSSI)
www.ams.com Revision 2.0 / 6/22/12 page 12/17
5.3 RF Path
The RF signal is output at the AS3993 pins 16, 17 and 20, 21. Since the output impedance of
the internal PA is not 50 ohm a load line matching network is required to deliver an maximum of
power. Through the matching circuit (L2, L3) the output driver stage of the internal Pa is supplied
by VDD_PA. The RF signal after the matching circuit is still differential therefore a 1:1 Balun is
used to transform the impedance to 50 ohm single-ended. The low pass filter (T2) is needed to
attenuate higher order harmonics of the carrier frequency. The low pass filter should have 30 dB
attenuation at the third order harmonic. After the low pass filter the RF signal is routed to the
directional coupler which should provide a directivity of ~22 dB. The input coupled port is
terminated by a 51 ohm resistor. The direct port is connected to the tuning circuit towards the
antenna and the isolated port is connected to the single ended receiver input pin of the AS3993.
The tuning circuit is essentially a PI structure consisting of two shunt capacitors and a series
inductor. To enable a tuning of this circuit the shunt capacitors are extended by digital tunable
capacitors (DTC
2
). The capacitance change is controlled by the MCU via the SPI interface. After
the tuning circuit the RF signal can be switched to the SMA connector in case an external
antenna is connected or to the onboard antenna
3
. Please note the onboard antenna is not
populated.
The returning tag signal again travels through the antenna switch, the tuning circuit and is
coupled (10 dB) to the receive pin of the AS3993. C59 and L5 provide a DC path to ground
which is required by the receiver inside the AS3993.
5.4 Power Supply
The Femto reader is supplied by 5V which is taped from the USB connector. 5 Volts can also be
supplied through pin 2 of pin header J5. Since the AS3993 is designed for mobile and battery
powered devices the 5 Volts need to be reduced. This is done by an LDO (U4
4
). The LDO has
an adjustable output voltage which is set by a resistor divider (R31, R32) at the LDO output. The
output voltage is set to 3.5 Volts and supplies the AS3993, the MCU, the DTCs and the TCXO.
The used LDO can deliver 1 A and has a low drop out voltage of only 140 mV @ 1 A.
2
DTC: PE64904 by Peregrine Semiconductor
3 0868AT54B0020 by Johannson Technology
4
AS1364 by AMS