The output is then split into two parts, one is fed into
the receiver chain and the other into the transmitter chain.
5.2.8.2 Transmit Chain
The transmit chain comprises a PSN0930A PLL as local oscillator and frequency hopping.
A 3 bit digitally controlled attenuator is used to adjust the RF output power level. The
attenuator controls the gain of the RF power ampli?¬?er (an M57785 IC from Mitsubishi
Semiconductors). That stage is followed by a ceramic bandpass ?¬?lter to suppress harmonics
and spurious frequencies. The module is capable of delivering up to 4 W of RF power
in the UHF band. The modulation is achieved by means of a mixer (MCL SBL-1Z) from
minicircuits. The modulating signal is generated using a DAC, which receives its input
from the DSP module directly.
5.2.8.3 Receive Chain
A three-port circulator (CN-6) from MECA Electronics Inc. is used to separate the transmitted
signal from the weak tag reply. The tag reply is then ?¬?ltered using a ceramic
bandpass ?¬?lter centered at 915 MHz. It is then split into two parts for I (Inphase) and Q
(Quadrature) demodulation. The two phases are then mixed with the local oscillator using
minicircuits double balanced mixers. The local oscillator signal for the Q channel is delayed
by a quarter of wavelength to generate a 908 lag in the Q path.
The mixed output signals are then ampli?¬?ed and ?¬?ltered by an antialiasing ?¬?lter.
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