Specifications

5.4. HARDWARE

result in a high received voltage at the transponder. As outlined in Section

5.6, the number of turns on the antenna had very little inﬂuence on the

receiver’s performance: if a receiving antenna with a few turns is used, the

receiver chain’s gain is simply increased to compensate and produce similar

results.

What is of consequence is the transmitter’s inductance: it should remain

low enough to pass a signiﬁcant current at 132.4kHz during the charging

phase from a 12V supply. As more turns are added to the transmitting

antenna, the inductance of the loop increases and since the supply voltage

and operation frequency remains constant, the current through the inductor

decreases. The undesirable consequence of such an action is a decrease

in transmitter power. The Texas Instruments Series 2000 antennae have

inductances of approximately 27µH [

15].

Many antennae were constructed and evaluated. Ultimately, it was decided

to use a single antenna for receiving and transmitting with a designed inductance

of 50µH. The number of turns on the antenna would ultimately depend on

the physical size, shape and diameter of the wire used. The proposed ﬁnal

design uses a rectangular antenna me asuring 260mm × 230mm.

5.4.5 Processor

Prior to this project, the only experience we had with microprocessors was

with Freescale’s HC08 series. After ﬁnding an 68HC908JK3 inadequate for

the task, Microchip’s PIC 18F452 and later a PIC 18F4620 were considered.

The details of the design as well as the beneﬁts and shortcomings of each

device are outlined in the sections which follow.

The microprocessor must be capable of:

• Generating a logic-level square wave at 134.2kHz

• Determining the frequency of a received square-wave

• Reporting the data to the RFID co-ordinator

The initial solution is to use two timer modules, one to generate the

transmitting pulse and the second to determine the elapsed time between