Specifications

5.6. DESIGN EVALUATION
clear that in order to achieve additional range, the receiver chain must be
optimised.
5.6.3 Reader Range and Optimisation
With the single-ended output stage, the range is limited by the charging
phase. At further distances, the tag does not become sufficiently charged
to fully transmit all 64 data bits and 16 BCC bits. This is easily verified
by using two independent antennae (for transmitting and for receiving) and
then varying the distance between the tag and the antennae individually.
Changing to the push-pull output stage rectifies this problem, and the
range is then limited by the SNR of the received signal. It was found
that range could either be increased by increasing the receiver chain’s total
gain or adding additional turns to the receiver antenna (thereby collecting
more of the transponder’s transmitted signal). This is because the receiver
only operates correctly when the output from the final amplifier stage has a
peak-peak amplitude of close to 5V (in order to trigger the digital circuitry
correctly). Increasing the chain’s total gain to 64 million times (156dB)
tripled read range to approximately 300mm. This gain is sufficient to amplify
the ambient noise in the laboratory to saturation, however, it still provided
the best read range.
It was noted that the majority of this noise was generated by surrounding
computer monitors and devices using switched-mode DC-DC converters. In
an effort to reduce noise, various filters were evaluated. First-order buffered
passive bandpass filters exhibited poor results and were dismissed. Second-
order active filters produced somewhat better results, but the roll-off was
still not as sharp as we would have liked. The solution was in the form of a
tuned filter, centred at 129kHz with a bandwidth of approximately 10kHz.
A gain of 156dB, is, however, still sufficient to amplify the ambient noise to
saturation of the final output op-amp. A tuned filter coupled with such high
gains resulted in a circuit which tended to break into oscillations. Two PCBs
were thus designed. One contained a high-gain amplifier without any filtering
for use in low-noise environments (such as Robben Island) and another with
reduced gain and aggressive filtering.
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