Specifications

5.4. HARDWARE
stage is AC coupled to the previous one to prevent DC bias offsets from
being amplified. Low value resistors are used to prevent noise from being
captured and amplified and to limit the possibility of the circuit breaking
into oscillation. The op-amps are thus not operated at high gains. The
initial design places the first two op-amps in series to provide a combined
gain of 100 times (10 times each). The final two amplifiers in the receive
chain are connected in parallel in order to provide a higher drive current to
the passive parallel tuned filter.
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The reader’s filtering is based around a passive parallel resonant circuit
consisting of a custom wound inductor and a capacitor bank. The filter is
tuned to 130kHz with a bandwidth of approximately 20kHz.
The final amplification stage is formed around an MCP602, which is a
rail-to-rail output 5V op-amp. This stage has a gain of 100 and is enough to
saturate the amplifiers, resulting in a 5V output square wave. These op-amps
are costly devices, however, it was not possible to replace them with LF353 s
as their output voltage swing was not sufficiently close to the supply rails to
trigger the 5V CMOS level digital logic. Furthermore, all attempts to use
a level crossing detector based on the LM311 comparator were unsuccessful
- the device would either break into oscillation (using low levels of positive
feedback), or would not trigger reliably (using too much positive feedback to
curb oscillations). We believe that the oscillation problem was due in part
to the switch-mode power supply’s rail noise (see section 8 for PSU details).
5.4.4 Antenna Design and Transponder Orientation
Antenna Selection
The antenna for this project should have a uniform read field, rather than
a strongly directional one. This will ensure that the birds are detected as
they pass the gate. Figure 5.8 illustrates the read field generated by the
two popular low frequency antenna designs (ferrite stick and gate antennae).
Recall that the transponders in use are of the 32mm glass variety with ferrite
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One of these parallel stages was disconnected in the final circuit design as it caused
the system to break into oscillation. Although range was affected slightly by the reduced
drive current to the resonant circuit, the reader operated more reliably.
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