A watch dog timer
may be used to reset the device after extended noisy periods or after long intervals of
inactivity.
An alternative embodiment to the switch is a parallel implementation that will include
multiple switches of differing frequencies where logic can be performed with the switches.
All of the logical conditions of AND, OR, etc., are candidates with the implementation
being the key problem. One such implementation is two switches in series where the
combined voltages at two frequencies are required to turn on the device. This particular
embodiment is especially interesting as it is totally passive, thus not reducing the shelf life
of the battery.
11.4.1.3 Results
To verify the burst switch capability, the burst switch was prototyped on a PC Board and
tested using a Samsys RFID reader to generate pulses of RF energy to be recognized by the
burst switch [16]. The system was tested using a RTSA from Tektronix, whose output is
shown in Figure 11.19. This test was taken with the reader at a distance of 1 m from the tag.
Burst
switch
Low-power
circuit
(filter)
Higher
power
circuit
FIGURE 11.18
Low-power ?¬?lter to avoid false wake-up from RF
noise.
FIGURE 11.19
Burst switch veri?¬?cation with a real-time spectrum analyzer.
Minimum Energy=Power Considerations 215
In this plot, time is on the Y-axis and frequency is on the X-axis.
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