Passive interrogator label system having offset compensation and temperature compensation for a surface acoustic wave transponder

In a system for interrogating a plurality of transponders, each carrying encoded information, said system comprising:

• (a) at least one transponder for receiving a first signal and transmitting second signal in reply thereto, said transponder comprising signal transforming means, coupled to receive said first signal as an input, for producing said second signal as an output, said signal transforming means including:
• (1) a plurality of signal delay means coupled to receive said first signal, each signal delay means providing an intermediate signal having a nominal, known delay time with respect to said first signal, different signal delay means providing different delay times which increase successively from shortest to longest;
• and
• (2) signal combining means, coupled to at least some of said signal delay means, for combining at least some of said intermediate signals in a known manner to produce said second signal, said signal delay means and/or said signal combining means imparting a known informational code to said second signal associated with said transponder;
• (b) interrogator/receiver means for transmitting said first signal to, and receiving said second signal from said at least one transponder;
• and
• (c) signal decoding and processing means, coupled to said interrogator/receiver means, for determining said informational code associated with each transponder;
• the improvement wherein said signal decoding and processing means includes means for compensating unknown variations in said nominal, known delay time of each signal delay means, said compensating means including:
• (1) means for measuring the actual difference in delay time between at least two successively longer delay times of respective signal delay means;
• and
• (2) means for determining said nominal, known delay times of the other signal delay means, using said actual delay time difference as a standard.