The invention is designed for application in an identification system that consists of a transmitter, a mobile bit-coded passive transponder that produces identification, and a receiver unit that in combination make up a processing system.
The transponder can be realized by means of SAW technology (Surface Acoustic Wave). This is a technology with many advantages for such an application, it is inexpensive, straightforward to produce, requires no maintenance, and is independent of external power sources etc. However a transponder realized in accordance with this technology has a characteristic which initially has one drawback: the high insertion loss. This means that the reflected return signal is weak in relation to the incoming interrogator pulse. The disadvantage is intensified as the transmitter unit utilizes a low output radar to meet the beam output requirements in the frequency used.
The combined result is that the output of the required return signal to the system's receiver unit is very low in relation to the unwanted signals, noise and the inherent noise in the receiver itself.
The traditional method for processing signals to improve the signal/noise ratio for the applications mentioned is to digitalize the reflected signal directly with an A/D converter and follow this by digital processing. The processing is done by averaging the digital values of the analog signals for several repeated return signal cycles. This will improve the signal/noise ratio because the required signal is identical for each cycle whilst the noise is different. On the other hand, this method necessitates a powerful digital processing unit. The unit has to be fast and have substantial calculating power to average an adequate number of cycles. At the same time, the A/D converter must be one of the fastest and most advanced types available. This solution means that the technology used clearly limits how efficient the processing is. This will also necessitate the use of expensive digital processors and A/D converters.
Another traditional method which is used in radar technology is using delay lines so that a signal is delayed one cycle and averaged by adding these signals analogously. This method has one limitation that the number of averages that can be made is very restricted and there is no synchronization with the cycle.
U.S. Pat. No. 3,706,094 describes a receiver for a low output radar surveillance system, where the receiver consists of equipment for analog processing of received, recurrent pulse signals whereby each pulse signal is divided up into a certain number of time slots in gate control circuits. Further, the signal from each gate circuit is led to an integrator for the recurrent integration of the pulse signals as the repetition frequency of the pulse signals is considerably higher than the integrator's low-pass filter frequency. The pulse signal can be reconstructed from the responder by reading the output from the integrator.