Experimental information-storage filter

The information-storage filter is a spectrum analyser which can examine a broad frequency spectrum continuously. The incoming signals are mixed with a sweeping frequency, and the resultant signals are processed by a tapped delay line matched to the swept signals. The delay line with associated detection circuitry is called an information-storage filter. The input to the filter is compressed and emerges as a narrow pulse at a time, relative to the start of the frequency sweep, dependent on the frequency of the signal under examination. The signal/noise ratio improvement relative to that of a crystal video receiver with a preamplifier is approximately ¿¿/2, where ¿ is the compression ratio. The system can operate over bandwidths of the order of 1 GHz, but to test the basic concept an experimental system was constructed with a 10MHz bandwidth on a 40MHz centre frequency. The experimental filter compressed c.w. and pulsed signals as expected, and enabled the frequencies of simultaneous signals to be measured. The investigation revealed that, under certain conditions, undesirable beat frequencies were produced when simultaneous signals were received. The major limitation to the use of the system as a broadband receiver is the sidelobe level. The best level obtained experimentally was ¿17dB. This could be reduced, but sidelobes lower than ¿30dB would be difficult to obtain.