GB1364975A - Spectral analysis - Google Patents

Abstract

1364975 Frequency analysis THOMSON-CSF 4 Feb 1972 [5 Feb 1971] 5388/72 Heading G1U A spectrometer of the type in which an input signal a, Fig. 1 and 2 is time compressed 1, amplitude modulated 4 on to a linearly rising carrier c, Figs. 1 and 3, and transmitted, via a matched dispersive delay line 6 and a detector 7, to an oscilloscope 8 to produce an analysis into N spectral lines, is modified in that the time compression factor is increased by a factor of n, each resulting compressed signal bo, Fig. 4, is transmitted through the system n times, and the carrier c, Fig. 5, is caused to sweep through an n-fold larger range in n equal linear steps, whereby, as a result, an analysis of the same input bandwidth into nNlines is achieved. In the basic system, input signals a, Fig.2, are notionally chopped in blocks T o long and compressed by a factor K into blocks T 1 /2 long separated by intervals of T 1 /2, in such a way that each compressed block has a bandwidth of B 1 /2, where B 1 and T 1 are the bandwidth and corresponding delay respectively, of the dispersive delay line 6. Thus, if the bandwidth of the input signal if B o then - and therefore Hence, since B o is not allowed to exceed B 1 /4, (otherwise expansion rather than compression would be necessary), in general the segments of length T o of the input signal overlap each other. Fig. 2 shows the special case when K= 2 and the segments are contiguous. Also in the basic system, each compressed segment is amplitude modulated on to a carrier c, Fig. 2, which is swept linear over a bandwidth of B 1 /2, so that by a suitable choice of frequencies either the sum or difference frequency from the modulator as it sweeps over a bandwidth of B 1 /2 can be made always to lie within the bandwidth B 1 of the delay line 6, and the resulting signals can be detected and displayed on a c.r.t. with an x-sweep that starts a time T 2 , equal to the mean delay of the line 6, after the beginning of the sweep of the carrier c. A blanking signal w, Fig. 1, is also applied to the c.r.t. between sweeps to avoid spurious blips. In accordance with the invention, the compression factor Kis increased by a factor of n, and each resulting compressed signal, which as shown in Fig. 4, which, like Fig. 2, shows the special case when K=2, is of the same duration T 1 /2 as in the basic system, is repeated n times. However, since the compression factor has been increased by a factor of n, the bandwidth of each compressed signal is also increased by a factor of n, and, as shown in Fig. 5, this is analyzed in slices of B 1 /2, by arranging that the carrier signal c is swept through a band of B 1 /2 for each repetition of a compressed signal, each sweep starting from where the previous one stopped. Thus, since as in the basic system, each sweep produces an analysis into N lines, n sweeps produce an analysis into nNlines, which will be spread over a corresponding time period of nT 1 . Hence providing the x sweep voltage, Fig. 7, for the c.r.t. display is also scaled by a factor of n, the result is that nNlines are produced instead of N lines. The Specification outlines a suitable compression circuit Fig. 6, (not shown) in which input signals a are sampled at a sufficiently high frequency, digitized (10) and fed into a first buffer (12). and while this is taking place signals from a second buffer (11) are repeatedly read out at an n-times higher frequency to a D-to-A converter (14) to form the compressed signal b. Once the next segment of the input signal has been digitized the roles of the buffers are interchanged.