Patent GB-1349366-A (GB)
Title: Signal processing apparatus
IPC: G01F1/66
Filed: 1971-09-23
Abstract: 1349366 Laser Doppler velocimeter; frequency detectors BBC BROWN BOVERI & CO Ltd 23 Sept 1971 [25 Sept 1970] 44442/71 Headings H4D and H3A In a laser Doppler velocimeter of the kind shown in Fig. 6, in which the scattering volume F is of significant size such that the velocity gradient V across the tube gives Doppler signals of various frequencies from within the volume, the various frequencies combine to give an amplitude and frequency modulated output waveform Fig. 7. The circuit of the invention thus seeks to analyse all such cyclic waveforms irrespective of origin to detect when the waveforms UE, Fig. 3, exceeds a threshold Uo and measure the average frequency of that part of the waveform so exceeding. The waveform is thus fed to two comparators 1, 2, Fig. 2, having respective comparison voltages of 0 (earth) and Uo. The comparator outputs trigger bi-stable circuits 7 and 8 to yield threshold crossing indicating waveforms U 7 , U 7  and U 8 , Fig. 3. Leading edge markers U 9 , U 10  and U 11  for the respective three waveforms are produced by monostable circuits 9, 10 and 11. The circuits basically distinguishes between the pulse sequence U 10 , U 9 , U 10  representing "threshold Uo not reached" and sequence U 10 , U 9 , U 11 , representing "threshold Uo reached". With only pulses U 9  and U 10  present, bi-stable 14 becomes permanently reset such that output A 14  enables gate 15 to pass the U 10  pulses to reset bi-stable 17 and give a 0 and its output A 17  indicating the absence of any crossovers at Uo. With all three pulses U 9 , U 10 , U 11  present, and with "start" switch S 1  closed (1 enabling signal fed to input E 12  of gate 12) and switch S 2  also connected to 1 generator 13, the U 11  pulses pass enabled gate 12 to cause-with pulses U 9 , alternate setting and resetting of bi-stable 14. The resultant U 14  signal enables gate 16 to pass the pulses U 10  of those cycles exceeding the threshold, whilst signal U 14  is 0 when pulses U 10  occur such that gate 15 give no output and bi-stable 17 is set for the duration of the threshold exceeding cycles. Output U 16  thus gives the number of threshold exceeding cycles and U 17  gives the duration of these cycles. By changing switch S 2  to "preset", an enabling output is received from counter 18 until it has counted a predetermined number of cycle pulses U 16 . The removal of the enabling output stops operation of the circuit. To determine the frequency of the threshold exceeding cycles, signal U 16 , Fig. 5, is passed to input E 4  of the circuit of Fig. 4 where series connected bi-stable circuits 19, 20 produce square waves U 19 , U 20  and their inversions U 19 , U 20 . And gates 21, 22, 23 receive respective combinations of these waves and produce respective switching pulse trains U 19 , U 20 , U 19 , U 20 , U 19 , U 20 . The first switching pulse train periodically closes switch S 3  to pass constant current from source 25 to a capacitor C 1  and result in a constant rate charge up of the capacitor up to a voltage proportional to the pulse spacing in signal U 16 . The second switching pulse train periodically feeds the capacitor charge voltage from the integrator circuit I to a sample and hold circuit H to give an output UT proportional to charge voltage. The third switching pulse train periodically discharges the capacitor C 1  to earth so that it can be recharged. Output UT represents the pulse spacing of signal U 1 . and to obtain the frequency, a circuit 27 supplies the negative of the logarithm of UT, i.e., the logarithm of 1/U T . Adding on the constant U k =log k at 28 gives log k/U 1 . An antilog circuit 29 gives signal U f  indicative of the pulse train frequency.