Patent GB-1273138-A (GB)
Title: Improvements relating to frequency change detection
Inventor: HEWLETT MICHAEL GILBERT TANNER
IPC: G01R23/00
Filed: 1968-08-09
Abstract: 1,273,138. Frequency multipliers; radar. PLESSEY CO. Ltd. 4 Aug., 1969 [9 Aug., 1968], No. 38253/68. Headings H3F and H4D. [Also in Division G1] In a system for detecting and/or measuring small frequency changes in high frequency signals, a high frequency input signal subject to small changes in frequency is utilized to modulate a substantially constant frequency or by other means to generate sidebands, and two of these sidebands are selected and mixed together to afford multiplication of said small changes in frequency of the modulation frequency to enable them to be detected and/or measured. In an embodiment, the input frequency f is applied to frequency-modulator FMO, Fig. 1, to frequencymodulate a constant frequency oscillation of frequency F. Filters FU and FL select from the resulting frequency spectrum the nth upper and lower sidebands respectively, i.e. having frequencies (F+nf) and (F-nf). These are multiplied together in the multipler M, from the output of which the difference term 2nf is selected by a filter. Thus if the input frequency f is varied by #f the output varies by 2n#f. Doppler radar receiver.-In a superheterodyne receiver for C.W. Doppler radar, the frequencymultiplying arrangement of the invention is used repeatedly, to increase the Doppler frequency shift due to a moving target. Intermediate frequency signal f, Fig. 2, at 5 MHz with frequency shifts of Œ100 Hz to Œ6 KHz due to moving targets is frequency modulated on to a carrier of 400 MHz in modulator FMO 1 and the tenth upper and lower sidebands, respectively centred on 450 MHz and 350 MHz, are separated out by filters FU1, FL1 respectively. These sidebands are multiplied together in multiplier M1 and the difference frequency, centred on 100 MHz separated out by a filter. This is then reduced to 5 Mcs. by combination in mixed M2 with 105 MHz from local oscillator LO1, and the process of multiplication is repeated with frequency modulator FMO2, upper and lower sideband filters FU2, FL2 respectively, and multiplier M3. The output from multiplier M3 at 100 MHz is reduced to 10 MHz by means of mixer M4 and 110 MHz oscillations from local oscillator LO2. The output from mixer M4 comprises 10 MHz with frequency shifts from Œ40 KHz to Œ2À4 Mcs. due to moving targets and is split, notch filter FB1 with a stop band 1 MHz wide providing an output at VS2 containing any signal above or below Signals with a smaller frequency deviation are passed by band pass filter FB2 which has a passband of 1 MHz centred on 10 MHz. Frequency multiplication is again applied to the output from filter FB2, using frequency modulator FMO3, upper and lower sideband filters FU3, FL3 respectively and multiplier M5. The output, centred on 200 MHz, from multiplier M5, is reduced to a signal centred on 20 MHz by means of mixer M6 in conjunction with 220 MHz local oscillator LO3. The 20 MHz-centred signal is passed to a notch filter 1 MHz wide, rejecting the frequency of 20 MHz due to fixed targets, and the residual signals due to moving targets may be detected at this stage and used as video for display purposes. Alternatively in view of the bandwidth of 20 MHz involved an autohelerodyne process may be used, i.e. a constant frequency output is derived having variations in amplitude but no variation in frequency: the fixed frequency signal is at 50 MHz, its amplitude being varied in accordance with the variation in amplitude of the signal fed to mixer M7 which also has a 50 MHz signal fed to it from local oscillator LO4. The output from mixer M7 and its input are fed to mixer M8. Pulse modulation systems: transmitter.-In an arrangement for simultaneously transmitting a pair of pulse trains, the frequency in the one continuously increasing in the one over the pulse period, while the frequency in the other continuously decreases by equal and opposite amounts (Fig. 4, not shown), oscillator FMO, Fig. 5, produces oscillations which vary continuously in frequency from 15 to 20 MHz over each pulse period. These are mixed in mixer M10 with 85 MHz from oscillator LO10 and the sum and difference frequencies i.e. 100 MHz rising to 105 MHz and 70 MHz falling to 65 MHz separated out by filters FU10, FL10 respectively. The outputs from filters FU10, FL10 are mixed in single sideband modulators M11, M12 respectively, with 300 MHz from local oscillator LO11. The outputs from modulators M11, M12 are again mixed in mixers M13, M14 respectively with 2600 MHz from oscillator LO12 to give 3000 MHz rising to 3005 MHz and 2970 MHz falling to 2965 MHz respectively. The two pulse trains are added together and amplified in amplifier AMP before radiation. Pulse modulation systems: receiver.-Received signals are reduced to a pair of simultaneous pulses, the one 100 MHz rising to 105 MHz at the end of the pulse period, and the other 70 MHz falling to 65 MHz in the same period, by means of successive reduction in mixers M15, M16 using local oscillators LO12, LO11 of 2600 and 300 MHz respectively. The two pulse trains are separated out by band pass filters FU11, FL11 respectively and applied to two multipliers M17, M18 respectively: the multiplier M17 gives the difference frequency 30-40 MHz as output, any Doppler shifts due to moving targets being absent from this output. Multiplier gives the sum frequency 170 MHz including any Doppler shifts, and this is passed via a filter FP to a discriminator DISC in which the Doppler shifts are increased as in the arrangements described with reference to Figs. 1, 2. The arrangement includes an auto-heterodyne circuit, the output f c  being an amplitude modulated constant frequency signal, and signals representing a moving target only are re-mixed in mixer M19 with the output from mixer M17. Thereafter the signals are processed in the normal way for pulse compression radar systems in a dispersive network DN after filtering at FD in order to extract the upper sidebands (i.e. f 0 +30 MHz to f 0 +40 MHz) from mixer M19.