GB1455832A - Doppler radar systems pulsed doppler radar systems - Google Patents

Abstract

1455832 Pulse doppler radar SIEMENS AG 16 Nov 1973 [17 Nov 1972] 53173/73 Heading H4D A Doppler radar system converts echo signals to digital form and supplies them to two parallel channels each containing a digital mixer, the mixers having different heterodyne frequencies, followed by respective digital low pass filters, having the same cut-off frequency, the channel outputs being fed to a comparator stage, the output of which is fed to a control circuit which generates control signals to lower the cut-off frequencies of the low pass filters and alter the heterodyne frequencies of the mixers. The arrangement enables a sequential narrowing of the system Doppler pass band around the received echo Doppler frequency. An antenna RA, Fig. 1 emits RF pulses supplied by transmitter ST, the echoes being passed to receiver ET. After being mixed the Doppler signals are passed to analogue to digital converter W feeding two channels UK1, UK2. Each channel comprises a digital oscillator 01,02, a digital mixer M1,M2, a digital low pass filter TP1,TP2, and a squaring stage B1,B2. An inverter IV1 is placed in channel UK1 and the inverted signal is passed to comparator AD together with the signal from channel UK2. A control device AS operates on the low pass filters and oscillators in response to the output of the comparator. In Fig. 2, the frequency characteristics DK1,DK2 of channels UK1,UK2 are shown together with frequencies f01,f02,fD of the oscillators 01,02 and the Doppler signal respectively. The low pass filters are initially given a cut-off frequency of e.g. 2 KHz by control AS and this, together with the mixers M1,M2 results in a pass band B of 4 KHz about frequencies f01,f02. Doppler frequency fD is for example 16À2 KHz and produces a relatively large signal S1 and a very small signal S2 at the outputs of channels UK1, UK2 respectively, f01,f02 being 15 and 11 KHz respectively. In the example, fD-f01=1À2KHz, fD-f02=5À2 KHz. Comparator AD therefore detects S1 is greater than S2 and control AS increases frequencies f01,f02 to 17 KHz (f01<SP>1</SP>) and 14 KHz (f02<SP>1</SP>) and lowers the cut-off frequencies to 1À5 KHz. This gives a band pass B<SP>1</SP> of 3 KHz and f01-fD=0À8 KHz, fD-f02<SP>1</SP>= 2À2 KHz. S1,S2 are again compared and AS alters the frequencies f01,f02 and cut-offs accordingly. In this case, the heterodyne frequencies are again increased to f01<SP>11</SP> = 17À4 KHz, and f02<SP>11</SP> = 15 KHz and the cut-off frequencies are lowered to 1À2 KHz, giving a band pass B<SP>11</SP> of 2À4 KHz, f01<SP>11</SP> - fD being 1À2 KHz and fD - f02<SP>11</SP> also being 1À2 KHz. Signals S1,S2 are now equal and doppler frequency fD is within the much narrower band pass filter characteristic. This enables a more accurate determination of fD and improves the signal to noise ratio. Frequency fD is indicated by device AF. Detailed circuitry suitable for oscillators 01,02 and filters TP1,TP2 is given.