Patent GB-818643-A (GB)
Title: Improvements in or relating to noise reduction in continuous wave doppler radar systems
IPC: G01S13/536
Filed: 1957-01-10
Abstract: 818,643. Radar. MARCONI'S WIRELESS TELEGRAPH CO. Ltd. Jan. 10, 1957 [April 14, 1956], No. 1000/57. Class 40 (7). In a C.W. Doppler radar in which the contemporanious transmitted and received signals are heterodyned in a first mixer to give a signal indicator of the Doppler shift frequency Fd, if the transmitter is frequency modulated with a modulation signal of frequency Fm then the output from the mixer comprises Doppler sidebands of the harmonics of the modulation frequency, i.e. nFmŒ Fd. If the noise spectrum of the transmitter oscillator has an effective width of ŒFn then the output from the mixer also includes noise signals in the frequency band O-Fn and according to the present invention interference by such noise signals is prevented by selecting from the mixer output signals in a frequency band centred on a particular modulation harmonic n<SP>1</SP> of frequency n<SP>1</SP>Fm>Fn. The Doppler frequency Fd is then determined by heterodyning the selected signals n<SP>1</SP>FmŒFd with a frequency n<SP>1</SP>Fm, Figs. 1-3, or by adjusting the value of Fm until n<SP>1</SP>Fm+Fd or n<SP>1</SP>Fm - Fd is equal to a predetermined value, Fig. 4. The Specification includes a mathematical analysis which shows that the effect of random reflections within the system, e.g. due to vibrations of component parts in an airborne system, are also eliminated. Theory indicates that for maximum signal/noise improvement n<SP>1</SP> should be in the range 2-5, but the choice of n<SP>1</SP>and Fm is also influenced by considerations of the noise characteristics of the receiver and the modulation characteristics of the transmitter oscillator and n<SP>1</SP>=3 and Fm=10 Mc/s. are suitable values. It is also shown that the optimum value of the transmitter frequency modulation index p = #F/Fm, where #F is the, frequency deviation is given by p=1/2 (n<SP>1</SP>   + 1), but this value is not critical. Figs. 1 and 4 and Fig. 2 (not shown) illustrate systems employing a single aerial suitable for airborne ground speed measurement. In Fig. 1 the transmitter reflex Klystron oscillator 1 is frequency modulated by the 10 Mc/s. output from a modulator 2 and is coupled directly to the aerial 4 and through an attenuator 5 to the first mixer 6 by means of a duplexer 3 which also couples the echo signal to the mixer 6, Output signals of frequency 30 Mc/s.ŒFd from the mixer 6 are selected by a band-pass amplifier 7 and heterodyned in a second mixer 8 with a 30 Mc/s. signal derived from the modulator 2 by a harmonic generator 9 and the resultant Doppler signal Fd is selected by a low-pass filter 10 and applied. to the utilization means 11. Frequency modulation of the oscillator 1 usually produces a very small third harmonic amplitude modulation side-band which results in a 30 Mc/s. signal in the first mixer 6 and if the amplifier 7 has sufficient gain this 30 Mc/s. signal can serve as the local heterodyne signal and in which case the harmonic generator 9 is not required. The duplexes 3 may be a magic-T, rat-race or zeromagnetic circulator. A modification, Fig. 4, employs a mixer 6 with a single input coupled to a circulator duplexer 3, the direct signal from the transmitter 1 being provided by a controlled mismatch produced by a matching unit 5 between the duplexer 3 and the aerial 4. The output from the band-pass amplifier 7 is applied to a frequency discriminator 12 whose centre frequency Fc is equal to 30 Mc/s. + 1/2 Fd (max.) where Fd (max.) is the maximum Doppler frequency to be encountered and the output from the discriminator 12 controls the frequency Fm of the modulating oscillator 2 such that the frequency (3 Fm+Fd) of the single sideband output from the amplifier 7 is maintained equal to the centre frequency Fc of the discriminator 12. The modulation frequency Fm is then related to the Doppler frequency Fd which may be indicated on a meter 11. Fig. 2 (not shown) illustrates a Fig. 1 type of system utilizing the type of duplexer shown in Fig. 4 and Fig. 3 (not shown) illustrates a highpower system using separate transmitting and receiving aerials. Specification 802,007 is referred to.