Patent GB-1017052-A (GB)
Title: Improvements in or relating to electric circuits primarily for use in doppler radar systems
Inventor: HARWOOD PETER JOHN
IPC: G01S13/58
Filed: 1961-07-12
Abstract: 1,017,052. Radio frequency systems; automatic phase control systems; radar. GENERAL ELECTRIC CO. Ltd. July 11, 1962 [July 12, 1961], No. 25320/61. Headings H3A, H4D and H4L. An electric circuit, for particular use in Doppler radar systems, suppresses spurious, i.e. inherently double sideband, components present in an input signal while the single sideband echo component of the input is converted to a double sideband signal. The input signal is mixed with each of two I.F. signals whose frequencies are symmetrically disposed about the frequency of the input signal. Subtraction of one mixer output from the other (provided the phase and amplitude relation is correct), removes the spurious components and leaves only the echo information. As shown, a Doppler radar system includes a transmitter 1 and receiver 2 utilizing a common aerial 3. The transmitted signal is supplied to aerial feeder 5 over arms 6, 7 of a circulator 4. The received echo signal is fed via arms 7, 8 of the circulator to mixers 9, 13 fed by way of further mixers 11, 15 from oscillators 10, 14 and the transmitter oscillator to derive an intermediate frequency signal at the output of mixer 13. This signal IF 1  contains unwanted interference signals in the form of double sidebands and the desired echo signal as a single sideband. Signal IF 1  is therefore fed direct to two mixers 32 and 33 and also to a phase-sensitive detector 22 phase controlling an oscillator 21. The output of oscillator 21 at frequency IF 1  is supplied via phase shifter 25 to a mixer 26 which also receives a signal IF 2  from an oscillator 27. The outputs from mixer 26, i.e. (IF 1  - 1F2) and (IF 1  + IF 2 ) are fed respectively via phase detectors 30, 31 and oscillators 28, 29, phase controlled thereby, to the mixers 32, 33. The necessary phase adjustment of the signals (IF 1  Œ IF 2 ) is effected with the phase shifter 25 normally set to zero, by the phase looking of the oscillators 21, 28, 29 so that when the output of mixer 32 is subtracted from that of mixer 33 the derived signal IF 2  is free from the original double sideband components while the echo signa is present in the form of a double sideband. The derived signal is then fed to the latter stags 36 of receiver 2 for determination of the Doppler frequency shift. The suppression circuit disclosed precludes the receiver from distinguishing between receding and approaching targets and may be switched out except when weak echo signals are being received. Undesired " break-through " signal in the circulator is substantially eliminated by reflecting back a portion of the transmitted signal via adjustable stub lines 16, 17 spaced along the aerial feeder 5 so that two portions in phase quadrature are reflected back to cancel the breakthrough signal. The stub lines 16, 17 are effectively varied in length by respective control signals derived by comparing the phase of the output of mixer 13 in phase-sensitive detectors 18, 19 with the output of the local oscillator 14 directly in the case of detector 18 and after it has been phase-shifted by 90 degrees in the case of detector 19.