Patent GB-991869-A (GB)
Title: A receiver for receiving a carrier wave
IPC: G01S1/54
Filed: 1962-07-31
Abstract: 991,869. Radio-navigation receivers. CSF-COMPAGNIE GENERALE DE TELEGRAPHIE SANS FIL. July 31, 1962 [Aug. 11, 1961], No. 29296/62. Heading H4D. A receiver for receiving, on board an aircraft, a beam sweeping in elevation and comprising a subcarrier frequency modulated as a function of its elevation, comprises demodulating means to produce the subcarriers and means to determine the instantaneous frequency of the subcarrier at the mid point of the passage of the beam through the aircraft. In one embodiment of the receiver, Fig. 3, the beam is picked up by aerial 1, converted to an I.F. signal by oscillator 3. amplified in A.G.C.. I.F. amplifier 4 and amplitude detected in detector 5. The frequency modulated low-frequency signal so reproduced is amplified in A.G.C. amplifier 6. The A.G.C. detector 6a also feeds its output to a threshold amplifier 6b which only passes that part produced from signals received from between the 3 dB. points of the aerial beam. The rounded pulse so produced, Fig. 4 (not shown) is squared off in amplifier/clipper 7, and fed to control a gated amplifier 8 to let through from amplifier 6, only signals above the said 3 dB. points. The frequency modulated signal from amplifier 8 is then squared off in limiting amplifier 9 and differentiated in differentiator 10. The alternating positive and negative spikes so produced are passed to respective positive and negative inputs of a narrow pulse monostable multivibrator 11, to produce a train of positive pulses on lead 17 at twice the rate of the output from amplifier 8. The frequency modulation is linear so that the average frequency f 0  and thus the elevation So of the aircraft could be found by adding up the number of pulses, it is preferred however to compare the actual elevation with a desired elevation S 1 , and for this purpose a manually set gated oscillator 12 generates a corresponding frequency f 1 . This is processed to produce a second train of pulses on lead 18. The two pulse trains are fed to means 16, wherein there is found: (a) the number of pulses in each train (N 0  and N 1 ); (b) the difference between these two numbers; (c) the value of f 0 -f 1 ; and (d) the elevation errors S 0  - Si, this last error being shown on meter 53. Alternatively, Fig. 6 (not shown), the error signal may be used to control a D.C. motor adjusting the oscillator 12 such that it generates a frequency f 0 . Calibration of the oscillator can thus give So. In a second embodiment, Fig. 7, oscillator 12 generates f 1  = f<SP>1</SP> 1  - F K , where f<SP>1</SP> 1  in the frequency corresponding to the desired elevation (f 1  of the previous embodiment), and F K  is a constant. The two frequencies are mixed in mixer 60 and the signal (f-f 1 ) is selected by filter means and fed to high-pass filter 61 and low-pass filter 62, the common cut-off frequency being F K . By suitable choice of F K  and the modulation all error signals due to S 0  being greater than S 1  pass through filter 61 whilst those due to So being less than S 1  pass through filter 62. Indicating means connected between integrating circuits at the outputs of the filters, gives (So - S 1 ) in sign as well as magnitude. This error signal can be used to control an autopilot. A suitable ground transmitter is described, Fig. 2, wherein an intermediate frequency signal produced by oscillator 103 is frequency modulated by for example a variable impedance 104 in accordance with the elevation of the aerial beam, the relationship preferably being linear. The frequency-modulated signal is then mixed in mixer 102 with a second intermediate-frequency signal from oscillator 101, to produce a frequency-modulated low-frequency difference frequency signal. The signal then passes via switch 106 during upward sweeps of the beam, to amplitude modulate the output of a highfrequency oscillator carrier wave. The modulated carrier wave is then radiated by an aerial having a radiation lobe with a vertical crosssection as shown in Fig. 1, the half-power beam width being 1 degree on either side of the axial surface. The beam may be fan shaped, having a width in azimuth of 20 degrees and be scanned in elevation between 2 degrees and 22 degrees, or it may be in the form of a hollow cone with a vertical axis, the apex angle of the cone being similarly variable. As the beam sweeps the aircraft M, the modulation frequency thereof varies from (f 0  - df ) to (f 0  + df ), f 0  being the modulation frequency as the axial surface E.S. sweeps the aircraft. The following modifications are mentioned: the frequency-modulated signal at the transmitter produced by mixer 102, Fig. 2, may be a square wave; the carrier may be modulated in frequency instead of amplitude; the frequency modulation need not be linear with respect to the elevation, but may have a logarithmic relationship; the scanning speed of the lobe need not be constant, the half power width of the lobe may vary with elevation, the receiver may accept the radiated beam on both the upward and downward scans; the azimuth of the aircraft may be measured by azimuth scanning of the beam which is formed with a vertical axial surface; the elevation and azimuth may be combined using independent transmitter and receivers or one reciever may be used, the two transmitters transmitting alternately with different centre modulation frequencies, the signals being separated at the receiver by filters.