Patent GB-541194-A (GB)
Title: Improvements in radio echo distance meters
IPC: G01S13/32
Filed: 1940-09-30
Abstract: 541,194. Indicating proximity of earth. BRITISH THOMSON-HOUSTON CO., Ltd. Sept 30, 1940, No. 14710. Convention date, Sept. 29, 1939. [Class 37] A radio echo distance meter comprises means for simultaneously propagating a pair of radio waves of different frequencies towards a surface the distance of which is to be measured to produce separate standing waves between each of the radiated waves and the corresponding wave reflected from the surface, and means for comparing the intensities of the separate standing waves. The transmitter for the two waves is shown in Fig. 2 and comprises a converter stage for obtaining a wave whose frequency is the difference of the frequencies of waves generated by a pair of oscillators. One of the oscillators of the heterodyne generator has a fixed frequency and the other has an adjustable frequency so that the frequency of the output may be controlled. The fixed frequency oscillator is shown in the upper part of Fig. 2 and comprises a valve 14 having its control grid connected through a condenser 17 and grid leak 18 to a resonant circuit formed by a condenser 15 and coil 16. The other end of the resonant circuit is earthed and also connected through a blocking condenser 19 to the second grid of the valve 14, which is supplied with operating current through a voltage dropping resistor 20. The cathode is connected to the central point of the coil 16, and the anode is connected through a resonant circuit 22, 23 to the source of operating potential. The adjustable frequency oscillator, shown below the fixed frequency oscillator, is similar and includes similar parts with similar reference numerals. The condensers 15, 23 are replaced by mechanically connected variable condensers 24, 25. The converter stage includes a pentagrid valve 26 of which the first grid is connected by a coupling condenser 27 to the anode of the valve 14 in the adjustable frequency oscillator, and the third grid is connected by a coupling condenser 28 to the anode of the valve 14 in the fixed frequency oscillator. The cathode is earthed through the secondary of a low-frequency transformer 29. The second and fourth grids are connected to a source of operating potential through a voltage-dropping resistor 33 and are also connected to the cathode through a highfrequency byepassing condenser 34. The anode is connected through a low-pass filter and the primary of an output transformer 35 to a source of operating potential. The transformer 29 introduces a fixed low frequency signal into the output of the heterodyne generator for facilitating recognition and measurement. The low frequency signal is produced by an oscillator formed by a valve 41 and a tuned .circuit consisting of the primary of the transformer 29 and a condenser 42. The transformer 35 is coupled to the circuit of a balanced or carrier suppressing modulator arranged to produce two very high-frequency waves separated in frequency by twice the frequency of the heterodyne generator. The modulator is excited by a high frequency oscillator comprising a pair of triodes 45, 46, the anodes and grids of which are connected by a pair of parallel transmission lines 47, 48. The centre point of the line 47 is connected to a source of operating potential, and the centre point of the line 48 is earthed through a grid leak resistance 49. The cathodes of the valves 45, 46 are earthed through high frequency choke coils 50, 51. The primary of a high frequency transformer 52, which may be a single loop, is connected to two points on the line 47 equally spaced on opposite sides of the centre. The balanced modulator comprises a pair of triodes 53, 54, the cathodes of which are connected together and through a condenser 55 to one side of the secondary of the transformer 52. The other side of this is connected through a condenser 57 to the centre tap of the secondary of the transformer 35, which is connected to the control grids of the valves 53, 54. The anodes of these valves are connected together through a load resistor 59, the centre tap of which is connected to a source of operating potential, and through coupling condensers 60, 61 to a dipole antenna. The balanced modulator produces two waves the frequencies of which are respectively the sum and difference of the frequency of the wave produced by the high forequency oscillator and that of the adjustable frequency wave produced by the heterodyne generator. These two waves are radiated from the dipole antenna and are directed towards the object from which they are reflected. The receiver and indicator for the two transmitted waves is shown in Fig. 3. A receiving dipole antenna excites the primary of a transformer 62, of which the secondary is tuned by an adjustable condenser 63 and a trimming condenser 64. This tuned circuit is connected. between the cathode and control grid of the heptode section of a duplex triode heptode valve 66. The triode section of the valve 66 forms an oscillator to produce a wave for converting the incoming signals to an intermediate frequency. The anode of this section is connected to the primary of a transformer 70, which is connected through a condenser 71 to the cathode. The secondary of the transformer 70 is tuned to the oscillator frequency by a condenser 72 and is connected between the cathode and control grid of the triode section. The grid of the triode section is connected directly to the third grid of the heptode section of the valve 66. The tuned circuits connected to the grids of the heptode and triode sections of the valve are adjusted so that the sum or difference of their frequencies produces the intermediate frequency required. The second and fourth grids of the heptode section are connected to the source of operating potential through a resistor 78, and the fifth or suppressor grid is connected directly to the cathode. The anode of the heptode section is connected to a source of operating potential through a resistor 80. Intermediate frequency voltage variations produced by the incoming signals are transmitted from the anode of the heptode section to two intermediate frequency amplifiers by two condensers 81, 82. The characteristics of these amplifiers are such that they amplify only waves produced by radiations of respectively higher and lower frequency than the frequency of the high frequency oscillator of Fig. 2. Each of the intermediate frequency amplifiers transmits its output to a separate second detector stage which demodulates its particular waves to produce a 500 cycle wave, the amplitude of which is a measure of the amplitude of the corresponding standing wave at the receiving dipole antenna. This demodulation is accomplished by a pair of duplex-diode-triode valves 91, 92 connected to the first and second intermediate amplifiers respectively. The diode anodes of the valve 91 are connected to a tuned circuit formed by a condenser 93 and a coil 94 coupled in the output circuit of the output valve 84. The other end of the tuned circuit is earthed through resistors 95, 96 shunted by a byepassing condenser 97. The junction of the resistors 95, 96 is earthed through a filter condenser 98. A resistor 99 is connected at one end through a condenser 100 to the junction of the resistors 95, 96, and is earthed at the other end through a condenser 101. A voltage is developed across the resistor 99 having a frequency of 500 cycles and an amplitude proportional to the amplitude at the receiver of the standing wave of higher frequency. One end of the resistor 99 is connected to the grid of the triode section of the valve 91, and the other end is connected to a source of bias voltage. The anode of the triode section is connected through the primary of a low-frequency transformer 102 to a source of operating voltage. This anode is also connected to earth through a byepassing condenser 103 which maintains the anode at cathode potential for intermediate frequencies but not for 500 cycles. The valve 92 is similarly connected to the second intermediate frequency amplifier and to the primary of a low-frequency transformer 105. The secondaries of the trans. formers 102, 105 are connected in opposition through a switch 106 to the control grid of a rectifying triode 107 which has a D.C. meter 112 in its anode circuit. The double-pole two-way switch 106 and a single-pole two-way switch 108 permit the output of the intermediate frequency amplifiers to be separately measured as a check on the operation of the apparatus. The switch 108 is then moved to supply grid bias voltage through both secondaries of the transformers 102, 105 in series, so that the difference in the output of the intermediate frequency amplifiers is obtained. The condensers 24, 25 are then adjusted until a minimum indication is obtained, corresponding to coincidence in the standing wave amplitudes. The dial attached to the condensers 24, 25 may be calibrated in distance units.