Patent GB-1023843-A (GB)
Title: Improvements in or relating to systems for measuring distances by electromagnetic waves
IPC: H03L7/06
Filed: 1963-08-15
Abstract: 1,023,843. Measuring distance by radio waves; automatic frequency control systems. ALBISWERK A.G. Aug. 15, 1963 [Aug. 8, 1962], No. 32285/63. Headings H3A and H4D. In a system for measuring distances between stations by means of electromagnetic waves in which a first station at one end of a distance to be measured emits electromagnetic waves of a first measuring frequency and a second station at the other end of the distance emits electromagnetic waves of a second measuring frequency which differs from said first frequency by a difference frequency, wherein a signal at the difference frequency is produced at each station by mixing the measuring frequency produced at said station with the measuring frequency received from the other station, and one such signal is transmitted from one station to the other for phase comparison with the other difference frequency signal, the frequency of the difference frequency signal at one station is compared with a constant reference frequency in a frequency discriminator and the resultant signal used to control the measuring frequency emitted from said one station to maintain the difference frequency constant. In the system of Fig. 1, station A, the " main " station, compares in unit 18 the phase of its own difference frequency signal produced by mixing a carrier produced by oscillator 7 and modulated in means 5 by the first measuring frequency produced by oscillator 3, in mixer 9, with a signal, received from station B, and comprising a carrier modulated by the second measuring frequency, with that of the difference frequency signal of station B transmitted to station A as a modulation signal, to produce an indication of the required distance. At station B its difference frequency signal is produced by mixer 10 and demodulator 15, and is fed to a modulator 6, to modulate carrier waves produced by oscillator 8 for transmissions to station A, and also to a frequency discriminator 20, wherein its frequency is compared with that of a signal from a reference oscillator 21, to produce a signal which controls the second measuring frequency produced by oscillator 4. At station A the difference frequency signals are respectively fed from demodulators 13 and 14 to pulse shapers 16 and 17. These produce pulses at corresponding parts of the oscillations such that the time interval between the occurrence of an output pulse from shaper 17, which operates on the difference frequency signal of station A, and the occurrence of the corresponding output pulse from shaper 16, represents the phase difference between the two difference frequency signals and the required distance. The second difference frequency from demodulator 15, Fig. 1, may be converted to a pulse train before being modulated on the carrier. An arrangement is illustrated in Fig. 3 (not shown), which, when substituted for means 12, 15, 20 and 21 in Fig. 1, connecting terminals of like notation, gives a station 13, which can be used as a main station or an auxiliary station. Means 11, 13, 14, 16, 17, 18, 19 and 20 are the same as in Fig. 1. With switches 32/35 as shown the station acts as main station A of Fig. 1. Switch 35 applies a constant bias voltage U2 to terminal D of Fig. 5 to control the centre frequency of the oscillations therefrom. With the switches in their second position, corresponding to auxiliary operation, the channel comprising means 13, 16 and 18 is no longer used and the output of counter oscillator 19, is frequency divided by means 31 to produce the reference frequency signal. Switch 35 now applies a different bias voltage U1 to terminal D to change the measuring frequency. The phase measuring unit 18 which receives the pulses from the two shapers, is illustrated in detail in Fig. 2 (not shown), and comprises an electronic pulse counter 29 fed with the output of a pulse shaper 28 operating on the output of oscillator 19 via AND gate 27, opened by a signal from NOT-AND gate 26. The pulse signal from shaper 17 is applied to one input of gate 25, which is kept open for a requisite number of oscillation cycles by a signal applied to its other input by electronic repeating counter 24. The output of gate 25 is then applied to an input of gate 26 to produce a corresponding output therefrom opening gate 27. The pulse shaper 28 then proceeds to pass pulses to the counter 29 through gate 27 until the pulse from shaper 16 occurs at the inhibiting input of gate 26, causing the output of the said gate to cease, and gate 27 to close. The count in counter 29 thus represents the required phase difference and distance. The frequency discriminator 20 is illustrated in greater detail in Fig. 4 (not shown) and is described as producing a signal proportional to the frequency difference between a reference signal B sin # 0 t and a test signal A sin # x t. The two signals are mixed in a mixer 42 to produce a difference frequency signal C sin( # x  - # 0 )t, which is then passed to a differentiator 43 to produce a signal D( # x  - # 0 )[sin( # x  - # 0 )t + #/2]. The reference signal is passed through a 90-degree phase shifter 41, to become E sin(# 0 t + #/2). This signal is then mixed in mixer 44 with the test signal to produce a signal F sin[(# x  - # 0 )t + #/2] or F sin[# x  - # 0 )t - #/2], depending on whether # 0  is less or greater than # x . The differentiator signal is amplitude demodulated in a phase controlled demodulator under the control of the output of mixer 44, to produce a signal G(# x  - # 0 ), the plus or minus sign depending on whether # x  is less or greater than # 0 . The controllable oscillator 4 is illustrated in greater detail in Fig. 5 (not shown) and comprises a grounded base transistor oscillator with a transistor H51, an output transformer T51 having a feed-back winding connected to a series resonant circuit comprising capacitor C51, frequency control crystal Q51 and a capacitive control circuit comprising a capacitor C52 and a capacitive diode G51. The frequency of the oscillator circuit is controlled by the output of the frequency discriminator 20 applied via terminal C and a low pass filter network, across the capacitive diode, together with a biasing negative signal on terminal D. Diodes G52 and G53 serve to limit the amplitude of the oscillations produced at output terminal E.