Patent GB-1476749-A (GB)
Title: Measurement of liquid level using a frequency modulated microwave signal
IPC: G01F23/284
Filed: 1974-11-18
Abstract: 1476749 Measuring liquid level SAAB-SCANIA AB 18 Nov 1974 [20 Nov 19731 49900/74 Heading H4D The level of the surface of a liquid in a tank is measured by an apparatus using a frequency modulated microwave signal, wherein to calculate the required distance the quotient is taken of a frequency difference signal obtained from mixing the transmitter signal and received reflected signal, and an internally generated reference frequency signal, corresponding respectively to the required distance and a known distance, the quotient obtained corresponding to the level of the surface. Suppression of spurious signals is enhanced by converting the analogue frequency signals to digital, obtaining the product of these signals and a statistical weighting factor, integrating the weighted signals over the time taken to perform a frequency sweep of the transmitted signal and forming the quotient to obtain the level measurement distance. As a measurement may be made over one sweep, several emitter units 1 may be time division multiplexed and each processed by signal processing unit 2. Each emitter unit comprises varactor tuned Gunn diode 6, triggered by a pulse from control unit 23, and feeding mixer 8 and antenna 10 with a frequency swept microwave signal, whereby the reflected signal received by antenna 10 is mixed with the instantaneous transmitter signal to provide the frequency difference signal. Diode 6 also feeds mixer 13 and delay line 14 whereby a reference frequency difference signal corresponding to a fixed known distance is generated. The reference signal is used, after multiplication in unit 16 of processor 2, to control the centre frequency of band-pass filter 19 to compensate for variations in sweep speed on the value of the difference frequency from mixer 8. The reference signal is then digitized at pulse generator 17 and fed to calculator 18 (see Fig. 2). The difference frequency is filtered at 19, digitized by pulse generator 20, and fed both to multiplier 28 and is also used to calculate at 26 the weighting factor 1 which is defined as (see i in Fig. 3, not shown) having amplitude n(N-n) where N is the number of equal interval pulses of the difference frequency signal h over the sweep time, and n refers to each numerical sequence in the event. The computed weighting factor is stored in register 27 and is multiplied at 28, 29 with difference signal h and reference signal f respectively, which signals are integrated over the sweep time in registers 30, 31 respectively, and the quotient taken at 32, to be displayed at 22 (Fig. 1). Amplitude comparator 25 detects the difference between the input and output of filter 19, i.e. the amount of spurious signal rejected by filter 19; when this is greater than the measurement signal passed through the filter, measurement is ceased. The apparatus is described in use on a cargo ship (Fig. 4, not shown) where a plurality of emitters are arranged above cargo tanks each emitter having encapsulated therein a horn fed parabolic antenna and the generator and mixing circuitry.