the first few cycles of the signal

the first few cycles of the signal become difficult to detect, and this will greatly increase the uncertainty with which the exact time of arrival of the signals can be detected.Electronic filters introduce small delays or phase shifts. In system designs where separate filters are used at either end of a path (i.e. where signal amplifiers are located close to or integral with thetransducers), the differential delays may be significant compared with the transit time differences. These shall be taken into account as described in 13.5.2.1, or else the use of such filters should be avoided.13.5.3 Signal maintenance (gain control)When signals are attenuated by sediment loading, seasonal weed growth, scattering, temperature gradients, debris or transducer deterioration, the first cycle may be so reduced that a simple detector may trigger off the second cycle. The resulting timing error may be equal to the period of the transducer frequency. If this occurs in only one direction, the error in the time difference could be serious (2 µs for 500 kHz transducers). For many gauges, designed for low flow determination, an error of many hundreds of percent could result.The receiving amplifier shall be able to cope with the resulting large dynamic range of signal. In practice this may be difficult without it becoming saturated when conditions are such that the attenuation is small. The saturation could lead to timing errors which would be serious in many applications.A system of automatic gain control (AGC) may be used to prevent the saturation of the amplifiers.When using an AGC, increasing the gain increases the interfering noise while attempting to maintain the signal. In order to cope fully with signal attenuation in the presence of noise, the transducer drive voltage needs to be set to maximum.The continuous use of maximum transducer drive voltage may be undesirable for the following reasons.a) There may be a detrimental effect on the operating life of the transducers.b) The “dead time” which occurs after the transducer is energized may be increased. This is particularly important when using transducers for depth measurement, since it determines the minimum depth of water that can be measured.c) The power consumption may be increased. This may be significant for systems designed to operate on batteries.A system of automatic adjustment of the transducer drive voltage to compensate for signal strength variations may be employed. It is unlikely that this would be acceptable if used alone without an AGC, because the gain would then need to be set to maximum to cope with the worst case, thus making the system susceptible to interference. A combined system, using an algorithm to adjust both the gain and transducer drive voltage, may be desirable. This would optimize the signal-to-noise ratio, while maintaining a minimum drive voltage.13.5.4 Signal detection13.5.4.1 GeneralHaving produced as good an electronic version of the ultrasonic signal as possible, the next process is to translate the point of arrival to an accurate time scale. The most important thing is the time difference, so the process applied to signals being received in both directions shall be identical. It is necessary to wait until a recognizable part of the waveform is received, usually after not more than a complete wavelength. To wait longer is to risk distortion from reflections from the water surface (see 6.2.5), and to suffer an increased uncertainty due to mismatch of the characteristic frequency of the transducers at either end of a path (see 13.2.1).If there is any doubt about the validity of the signal, it is better to reject it than to produce a potentially grossly erroneous determination of velocity