In particular, the invention relates to electronic systems that include analogue components, which are especially sensitive to noise. Analogue components are used in circuitry for measuring and monitoring parameters including temperature, time and gas flow. For accurate measurements, it is thus desirable that interference from noise sources be minimised.

Noise can either be random or systematic (non-random) in nature.

Random noise is mainly due to thermal effects of resistive elements of electronic circuits. Other types of random noise include shot noise and 1/f noise (or'pink noise'). Random noise can be minimised by careful selection of components and analogue circuit design, for example by careful power supply routing and decoupling or by employing a single point ground. In addition, the effects of random noise can be averaged out by taking several measurements of the parameter of interest.

Interference from defined events such as the switching of digital circuits causes systematic noise effects. In many circuits the main source of such defined events is the system clock. If a noisy digital signal or system clock initiates measurements of the parameter of interest, systematic noise effects cannot be averaged out by taking several measurements.

Circuits containing both analogue and digital components are known as mixed signal circuits. Many electronic applications, particularly applications involving microcontrollers, use mixed signal

circuits, for example mixed signal application specific integrated circuits (ASICs). Systems that employ mixed signal circuits are particularly sensitive to systematic interference.

In mixed signal ASICs, signals in the digital components cause interference currents to flow within the ASIC substrate which in turn couple into the sensitive analogue components used to process the signal of interest.

Mixed signal ASICs are used in flow metering. The time of flight (TOF) of a signal, for example an ultrasonic signal, is measured using a system clock-a digital component. If the received signal is subjected to noise, then both the TOF measurement and meter accuracy are affected.

This is particularly true for ultrasonic gas meter applications where the signal of interest is severely attenuated by the gas within the measuring tube, resulting in a poor signal to noise ratio. As elsewhere, the influence of random noise can be cancelled out over time by sampling the gas flow on a regular basis and averaging. However systematic noise, for example noise caused by the system clock pulse-edges, will not average out, and the interference can only be reduced by taking special steps.

The analogue and digital components of a mixed signal ASIC cannot be separated. External circuit changes can improve system performance in the presence of systematic interference but not in every case. When improved system performance cannot be achieved by external circuit changes, expensive and time-consuming re-engineering of the ASIC has to be considered. Due to the complexity of mixed signal ASICs, there is no guarantee that re-engineering the ASIC can cure the problem of systematic interference. The signal to noise ratio in the re-engineered ASIC may still be too low for measurements to be meaningful.

Ideally, apparatus that reduces systematic noise in mixed signal metering circuitry will be provided, thereby rendering ASIC design less critical and increasing the chance of successfully curing the systematic noise.

It is therefore an object of the invention, when applied to a metering apparatus, to shift the signal of interest in relation to a fixed clock, in order to reduce systematic interference from the fixed clock by taking several measurements and averaging the result.

It is a further object of the invention to shift the signal of interest in relation to the fixed clock in order to reduce systematic interference from the fixed clock by predicting the coincidence of a given signal of interest and systematic noise.

According to the present invention there is provided an electronic metering system including: a first clock, which has a substantially constant frequency; a second clock, which has a higher frequency than the first clock; and a measurement apparatus, which measures the time of flight of at least one measurement signal; the measurement apparatus including a delay means, which delays signal transmission in order to reduce the possibility of the at least one measurement signal being received at substantially the same time as any first clock pulse edge.

Preferably, the measurement apparatus includes a first transducer, which transmits a first measurement signal, the signal transmission coinciding with a given edge of a pulse of the first clock; and a second transducer, which receives the first measurement signal. The second transducer can transmit a second measurement signal and the first transducer can in turn receive the second measurement signal.

Advantageously, a plurality of further first measurement signals and further second measurement signals are transmitted between the first and second transducers.

It is preferred that the first transducer and the second transducer are both ultrasonic transducers and that the time of flight measured is the time of flight of a given measurement signal through a medium from one transducer to the other. The medium is preferably a gas.

The delay means advantageously delays successive transmissions of the at least one measurement signal in a predetermined sequence of delays to avoid the at least one measurement signal being received at substantially the same time as any first clock pulse edge.

A preferred sequence of delays starts at a minimum number of second clock periods, the number of second clock periods by which the transmission is delayed is then incremented by a first integer number of second clock periods for each further measurement until the delay reaches a maximum number of second clock periods, thereafter the delay in subsequent time of flight measurements is decremented by a second integer number of second clock periods for each further measurement until the delay reaches the minimum number of second clock periods.

Preferably, the first integer number is one and the second integer number is also one.

The minimum number of second clock periods is advantageously zero.

Preferred maximum numbers of second clock periods include seven and sixteen.

The delay means can advantageously prevent a given measurement signal from being transmitted whenever the given measurement signal would be received at the same time as any first clock pulse edge.

Preferably, the delay means prevents transmission of the given measurement signal by imposing a delay of a predetermined number of second clock periods on the transmission of the given measurement signal.

The electronic metering apparatus advantageously further includes a microcontroller which controls the delay means, the microcontroller calculates when a received signal will coincide with a first clock edge and as a consequence delays the transmission by a number of second clock periods, the delay preventing the coincidence of the measurement signal with said first clock edge.

In a further aspect of the invention, there is provided a method of delaying the transmission of at least one measurement signal in an electronic metering apparatus having a first clock and a second clock, the second clock having a higher frequency than the first clock, the method having the steps of : transmitting the at least one measurement signal with a predetermined delay; and receiving the at least one measurement signal; the predetermined delay being arranged to delay the reception of the at least one measurement signal to a time other than the time when any first clock pulse edge is present.

Advantageously, the predetermined delay consists of a delay of a predetermined number of second clock periods.

Preferably, the predetermined number of second clock periods starts at a minimum number of second clock periods, the number of second clock periods by which the transmission is delayed is then incremented by a first integer number of second clock periods for each further measurement until the delay reaches a maximum number of second clock periods, thereafter the delay in subsequent time of flight measurements is decremented by an integer number of second clock periods for each further

measurement until the delay reaches the minimum number of second clock periods.

Preferably, the first integer number is one and the second integer number is also one.

The minimum number of second clock periods is advantageously zero.

Preferred maximum numbers of second clock periods include seven and sixteen.

The predetermined delay advantageously prevents a given measurement signal from being transmitted whenever the given measurement signal would be received at the same time as any first clock pulse edge.

Preferably, the predetermined delay prevents transmission of the given measurement signal by imposing a delay of a predetermined number of second clock periods on the transmission of the given measurement signal.

More preferably, the predetermined delay is determined by calculating occasions when a received signal will coincide with a first clock edge and using the calculation to determine the delay by which the transmission is delayed in order to prevent the coincidence of the measurement signal with any first clock edge.

Advantageously, the electronic metering apparatus is provided in a gas meter.

For a better understanding of the present invention, reference will now be made, by way of example only, to the accompanying drawings: Figure 1, shows a ultrasonic gas metering system to which the present invention applies;

Figure 2, shows the coincidence of signal pulse and sub-clock signal edge which the present invention seeks to avoid; and Figure 3, is a diagram illustrating one method of delaying the transmission of a signal as used in the present invention.

The application of the present invention to ultrasonic gas metering systems can be seen in the following embodiment.

Figure 1 shows an ultrasonic gas metering system to which the invention applies. Within an ultrasonic gas flow meter 100 there is a measuring tube 138 with an ultrasonic transducer at each end 132,134.

The transducers 132,134 alternate between transmitting and receiving. To measure flow in a medium 136, a first transducer 132 transmits a signal which is received by a second transducer 134, and the time of flight (TOF) is determined using the system clock 124. Thereafter the second transducer 134 transmits a signal which is received by the first transducer 132. The transmission pattern is repeated continually. Over time the effects of random noise on the TOF measurements will average out. If the path length and cross sectional area of the measuring tube are accurately known, the flow can be calculated from the TOF measurements.

In order to measure TOF accurately a 32,768Hz (32kHz) watch crystal is used to generate a stable clock (or'sub-clock') 122. However to get the required time resolution this sub-clock signal has to be multiplied up to 2.097MHz (the'main clock'frequency) using a phase locked loop 124.

A conventional meter produces a signal"FIRE"which initiates the relevant transducer to transmit a signal. The FIRE signal is transmitted coincident with a 32kHz sub-clock edge. If the received signal also happens to be coincident with another 32kHz sub-clock edge, then

systematic interference can occur. This type of systematic interference has proved to be a problem for gas meters that use mixed signal ASICs.

Figure 2 shows two waveforms which illustrate this problem and the inventive solution. The top waveform 202 is the signal from the transducers and the bottom waveform 206 is the noisy sub-clock signal.

When a sub-clock edge 208 occurs at the same time as a signal pulse 204, as illustrated, interference may corrupt the signal.

Now consider the situation when the sub-clock 206 is steady; if the transmission signal (the FIRE signal) is shifted causing the signal pulse 204 to be shifted in the direction of the arrows 210, between"A"and"B", then the influence of the sub-clock edge 208 will be lessened.

Thus in the gas metering embodiment of the invention, the FIRE signal is shifted in time with respect to the sub-clock, which in turn allows the received signal to be shifted away from systematic noise. A convenient reference for the amount of shift can be one or more multiples of the main clock period. So, for example, the FIRE signal could be delayed until one main clock period after a sub-clock edge.

Variable delay will be of little use if the delayed FIRE signal still causes the received signal to be shifted to coincide with a noisy clock edge. To get round this difficulty, one of two alternative methods can be adopted.

In the first method, the FIRE signal can be shifted in a rotating fashion. By making the amount of shift variable (either random or incremental between successive measurements) the received signal cannot be coincident with systematic noise for every measurement made. So the effect is to reduce the influence of the noise. The more variation in the shift the less the influence should be.

By way of illustration consider the example of a rotating shift of seven periods shown in Figure 3. Here the amount of shift increments up to seven then decrements to zero. Then the whole process repeats.

Of course, this is only one example from amongst many appropriate shift patterns. By allowing the signal to move, in a number of steps, and then back again, and making a measurement at each step, the effects of the sub-clock edges can be reduced by averaging. It will be understood that the maximum and minimum shifts, the magnitude of the increments and even the pattern of applications of increments are all predetermined so that there is a range of delays over a cycle of rotation. The minimum delay imposed on the measurement signal is not necessarily zero, nor is the maximum delay limited to seven (another common value is sixteen periods). The increments need not be in single clock periods; increments can equally well be two or three clock periods. The increments do not need to be made at each successive measurement: for example three successive measurements can be made at a given delay before incrementing.

For each measurement, the transmission of the signal is delayed by an integer multiple of the main (2.097MHz) clock period (i. e. 477 ns).

This delay is varied over many measurements and the effects of systematic interference are averaged out to an acceptable level. The delay is incremented up to a maximum number of main clock periods, and is then decremented back to zero, thereafter the process repeats.

With this rotating shift method, the gas flow meter does not have to know where clock edges are. Under steady state conditions for temperature and flow, the received signal will be moving in relation to the sub-clock edges; the systematic interference of the sub-clock will be

reduced over several measurements. The shift steps and shift range may be changed to reduce the systematic effects to an acceptable level.

The second method requires knowledge of where clock edges are and consequently prevents the received signal from coinciding with clock edges.

Metering systems generally have microcontrollers for a variety of control purposes. Systems with microcontrollers can be programmed to use the main clock for the FIRE signal delay and to measure TOF, such systems can calculate when a received signal is likely to coincide with systematic noise, such as sub-clock edges, and thus modify the signal delay accordingly. A TOF estimate can be generated from recent TOF measurements. If this estimated time proves to be sufficiently close to a multiple of the half period of the sub-clock (when a clock edge occurs) an evasive transmission delay is imposed. Referring again to Figure 2, by effectively moving the signal to positions"A"or"B", coincidence with a sub-clock edge 208 can be avoided. It is thus possible to avoid systematic interference in a single measurement, provided systematic noise is confined to one signal source (sub-clock signal or a harmonic of the main clock signal).

In this way systematic interference in mixed signal ASICs can be overcome or at least reduced.

If the transmission delay is under the control of a microcontroller, the delay is thereby known and can be compensated for in the TOF measurement, whether the rotating shift or the clock edge avoidance method is applied.

It will be understood that although the preceding discussion relates to mixed signal application specific integrated circuits, the invention

applies equally well to other types of mixed signal circuitry that share the problems of systematic noise.

Although the above discussion deals with ultrasonic gas metering applications it is not intended that the scope of the discussion is limited solely to applications to gas metering. The methods described can equally be applied to the metering of the flow of fluids in general, whether gaseous or liquid. In a similar vein, the transducers are not to be considered restricted in number to just two transducers or in type to ultrasonic transducers alone. The present invention is relevant to measurement using any conventional transducers, whether they be electromagnetic (e. g. RF, infrared, optical) or ultrasonic in nature. Indeed different types of transducers will be selected to measure flow under different temperature ranges and flow conditions.