GB2433367A

GB2433367A - Filter using tuning fork crystals

Info

Publication number: GB2433367A
Application number: GB0524758A
Authority: GB
Inventors: Peter John Jones; Philip Elphee Williams
Original assignee: Individual
Current assignee: Individual
Priority date: 2005-12-03
Filing date: 2005-12-03
Publication date: 2007-06-20
Anticipated expiration: 2025-12-03
Also published as: GB0524758D0; GB2433367B

Abstract

A filter comprises miniature crystal tuning fork oscillators H, I to provide low frequency and narrow band characteristics and uses an operational amplifier G to neutralise stray capacitance. The tuning fork crystals may include two identical crystals in parallel to reduce series resistance and increase bandwidth. Crystals with slightly differing resonant frequencies may also be used to widen the bandwidth. A suggested application for the filter is in a system sending data over a noisy network such as a domestic electricity supply circuit.

Description

An Enhanced Filtering Technique The invention describes the innovative

application of tuning fork crystals to provide an economic means of filtering signals to allow the accurate detection of small signals in the presence of noise and adjacent channel interference in signal transmissions along mains power wiring. It includes the techniques for enhancing the characteristics and performance of the tuning fork crystals by neutralising the effects of stray capacitance and optimising system bandwidth.

The principles described in the invention can be applied to the filtering of transmissions in a wide range of media such as infra-red or modulated laser light, ultra-sonic, and radio paths. A particular application of this invention is in the transmission and reception of low data-rate signals through domestic or commercial mains power wiring, and the invention will be explained below with special relevance to its use in this area.

The domestic mains wiring consists of three cables, Live, Neutral and Earth and there exists examples of utilising these cables for communication purposes. The best known being the voice intercom, frequently in the form of a baby alarm.

These intercoms could be plugged into any mains sockets and used Live and Neutral Lines, onto which was superimposed the audio signal, either as plain audio, or, in more elaborate arrangements, the audio was incorporated as part of a modulated signal. The disadvantages of this arrangement were twofold: both mains sockets must be wired to the same phase of the mains supply, and the ever present or 100 Hz hum had to be eliminated from the audio circuits.

Nowadays there is a lot more noise superimposed on the mains wiring due to the huge increase in domestic appliances that rely on switch-mode power supplies.

Examples are: light dimmers, fluorescent lights, motor speed controls and computer power supplies, as well as thermostats, all of which can superimpose high amplitude spikes on the mains. The mains wiring is, therefore, much noisier now than it was when the mains intercoms were introduced.

There remains a requirement to use the infrastructure in a building as a means to communicate data between control points, an example being for instance, the switching on and off of lighting or thermostatically controlled devices, where to retrofit cables would be costly and inconvenient.

A protocol known as Xl 0 exists, but in practise it has been found to be limited in range and easily degrades in the presence of electrical noise.

The arrangement that is the subject of this application demonstrates an economical way to improve the range and reliability of mains wiring communications by a factor of more than 100 times.

As mentioned, it is necessary to establish the communications link in the presence of high amplitude noise, which is both pulsed and frequency agile and results in a broad spectrum of interference from 50Hz to tens of Megahertz.

by reducing the bandwidth of the equipment, much of this interference can be avoided. In addition, by choosing a suitable operating frequency, then a compromise can be reached where the necessary filters can be made practical and economical, and the propagation of the signals in the wide ranging impedances, encountered on mains wiring, acceptable.

For the purposes of the description that follows, an operating frequency of 100kHz has been chosen, as this provides a practical compromise between available components and good propagation within the mains wiring environment, It should be remembered that the principle to be described applies to a wide range of operating frequencies and is not limited to the region around 100kHz. The description focuses on the receiver filtering techniques. The sender is assumed to be a high stability signal source switched at the data rate as required.

The use of narrow band filtering is well known, and is widely used. However, at frequencies below 1 MHz, the physical size of conventional quartz crystal components required to achieve suitable filtering becomes cumbersome. Below 200kHz, it is especially difficult to make them an economically viable component to manufacture and use. To offer an economic solution to this problem, tuning fork crystals are used as an alternative to the conventional piezo quartz crystal.

A range of mechanical Tuning-Fork devices, developed -largely in the Far East -for use in the watch and clock industries, are components with a characteristic similar to conventional Quartz Crystals. They offer a readily available, small and economic component. A typical fundamental frequency for use in oscillators for electronic timepieces is 32.768kHz, which divides neatly down to 1 second increments. These are often referred to as Crystals, but differ from the conventional piezo quartz crystal, which, at 32kHz, would be unacceptably large.

These tuning-fork crystals are universally used in oscillator circuits because of their small size and good frequency stability characteristics. However, we have now established that they may also be used as effective narrow band filters when properly fed and neutralised, and it is the method of enhancing their characteristics, which forms an integral part of this invention and its suitability for communications using mains power wiring.

It should be remembered that the procedures to be described could equally be applied to conventional Quartz Crystals operating at different frequencies, but for the purposes of the explanation, tuning fork crystals are referred to, as they are suited to the mains communications requirements.

Tuning Fork Crystals are available in the frequency range 12kHz to 300kHz and, as mentioned, the centre frequency in use for this description is 100khz. However, these techniques apply to any frequency for which practical devices can be made.

In order to use any filter in a signalling application, it is essential to consider the nature of the signals that are to be passed through the filter, and its ability to handle them. In addition, the relationship between the fundamental frequency of the noise on the transmission medium and the choice of bandwidth for the data and bandwidth of the filter stage is critical to ensure reliable signal detection and decoding. The chosen data and filter bandwidths must be much lower than the 5OHzI6Ohz fundamental frequency of the a.c. mains voltage. Similarly, they should not be harmonically related to these frequencies in order to avoid the mixing products from mains-synchronous switching pulses passing though the filter.

To transmit meaningful data, the signal at 100kHz needs to be modulated or switched on and off at a predetermined rate, which will determine the data bandwidth.

In our working example, we have chosen to use a 12 bit data word to convey the information, although it must be appreciated that it is up to the user to decide what is most suitable for any particular application.

Figure 1 shows the signal waveform representing a logic 0 bit and Figure 2 shows a logic I bit. Each bit comprises of 3 time elements, the first to indicate that the signal is present and is set to 1', the third element is a 0, and the second element is either a I or a 0 and is the data value for that bit. Therefore in a 12-bit word we need to look at sending and filtering 36 time elements and this has to be achieved in an

acceptable time.

If we select each element to be 55ms then a 12 bit word will take 3 x 12 x 55 1980ms, or just under 2 seconds, which, whilst being unacceptably slow for many applications, is considered suitable for sending a 12 bit word once.

The filtering stage must have sufficient bandwidth to preserve the rise time of the data stream by allowing the filter to reach ninety seven percent of its output. Ideally we would be looking for a bandwidth equivalent to three time constant periods in order to preserve the integrity of the incoming data and to correctly determine the status of the individual bits. Therefore, each time element within a data bit must be thiee time constants in duration. With a time slot of 55ms, then a time constant off 55ms13 is required, which is approximately 2Oms.

The ideal data bandwidth then is: I I (2 x pi x time constant).

With a time constant of 0.02s this gives a data bandwidth of 8Hz. To correctly filter out data with a bandwidth of this value, the associated bandwidth required in the filtering stage is twice this amount, so we need to achieve a filter bandwidth of 16Hz at 100kHz, with good out-of band attenuation.

The choice of 8Hz for the data bandwidth and, therefore, 16Hz for the filter bandwidth places both these values well below the fundamental frequency of the mains power of 50Hz, and avoids the mains synchronous mixing product harmonics of that frequency.

The normal characteristic of a single tuning fork crystal on its own as a filter shows a bandwidth equal to its resonant frequency (f) divided by its Quality Factor (Q). For tuning fork crystals designed to resonate at 100 kHz their bandwidth is approximately 1 Hz as the Q is quoted as I 0. This level of filter bandwidth would only detect data with a bandwidth of half this figure i.e. 0.5Hz. The filter bandwidth for a simple crystal is too small to work as a filter for data with a required bandwidth of 8Hz.

Referring to the Figures, Figure 3 shows the equivalent circuit of a tuning fork crystal.

The inductance (A) and capacitance (B) represent the primary tuned frequency. In addition there is a series resistance (C) and a parallel capacitance (D) sometimes rererred to as the Stray' or Shunt' capacitance, which comprises mainly of the capacitance associated with the leads and housing of the device itself.

Figure 4 shows the electrical characteristics. The impedance off-resonance at a frequency in the region of 100kHz is around 884k Ohms and is that associated with the stray capacitance (D) which is typicallyl.8pF. The impedance in the series mode is the value of resistance (C), typically 30k Ohms, which is represented by point (F).

Finally, the impedance at parallel resonance is the device's quality factor Q x the series resistance of 30k, which is very high indeed. The typical Q can be 100,000, giving a parallel impedance of 3000 Mega Ohms. This is represented by point (E) in Figure 4.

The other vital consideration is the bandwidth of the device. As mentioned above, this is of the order of 1 Hz.

Using such a crystal as a filter in an operational amplifier circuit to provide the gain and frequency characteristic necessary for use in a mains signalling application, then the parameters of the circuit components will all have an effect on the performance of the stage.

Operating at 100kHz, the stray capacitance of a crystal looks like an impedance of 800k Ohms and the series resistance looks like 30k Ohms. In order to get the filter stage to provide both gain and filtering at acceptable impedances then we need to create a suitable circuit design to reduce these values.

l1gure 5 shows an operational amplifier circuit with additional components to effect the necessary improvements.

Firstly two crystals are shown in parallel, (H) and (I). The immediate effect of this is to place the two series resistances in parallel so that the 30k is reduced to 15k.

However we now have 2 shunt capacitors in parallel and this has an adverse effect on the ability to reject frequencies above and below the operating frequency, which in our case is 100kHz. The extra capacitance allows these frequencies to pass though and be amplified by the operational amplifier. This effect is shown in Figure 6 by graph Q. However, as the operational amplifier has both inverting and noninverting inputs, then by adding capacitor (J) and making its impedance appear the same as that associated with the crystals, then the operational amplifier sees equal signals fed to both the inverting and non-inverting inputs. These signals are equal and, once passed through the operational amplifier, are opposite in phase and cancel each other out, so do not appear in the output at point (P).

All signals on the series-resonant crystal frequency of 100kHz are passed through the crystal to the non-inverting input only, where they are suitably handled by the Operational Amplifier. In practice, the characteristics of operational amplifiers are slightly different when using these two channels and capacitor (0) is used to trim out this offset. The result is that the input impedance now looks like 15 kOhms and the out-of-band rejection is improved by more than 50dB. This improvement is indicated in Figure 6 by comparing the original graph (Q) with the enhanced performance characteristics in graph (R).

The input resistor (K) is a critical component for achieving the required filter bandwidth of 16Hz. With the crystal in series resonant mode, it acts as the load resistor in series with the crystal. If, however its value is too high then the crystal will tend towards operation in its parallel resonant mode and drift off frequency and thus away from the fixed transmit frequency of 100kHz.

The crystals must operate in series resonant mode, so the value of (K) needs to be low. However, at the same time the bandwidth of the stage is also important. We have shown that the characteristic bandwidth of the crystal on its own is approximately 1 Hz. Within the circuit shown in Figure 5, the stage bandwidth is modified by the presence of the input resistor (K). The stage bandwidth is given by: (RL+RS) multiplied by the crystal characteristic bandwidth of f/Q

RS

where RL is the load resistor (K) and RS is the crystal(s) series resistance of 15k Ohms.

Referring to Figure 5, a suitable value for RL or (K) is 220k Ohms. It maintains the crystal in the series resonant mode and gives a value for the RF Bandwidth of: (220+15) x 1Hz = 15.6Hz A further enhancement of the system to expand the bandwidth can be achieved by shifting the fundamental frequencies of the tuning fork oscillators so that one is slightly higher and the other slightly lower that the transmit frequency of 100Khz.

This effect is illustrated in Figure 7.

The result is a characteristic double-hump, but very sharp, 16Hz filter with very much enhanced out-of band attenuation characteristics, capable of handling a 12 bit data The embodiment of the invention is a device that is capable of receiving data via the mains wiring in the presence of the strong and random electrical noise to be encountered on modern mains wiring circuits. When designing such communications devices, due care needs to be given to impedance matching and the requirement for high voltage rating capacitors.

The communication system can further be improved by the use of FM techniques using two separate frequencies, for example 100kHz and 110kHz, corresponding to the 0' and 1' logic bits. In addition, a number of parallel channels spaced in frequency could be used in a system with suitable control equipment to distribute and recombine the data.

Establishing the signal between Neutral and Earth can improve the signal to noise ratio as much of the mains borne noise is to be found on the Live circuit. The use of Neutral and Earth has an addition benefit in 3-phase installations where Neutral and Earth are usually common and only the live phase is distributed separately. I0

Claims

Claims 1) An Electronic filter incorporating miniature tuning-fork

oscillator crystals in a low frequency very narrow band configuration, made possible by the neutralisation of stray capacitance using an operational amplifier.

2) A Filter as in 1) incorporating 2 or more identical tuning-fork crystals in parallel to reduce the equivalent series resistance thus widening the bandwidth.

3) A filter as in 1) and 2) using crystals of slightly differing frequencies to further widen the overall bandwidth. * I * * * Is.. * S S...

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