GB2094598A - Transmission systems for transmitting signals over power distribution networks, and transmitters for use therein - Google Patents

Abstract

A transmission system for transmitting digital signals over an electrical power distribution network, eg for load management and/or remote meter reading purposes, comprises means for generating spread spectrum signals (as described chirp signals) whose lower frequency lies above 15 kHz. Modulating means is provided to modulate one or more chirp signals onto the network voltage to represent each occurrence of a particular bit value (ie a 0 or a 1) of the digital signal to be transmitted. The frequency range of the chirp signals preferably covers 64kHz to 150 kHz. If desired, a different kind of spread spectrum signal, for example a pseudo-random signal, can be used in place of a chirp signal. With the use of spread spectrum signals it is more probable that at least part of the frequency range will be received and hence that data will be received correctly despite changing nulls in the power network. <IMAGE>

Description

SPECIFICATION Transmission systems for transmitting signals over power distribution networks, and transmitters for use therein This invention relates to transmission systems for transmitting signals, such as remote control signals and/or data signals, over electrical power distribution networks, and to transmitters and receivers for use in such transmission systems.

It has been proposed that such functions as tariffswitching, load-shedding and remote meter reading in electrical power distribution networks could be effected by means of radio-frequency carrier signals, which are modulated with digital signals and then transmitted over the wires of the network by modulating the network supply voltage with the modulated carrier signal. Our co-pending United Kingdom Patent Application No. 8104777 describes and claims a receiver suitable for receiving such signals, even when they are relatively weak and heavily contaminated by noise.

However, the abovementioned proposal suffers from the serious drawback that for any given carrier frequency, and in a transmission system where respective transmitters located in or adjacent individual power consumer's meters are arranged to transmit data such as the meter reading to, for example, the power supplier's sub-station, the receiver may be located at a null in the network with respect to one or more of the transmitters: similarly, in a transmission system where a transmitter is arranged to transmit signals from, for example, the power supplier's sub-station to respective receivers located in or adjacent individual power consumer's meters, one or more of the receivers may be located at a null in the network with respect to the transmitter.The drawback is made worse by the fact that the positions of such nulls change continually as the transmission characteristics of the network vary in response to the continually changing load on the network.

It is an object of the present invention to provide transmission systems for transmitting signals over electrical power distribution networks, and transmitters and receivers for use in such systems, in which the effects of the abovementioned drawback are substantially alleviated.

According to the present invention, there is provided a transmission system for transmitting digital signals over an electrical power distribution network, the system including means for generating at least one kind of spread spectrum signal whose frequency spectrum lies substantially wholly above 15 kHz, and means responsive to at least one possible bit value of a digital signal to be transmitted over the network to modulate at least one spread spectrum signal of said one kind onto the network voltage to represent said bit value.

Thus at least a portion of each such spread spectrum signal is likely to be successfully transmitted and received, and the received portion will usually be sufficient to enable the bit value represented thereby to be correctly determined.

The spread spectrum signal preferably has a frequency spectrum lying substantially wholly below 150 kHz and a upper to lower frequency ratio of at least 2:1, and may typically be either a pseudorandom signal, or a swept frequency signal such as a chirp signal (that is, a signal whose frequency is swept linearly between first and second values).

Where messages are transmitted using swept frequency signals such as chirp signals as pulses, the pulses can be modulated with the message using any convenient form of pulse modulation, for example pulse position modulation, pulse presence modulation or pulse amplitude modulation. Of these, pulse position modulation and pulse presence modulation can both offer certain advantages in a noisy environment.

The system about to be described as a particular example uses 1 millisecond wide chirp pulses sweeping from 64kHz to 150kHz and having a repetition rate of 500 pulses per second. This is a suitable carrier for 50 baud information rates, and may be used to carry messages whose format is exactly analogous to that of the messages described in the aforementioned application No. 81 04777.

Advantageously, the transmitters and receivers of the transmission system are each controlled, as described in our co-pending United Kingdom Patent Application No.81 04776, by a respective radio receiver of the kind described in the aforementioned application No.81 04777, so that for example the nominal start positions of the chirp or other swept frequency pulses are locked to the carrier frequency of the broadcast radio signal at defined temporal positions in each bit represented thereby, these bits typically being transmitted at 50 baud and having their possible bit positions synchronised with those of the bits modulating the broadcast radio signal, and making up messages in frames also synchronised with those of the messages of the broadcast radio signal.

The invention will now be described, by way of example only, with reference to the accompanying drawings, of which: Figure lisa simplified block circuit diagram of a transmitter in accordance with the present invention, for transmitting signals over an electrical power distribution network; and Figure 2 is a simplified block circuit diagram of a receiver, also in accordance with the present invention, for receiving the signals transmitted by the transmitter of Figure 1.

The transmitter shown in Figure 1 is indicated generally at 10, and is intended, for example, for incorporation in an electricity meter (not shown) coupled to the aforementioned power distribution network to measure the amount of electrical energy supplied to a power consumer via the network. Thus in practice, there would be a plurality of the transmitters 10 coupled to the network, one for each con sumer.

The transmitter 10 comprises a radio receiver 12 of the kind described in the aforementioned application No. 81 04777, having four outputs 14 to 17 at which it produces FRAME CODE DETECTED signals, BIT SYNC pulses, a TRANSFER READING signal and ADDRESS DETECTED signals, as described in our co-pending United Kingdom Patent Application No.

81 04776. The receiver 12 also has a fifth output 18, at which it produces a 500 Hz square wave signal.

This 500 Hz signal is derived from the internal 4.608 MHz osciliator (not shown) of the receiver 12 by frequency division, and has a predetermined phase relationship with the BIT SYNC pulses.

The 500 Hz pulses are applied to a pulse modulator 20, typically a pulse presence modulator, where they are modulated by a digital signal, eg representative of the meter reading and/or other information as described in the aforementioned application No.

81 04776, contained in a message source 22, the modulation being effected underthe control of a control circuit 24 responsive to the signals at the outputs 14 to 17 of the receiver 12 in a manner analogous to that described in the aforementioned application No.81 04776. Thus a digital 1 signal may, for example, appear at the output of the pulse modulator 20 as a train of ten 500 Hz pulses, while a digital 0 may be represented by an absence of the 500 Hz pulses for ten successive periods thereof. The message and the individual bits thereof represented by the presence or absence of the 500 Hz pulses are synchronised with the frame and individual bit positions of the broadcast signals received by the radio receiver 12.

The output of the pulse modulator 20 is applied to the input of an integrator 26, which produces a substantially linear ramp voltage output in response to each 500 Hz pulse. This ramp voltage is applied to a voltageto-frequency converter 28, whose frequency may typically vary from 1.28 MHz to 3.0 MHz from the beginning to the end of the ramp. The varying frequency produced by the converter 28 is applied to the clock input of a CCD 30, whose sampling input is connected to receive a 64 kHz sinusoidal signal derived by frequency division and filtering (in filter 31) from the aforementioned 4.608 MHz oscillator in the receiver 12.The CCD 30 is arranged to produce, at the output of its output smoothing amplifier 32, a respective one millisecond chirp signal corresponding to each 500 Hz pulse, the frequency of each such chirp signal varying linearly from 64 kHzto 150 kHz.

These chirp signals are amplified in a power amp lifier 34, typically having a power output in the region of 1 watt, before being applied to the network as a modulation of the 240 volt, Hz, network supply voltage.

The receiver of Figure 2 is indicated generally at 40, and comprises a wide band input amplifier 42, having an input which is inductively coupled to the network, typically near the secondary winding of the electricity supplier's normal stepdown transformer, to receive from the network the respective chirp signals transmitted by all the transmitters 10. The amplified versions of the chirp signals produced by the amplifier 42 are applied to the sampling input of a CCD 44, which is substantially identical to the CCD 30 of Figure 1 and whose clock input is connected to the output of a voltage-to-frequency converter 46 substantially identical to the converter 28 of Figure 1.

The receiver 40 also includes a radio receiver, indicated at 50, the radio receiver 50 being substantially identical to the radio receiver 12 of Figure 1. In particular, the radio receiver 50 has outputs 51 to 55 corresponding to the outputs 14 to 18 respectively of the receiver 12, and thus produces at its output 55 a 500 Hz square wave signal substantially synchronised with the corresponding 500 Hz signal produced by the receiver 12. The output 55 of the receiver 40 is connected to the input of an integrator 58, which is arranged to produce a substantially linear ramp voltage in response to each 500 Hz pulse. This ramp voltage is applied to the input of the converter 46.

However, the ramp voltage produced by the integrator 58 is of the opposite sense to that produced by the integrator 26 of Figure 1, so it causes the frequency of the converter 46 to vary linearly from 3.0 MHz to 1.28 MHz from the beginning to the end of the ramp.

Thus each 1 millisecond chirp signal arriving at the CCD 44 in the correct time relationship with the 500 Hz pulses at the output 55 of the radio receiver 50 has all its higher frequencies reduced in the CCD, back to the original 64 kHz. The CCD 44 then produces at the output of its smoothing amplifier 60 an approximately 1 millisecond "burst" of 64 kHz in response to each received chirp signal. These 64 kHz "bursts" are filtered in a narrow band filter 62 tuned to 64 kHz, then amplified at 64 and rectified by a diode 66 to produce on a smoothing capacitor 68 a waveform approximating to that appearing at the output of the Butterworth filter 36 of Figure 2 of the aforementioned application No.81 04777.This waveform is then processed in a digital correlator70 as described in application No.81 04777, but under the control of the outputs 51 to 54 of the radio receiver 50 as described in application No.81 04776.

The use of chirp signals, each containing a wide range of frequencies, to transmit messages over the network ensures that significant portions of most if not all of the chirp signals are likely to be detectable at the receiver 50, despite the varying transmission characteristics of the network.

Many modifications can be made to the described embodiment of the invention. For example, other, more sophisticated, methods of pulse detection can be used in place of the rectifier 66 and capacitor 68, while forms of pulse modulation, eg pulse position modulation, pulse amplitude modulation, can be used instead of pulse-presence modulation. Further, although desirable, it is not strictly necessary that the transmitter 10 and the receiver 40 be controlled by the radio receivers 12 and 15.

Although the specific embodiments use chirp signals whose frequencies cover the range 64 kHz to 150 kHz, it is possible to use chirp signals covering a lower frequency range, down to a minimum fre quencyof 15 kHz. Frequencies below this limit should not be used, since they may propagate through the supplier's distribution transformer from the secondary side to the primary side. This is unde sirable, since in the transmission system of the present invention, these transformers are effectively being used to isolate the respective groups of consumers connected to each distribution transformer from each other.

Finally, although the use of chirp signals has been described, other forms of spread spectrum signal can be used, for example pseudo-random signals, where a respective multi-bit pseudo-random signal, containing up to say fifteen bits, can be used to represent each bit of the digital signal to be transmitted.

Claims (10)

1. Atransmission system for transmitting digital signals over an electrical power distribution network, the system including means for generating at least one kind of spread spectrum signal whose frequency spectrum lies substantially wholly above 15 kHz, and means responsive to at least one possible bit value of a digital signal to be transmitted over the network to modulate at least one spread spectrum signal of said one kind onto the network voltage to represent said bit value.

2. Atransmission system as claimed in claim 1, where the frequency spectrum of said spread spectrum signal lies substantially wholly below 150 kHz.

3. A transmission system as claimed in claim 1 or claim 2, wherein the upper and lower frequencies of the frequency spectrum of said spread spectrum signal have a ratio of at least 2:1.

4. Atransmission system as claimed in any of claims 1 to 3, wherein said spread spectrum signal is a pseudo-random signal.

5. A transmission system as claimed in any of claims 1 to 3, wherein said spread spectrum signal is a swept frequency signal.

6. Atransmission system as claimed in claim 5, wherein the means for generating a swept frequency signal comprises an oscillator for producing a signal, preferably a sinusoidal signal, of fixed frequency, a charge-coupled device (CCD) having its sampling input coupled to receive the fixed frequency signal, and means for varying the shift frequency of the CCD to produce the swept frequency signal at the output of the CCD.

7. Atransmission system as claimed in claim 5 or claim 6, wherein said swept frequency signal is a chirp signal.

8. Atransmission system as claimed in claim 7, wherein the chirp signal is produced by an impulsed CCD chirp filter, and is detected by a matching CCD chirp filter.

9. A transmission system as claimed in any preceding claim, wherein said spread spectrum signal is modulated onto the network voltage using a modulation technique selected from pulse position modulation, pulse presence modulation and pulse amplitude modulation.

10. Atransmission system substantially as hereinbefore described with reference to the accompanying drawings.