GB2121191A

GB2121191A - Phase sensitive detector and frequency component measuring apparatus

Info

Publication number: GB2121191A
Application number: GB08221186A
Authority: GB
Inventors: Stephen John Kearley
Original assignee: Electricity Council
Current assignee: Electricity Council
Priority date: 1982-05-07
Filing date: 1982-07-22
Publication date: 1983-12-14
Also published as: GB2121191B

Abstract

A phase sensitive detector has a multiplier circuit (10-12) providing an output signal representing the product of an input signal and a predetermined switching waveform having a selected fundamental frequency. The detector includes switching means (13) arranged to form the switching waveform with 2n discrete levels (e.g. light levels), where n is greater than or equal to 2. The discrete levels are selected to minimise the amplitudes of a maximum number of lowest order harmonics in the switching waveform. A low pass filter (14) filters periodic components from said output signal. Apparatus (Fig. 3) for measuring the amplitude of a component at a predetermined frequency on an input signal comprises a pair of the above detectors, each being connected to receive said input signal as the input. The switching waveforms for said detectors have the same fundamental frequency equal to said predetermined frequency, but are in phase quadrature to one another, and a vector adder combines the output signals of the two detectors to produce a signal representing the amplitude of said component. <IMAGE>

Description

SPECIFICATION Phase sensitive detector and frequency component measuring apparatus The present invention relates to phase sensitive detectors and also to apparatus for measuring the amplitude of a selected frequency component of a signal.

It is often necessary to measure the amplitude of a signal at a known frequency in the presence of signals at other frequencies and random noise.

A common instrument for making such measurements is a phase sensitive detector (PSD). Two different types of PSD are known. In the first type an input signal is electronically multiplied by a sine wave at frequency fs, the frequency of the frequency component to be measured. It can be shown that the output V out of such a device will be of the form: V out a V cos 0 where V is the amplitude of the measured frequency component and 4 is the phase difference between the measured frequency component and the multiplying waveform.

In the second known type of PSD, the polarity of the input signal is alternately switched positive and negative at frequency f,of the measured frequency component. The output voltage will again be of the form: V out a V cos 0 as unwanted frequency components and noise will be alternatively switched positive and negative and will cancel on time averaging. However, if the input signal contains frequency components at odd harmonics of the frequency f, there will be a non-zero contribution to V out from these components.

From the foregoing, it would seem that the PSD employing a sine wave and linear multiplier is ideal. However, in practical applications, where cost is an important consideration, the disadvantages of devices employing linear multipliers become apparent, as they are expensive. The present invention seeks to provide an improved phase sensitive detector which is less affected by odd harmonics of the measured frequency component than the second described PSD and yet employs cheaper circuit components than the sine wave PSD.

According to the present invention there is provided a phase sensitive detector having a multiplier circuit arranged to provide an output signal representing the product of an input signal and a predetermined switching waveform having a selected fundamental frequency and including switching means arranged to form said switching waveform with 2 n discrete levels (where n > 2) selected to minimise the amplitudes of a maximum number of lowest order harmonics in the switching waveform, and a low pass filter to filter periodic components from said output signal.

Preferably the switching means is arranged to form said switching waveform with eight discrete levels, and also with level changes synchronised with a switching rate equal to 2 nf where n is the number of levels in the waveform and f is the fundamental frequency of the waveform.

Advantageousiy, for eight discrete levels, there are four pairs of levels which are equal but of opposite sign, and three of the levels of a respective sign are related to the fourth level by the fraction k, k2 and k3, where Sin 5x Sin 7x Sin 3x k Sin 7x and Sin x k3= Sin 7x where x=7r/1 6.

An example of a phase sensitive detector according to the present invention will now be described with reference to the accompanying drawings, in which: Figure 1 shows a phase sensitive detector; Figure 2 shows the switching waveform; and Figure 3 shows an embodiment of the phase sensitive detector in use in a frequency component measuring apparatus.

In Figure 1, a phase sensitive detector employing a multiplier circuit to multiply an input signal by a predetermined switching waveform is shown. The multiplier circuit comprises a unity gain, inverting amplifier 10, and potential divider chains 11 and 12 arranged to give sets of positive and negative fractions of the input signal respectively. The outputs of the potential divider chains 11 and 12 are connected to a muiti- position switch 1 3 which is switched to provide an output signal representing the product of the input signal and the switching waveform. The output signal consists of a series of positive and negative fractions of the input signal selected by the switch 1 3 according to a determined sequence by the desired switching waveform.The output signal from the switch 13 is passed through a low pass filter 14 whose function is to filter periodic components from the output signal.

The first potential divider 11 consists of four resistors 11 A, 11 B, 1 1 C and 1 1 D connected in series between the input and ground, and, similarly, the second potential divider 12 consists of another four resistors 12A, 12B, 12Cand 12D connected between the output of the inverting amplifier 10 and ground. The resistance values and the method of their selection will be described later. There are four outputs from each potential divider, at the three intermediate positions between series resistors and at the input or output respectively of the inverting amplifier.

The multi-position switch 1 3 is shown having eight different positions, A-H. In operation switch 1 3 is operated in the following sequence: ABCDDCBAHGFEEFGHA. . .

It can be seen that this sequence of switching provides on the output line from the switch 13 a signal which in effect comprises the product of the input signal to the circuit, i.e. from the low pass filter 1 5, and a switching waveform of the form illustrated in Figure 2. Figure 2 shows two waveforms shifted in phase by or/2. Selection of the values of the resistors 1 A to 11 D and 1 2A to 1 2D enables fractions kt, k2 and k3 illustrated in Figure 2 to be selected. The rate of switching the switch 1 3 is controlled to be equal to sixteen times the frequency of which the component in the input signal is to be measured.Selecting fraction k1, k2 and k3 as illustrated generally in Figure 2 produces a switching waveform approximately a sine wave at the frequency to be measured. By appropriately selecting the values k1, k2 and k3, it can be shown that the switching waveform illustrated in Figure 2 contains no harmonics of the fundamental sine wave below the fifteenth harmonic. It can be shown that the Fourier analysis of the switching waveform has no harmonic below the fifteenth if: Sin 5x kt= Sin 7x Sin 3x k2= Sin 7x and Sin x k3= Sin 7x Where x=ti16.

Furthermore, the 1 5th harmonic is at less than 7% of the fundamental.

It is possible to select the values of k1-k3 differently to minimize different harmonics in the switching waveform. It is also possible to use more switching levels to produce a closer approximation to a sine wave, which will have more harmonics minimized.

It can be seen, therefore, that employing the above-described apparatus enables the aforementioned problem with switching type phase sensitive detectors to be overcome without the requirement for expensive linear multipliers. If the input signal to the phase sensitive detector is first fiitered as in filter 1 5 to remove frequencies above the fourteenth, harmonic the output of the detector is an accurate representation of the amplitude and relative phase of the desired frequency component.

Figure 3 illustrates a complete apparatus for measuiring the amplitude of a desired frequency component of an input signal and employing two such phase sensitive detectors 21 and 22 supplied with control signals to operate their switches to form switching waveforms in phase quadrature such as shown in Figure 2. If the filtered outputs of the phase sensitive detectors are combined in a vector adder 27, the output signal is representative of the amplitude of the desired frequency.

An harmonic analyser employing phase sensitive detectors of the present invention will now be described by way of further explanation.

Harmonic analysers are particularly useful when measuring the effect of a non-linear load on the mains distribution system. Non-linear ioads lead to voltage distortion at the point of common coupling, and this distortion, if supplied to other consumers may cause overloading of power factor correction capacitors, overheating of transformer windings, maloperation of thyristor circuits or interference with communication or ripple control equipment.

Due to the listed problems, electricity supply authorities restrict the amount of voltage and current distortion a consumer is permitted to cause at the predominant harmonic frequencies.

It is therefore useful for the supply authorities to have an harmonic analyser to establish what effect a consumer is having on the mains.

A typical analyser has a frequency component measuring apparatus of the type illustrated in Figure 3, a programmable frequency loop which can provide the necessary reference frequency at 16 times the harmonic to be measured, a microprocessor for various control and data manipulation functions, and recording means and a display or recorder. When analysing the harmonics of the mains, the reference signals are derived from the mains.

In the particular example, the vector addition required to give the phase independent amplitude of the harmonic being measured is performed by the microprocessor. For this to happen, the voltage outputs from the low pass filters 23 and 24 are converted into digital form using a slow, high-accuracy, ramp type analogue to digital converters controlled by the microprocessor to measure the output of each channel in turn.

Each calculated harmonic could be sent as a multiplexed BCD output to a LED display, as a serial output in ASC II code to drive printers, digital cassette recorders and other RS 232 compatible data monitoring equipment and as a 6 kHz serial phase encoded data stream to a low cost cassette recorder.

After completion of the analysis of one harmonic the microprocessor programs the frequency locked loop circuit to select the frequency of the next haromonics to be measured, and then the analysis is repeated for the next harmonic.

Claims

1. A phase sensitive detector having a multiplier circuit arranged to provide an output signal representing the product of an input signal and a predetermined switching waveform having a selected fundamental frequency and including switching means arranged to form said switching waveform with 2n discrete levels (where n > 2) selected to minimise the amplitudes of a maximum number of lowest order harmonics in the switching waveform, and a low pass filter to filter periodic components from said output signal.

2. A phase sensitive detector as claimed in claim 1 wherein the switching means is arranged to form said switching waveform with eight discrete levels.

3. A phase sensitive detector as claimed in claim 1 or claim 2 wherein said switching means is arranged to form said switching waveform with level changes synchronized with a switching rate equal to 2nf where n is the number of levels in the waveform and f is the fundamental frequency of the waveform.

4. A phase sensitive detector as claimed in claim 3 wherein, for eight discrete levels, there are four pairs of levels which are equal but of opposite sign, and three of the levels of a respective sign are related to the fourth level by the fractions k, k2 and k3, where Sin 5x Sin 7x Sin 3x k2= Sin 7x and Sin x k~ ; Sin 7x where x=7r/1 6.

5. A phase sensitive detector as claimed in any preceding claim, wherein the multiplier circuit comprises a unity gain inverting amplifier connected to provide a signal which is equal and of opposite sign to said input signal, a first potential divider chain connected between the input of the amplifier and ground and selected to provide positive fractions of said input signal corresponding to the positive levels of said switching waveform, a second potential divider chain connected between the output of the amplifier and ground and selected to provide negative fractions of said input signal corresponding to the negative levels of said switch waveform, a multi-position switch for feeding to an output terminal a selected one of said positive and negative fractions and switch control means arranged to cycle said switch repeatedly through its positions in accordance with the switching waveform so as to produce said output signal at said output terminal.

6. A phase sensitive detector as claimed in any preceding claim and including a low pass filter connected to filter from said input signal frequencies greater than said maximum number of lowest order harmonics.

7. Frequency component measuring apparatus for measuring the amplitude of a component at a predetermined frequency in an input signal; comprising a pair of phase sensitive detectors as claimed in any preceding claim, each detector being connected to receive said input signal as the input signal to the detector, means for generating switching waveforms for.said detectors having the same fundamental frequency equal to said predetermined frequency but in phase quadrature to one another, and a vector adder arranged to combine the output signals of the two detectors to produce a signal representing the amplitude of the said component.