BS EN 60440-2012 pdf download.Method of measurement of non-linearity in resistors.
3.3
third harmonic ratio
A3
ratio of the fundamental voltage over the e.m.f. of the third harmonic
Note 1 to entry The third harmornc rateo is expressed in dB
Note 2 to entry The third harmonic ratio has been addressed before as third harmonic attenuation This historic convention is misleading as it wrongly suggests harmonic frequencies originating from the test equipment being attenuated or liltered by the components under lest. The misleading term should therefore be avolded
4 Method of measurement
4.1 Measurement principle
A pure sinusoidal current is passed through the component under test. If the impedance of the component is not perfectly linear, the voltage across the component will be distorted and contain harmonics. One or more of these harmonics can be measured and the magnitude of these distortions is a measure of the non-linearity in the component. It is recommended to measure the third harmonic, as it is the dominant one.
The third harmonic voltage appearing across a component needs to be separated from the fundamental voltage and from any other harmonic voltage for the measurement. This is accomplished by a filter circuit letting the harmonic voltage pass through while featuring very high Impedance at the fundamental frequency. Also, the generator of the fundamental frequency needs to feature very high impedance at the third harmonic frequency so as not to act as a load to the generated distortions.
Hence, the equivalent circuit of the generator part operating at the fundamental frequency is quite simple, as shown in Figure 1.
A suitable range for the fundamental frequencyf1 for measurements on resistors is between
10 kHz and 40 kHz. This frequency range enables the test circuit to be set up without too
much difficulty.
NOTE 2 Another methoi i usiny a bridyc which i balanced at the fundamental frequency, where (he harmonica appear across the bridge diagonal. This method requires Individual balancing of the bridge for each specimen, which may be suitable for occasional use in a laboratory environment
4.2 MeasurIng circuit
Figure 4 shows a block schematic of a suitable measuring circuit.
A distortion-free impedance matching device may be used to switch R3 in order to achieve
good matching to the test specimen RT. Examples of suitable values of R3 are 10 1): 100 ft
1 kc 10 k and 100 kft these values are used for specifying the test conditions in Table 1
The suitability of the measuring circuit for measurements on resistors with resistance values covering a wide range depends on the lowest and highest available impedance R3 of the circuit. The range of values for R3 proposed above grants suitability for measurements on specimen RT with their resistance being in the range of 1 to at least 10
However, there is an overriding influence of (he correcting term i depending on the ratio of resistance under test RT over impedance R3. see Table 1 and Figure 3.
Precautions shall be taken to avoid non-linear distortion from the components near the test specimen in the low-pass and band-pass filters. The filter inductors for instance shall not contain cores of magnetic material.
4.3.7 Test fixture
The test fixture for the specimen RT shall be capable of providing safe electrical connection.
4.4 VerificatIon of the measuring system
Reference resistors with known non-linearity shall be used to verify the integrity of the measuring system.
5 Measurement procedure
5.1 Environmental conditions
Unless otherwise specified, all tests shall be carried out under standard atmospheric
conditions for measurement and tests as specified in IEC 60068-1.
5.2 Preparation of specimen
The specimen shall be kept for at least 2 h in the environmental conditions prescribed in 5.1.
5.3 Measurement conditions
The choice of system impedances R3 is determined by the properties of the actual measurement system. Table I is based on examples of suitable values for R3.
The fundamental test voltage U1 shall be chosen from Table 1, unless otherwise specified in the relevant component specification. e.g. relative to the rated dissipation.
Analysis shows that the third harmonic ratio depends significantly on the choice of the fundamental voltage as the readings of the third harmonic voltage C]3 show an exponential relationship over the ratio of applied fundamental voltages. Comparison of the non-linearity of different products should therefore always be based on identical prescriptions for dissipation and voltage limitation in order to define an identical fundamental voltage for each resistance value.
The application of the fundamental voltage results in a dissipation, and thus in a temperature rise within the specimen.BS EN 60440-2012 pdf download.