User manual

3 Design and measurement methods
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Locked Loop circuit as a reference signal. PLL circuit generates a frequency which is a multiple of
the reference frequency required to clock the ADC.
The need for the phase-locked loop results directly from the requirements of IEC 61000-4-7
standard, which describes the methodology and acceptable errors when measuring harmonics. This
standard requires that the measuring window (which is the basis for a single measurement and
evaluation of the harmonics) is equal to the duration of 10 mains cycles for 50 Hz systems and 12
cycles for 60 Hz systems. In both cases, it corresponds to approx. 200 ms. Since the frequency of
the mains may be subject to periodic changes and fluctuations, the duration of the window may not
be exactly 200 ms, and for example for frequency 51 Hz it will be approx. 196 ms.
The standard also prescribes that before applying the Fourier formula (in order to extract the
spectral components) data should not be subject to windowing. No frequency synchronization and
a situation where FFT is performed on the samples not covering integer number of cycles, may lead
to spectral leakage. This would cause blurring of the harmonic line over a few adjacent interhar-
monic bands, which may lead to loss of information about the actual level and power of the tested
line. It is allowed to apply Hann weighting window, which reduces the adverse effects of spectral
leakage, but this is limited only to situations when PLL loses synchronization.
IEC 61000-4-7 specifies also the required accuracy of the synchronization block. This is ex-
pressed as follows: the time between the rising edge of the first sampling pulse and (M+1)-th pulse
(where M is the number of samples within the measuring window) should be equal to the duration
of specified number of periods in the measuring window (10 or 12) with a maximum allowable error
of ± 0.03%. To explain it in a simpler way, consider the following example. Assuming network fre-
quency of 50 Hz, the measuring window lasts exactly 200 ms. If the first sampling pulse occurs
exactly at time t=0, then the first sampling pulse of the next measurement window should occur at
t=200 ± 0.06 ms. This ±60 µs is the permissible deviation of the sampling edge. The standard also
defines the recommended minimum frequency range at which the above-stated accuracy of the
synchronization should be maintained and defines it as ± 5% of nominal frequency, i.e. 47.5...52.5
Hz for 50 Hz and 57...63 Hz for 60 Hz .
Another issue is the input voltage range for which PLL will work properly. For this issue, 61000-
4-7 standard does not provide any specific guidance or requirements. However, 61000-4-30 stand-
ard defines the input voltage range in which the metrological parameters cannot be compromised
and for class A the range is: 10%...150% U
din
. The analyzer meets the requirements listed above
relating to the operation of PLL, for the rated voltage U
nom
>= 100 V, i.e. up to approx. 10 V.
3.6 Frequency measurement
The signal for measuring 10-second frequency values of the network, is taken from L1 voltage
channel. This is the same signal that is used to synchronize the PLL. L1 signal is sent to a 2nd order
band-pass filter, for which the passband was set at range of 40...70 Hz. This filter is designed for
reducing the level of harmonics. Then, a square signal is formed from the filtered waveform. The
signal cycles number and their duration are counted during the 10-second measuring cycle. 10-
second time intervals are determined by the real time clock (every full multiple of 10-second time).
The frequency is calculated as the ratio of the number of cycles counted and their duration.
3.7 The method for measuring harmonics
Harmonics measurement is carried out according to IEC 61000-4-7.
It defines the method for calculating individual harmonics.
The whole process consists of several steps:
 synchronous sampling (10/12 periods),
 FFT (Fast Fourier Transform),
 grouping.
FFT analysis for the test window of 10/12 period (approx. 200 ms). As a result of FFT, we
receive a set of spectral lines from 0 Hz (DC) to the 50-th harmonics (approx. 2.5 kHz for 50 Hz or
3 kHz for 60 Hz). The distance between successive lines directly results from the duration of the