AN3213 Datasheet, PDF (39/64 Pages) STMicroelectronics – ST7570 S-FSK power line networking system-on-chip

AN3213
ST7570 demonstration board description
7.2.3
Noise and signal are measured independently on the RX_IN pin of the ST7570 through a
spectrum analyzer with a differential 1 MΩ probe. When measuring the signal level S_S
(expressed in dBµV rms), the transmitting ST7570 device is set in continuous transmission
of a single sine tone, allowing the measurement of the signal level to be performed with a
resolution bandwidth (RBW) of 100 Hz. The probe was removed during communication tests
to avoid any interference.
AWGN tests
These tests are aimed at testing the demonstration board performance against AWGN and
guaranteeing compliance with IEC61334-5-1 requirements (References 5).
The value of injected noise has been kept constant during all performance tests; different
SNR values have been set by changing the signal level on the transmitting ST7570 device.
The measured noise level has been obtained with an RBW equal to 3 kHz. As the spectrum
analyzer is integrating the noise spectral density over its RBW, the actual noise level S_N
(expressed in dBµV rms) can be obtained by scaling the measured value to the modulated
signal bandwidth SBW (that is, 1200 or 2400 Hz according to the baud rate). Equation 10
expresses the relationship between measured and actual noise level.
Equation 10
Table 6 gives the actual noise level at each S-FSK tone frequency in the test setup of
Figure 33.
Table 6. Noise level S_N0 and S_N1 during AWGN tests
SBW [Hz]
S_N0 [dBμV rms]
1200
41.12
2400
44.13
S_N1 [dBμV rms]
39.82
42.83
The signal-to-noise ratio (SNR) values for the two S-FSK tones can be calculated as:
and
. The average SNR_AV is calculated in accordance with
Equation 11 as in References 3:
Equation 11
The difference found between noise levels on f₀ and f₁ frequencies is mostly due to non-flat
response of transmission and reception filters. Nevertheless, these values can be
considered as a real case for evaluating the ST7570 demonstration board performance in
communication tests.
S_S0 and S_S1 have the same unbalancing, so the resulting difference between SNR₀ and
SNR₁ is about 1 dB.
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