AN3213 datasheet(39/64 Pages) STMICROELECTRONICS

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AN3213

ST7570 demonstration board description

Doc ID 17429 Rev 1

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Noise and signal are measured independently on the RX_IN pin of the ST7570 through a

spectrum analyzer with a differential 1 M

Ω probe. When measuring the signal level 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.

7.2.3

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 SN

(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.

The signal-to-noise ratio (SNR) values for the two S-FSK tones can be calculated as:

and

. The average SNRAV 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.

SS0 and SS1 have the same unbalancing, so the resulting difference between SNR0 and

SNR1 is about 1 dB.

Table 6.

Noise level SN0 and SN1 during AWGN tests

SBW [Hz]

SN0 [dBμV rms]

SN1 [dBμV rms]

1200

41.12

39.82

2400

44.13

42.83

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