AN1714 Datasheet, PDF (15/46 Pages) STMicroelectronics – transceiver demonstration kit description

AN1714
Demonstration board for ST7538Q
2.2.1
Transmitting section
The function of the transmitting coupling circuits is to inject the transmitted signal coming
from the power amplifiers (ATOP1/ATOP2) to the mains with the maximum efficiencies and
filter noise and spurious signals over the Cenelec mask (EN50065-1, section 7:
disturbances limits).
The critical frequencies of the conducted disturbances emitted are the 2^nd and 3^rd
harmonics of the transmitted signal (265 kHz and 497.5 kHz for the channel at 132.5 kHz)
the harmonics of the working frequency of the power supply regulator and two spurious
tones centered at 1.3 MHz (+/- the channel frequency) produced by the direct synthesis
technique used for the transmitted signal generation.
The configuration used for the transmitted circuit uses a 4th order band pass filter (four
poles and two zeros). In order to have good immunity to the components spread (accuracy
and temperature) and to the load variation, the filter has a band of about 60 kHz (see
Figure 14). To obtain this characteristic two poles can be put at a frequency of about 100
kHz and the other at a frequency of about 160 kHz.
Figure 12. Demonstration board ST7538Q transmission coupling circuit
ST7538Q
RAI
C33
32
100K Ω
C36
2.5V
ATOP1
19
C13
ATOP2
21
LC12
R11
Artificial Network
L7
Transmission
CISPR 16-1
Coupling Section
L4 C11
50 µ H
1:1
R10 D16
D17
CR9 _D15
T1
50 Ω
5Ω
R8
50 Ω
5Ω
50 µ H
For a correct dimension of the filters the mutual influence of the various components has to
be considered, as well as the influences due to the other elements: the leakage inductance
of the transformer (from 0.1 µH to 10 µH), the capacitance of the transil diode (about 2 nF),
the ESR of the series components C₁₃, LC₁₂, T₁, L₄, C₁₁ (from 100 mΩ to 1 Ω).
For a first approximate rate of the components’ values, only the reactive components are
used in the simplified circuit of Figure 13 and the transformer (1:1 ratio) is considered ideal.
For the correct dimensioning of the filter it is better to consider the typical impedances
expected for the mains network (usually an inductive load). If an impedance characterization
of the network is not available it is possible to use a reference load like the artificial network
CISPR16-1 (50 ohms parallel 5 Ω plus 50 µH). In the simplified circuit only the reactive part
of the CISPR 16 artificial network (2 x Lc = 100 µH) has been considered
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