docu: signal amplitude collection

doc/plcSignalAmplitude.md

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+# PLC signal amplitudes
+
+To have comparable results, we measure the peak-to-peak voltage of the preamble of the homeplug packet.
+
+1. Homeplug modem with AR7420, supplied by 5V
+10Vpp at the transformer
+
+2. Homeplug modem with AR7420, supplied by 5V, with 150ohm series resistor between transformer and CP.
+Foccci v3 (including transformer with RIK10, 2x5 turns) connected via 1m twisted pair.
+3Vpp at the CP line
+
+3. Like 2, but in the homeplug modem two additional series resistors 680ohms in the transmit path, between
+the power amplifier and the transformer.
+
+- EVSE packet: 250mVpp at the CP line 
+- Foccci packet: 500mVpp at the CP line
+
+4. Like 3, but in the homeplug modem the two 680ohm in serial and a 10ohm in parallel to the transformer.
+
+- Foccci packet: 500mVpp at the CP line (no change, normal)
+- EVSE packet: hardly measurable, in the 20mVpp range.
+- Communication still works perfect.
+
+5. Compleo 20kW triple charger
+
+Sends very loud packets, even if no vehicle is connected.
+17Vpp
+
+6. ISO 15118-3
+
+On both, EVSE side and EV side of the charging cable, the typical voltage is
+1.3Vpp.
+This is specified as "power spectral density" of typical -75dBm/Hz at 1.8MHz to 30MHz, receiver band width 9kHz with 50 ohms.
+
+Example states the following levels:
+- Transceiver output: -72dBm/Hz
+- CP on EVSE: -76dBm/Hz
+- CP on PEV: -78dBm/Hz
+- Transceiver modem input: -81dBm/Hz
+This would be 9dB between transmitter output and receiver input.
+