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In order to measure the broad band noise level near 127 kHz it is necessary to utilize a circuit that will separate a ~127 kHz noise amplitude that is typically in the range 10 mV to 100 mV RMS from the 60 Hz AC power signal that is in the range 120 V to 347 V RMS. It is also necessary to approximately simulate the PLC receiver bandpass. The schematic of a circuit for performing this function is illustrated below.

In this circuit 1500 pF 1000V capacitor Ca resonates with inductor L at about 130 kHz. The resonance condition is given by:
W^2 = 1 / (LC).

Inductor L is chosen as 1000 uH with a Q>60 @ 790 kHz. This is a standard value inductor with a maximum current rating of 70 mA that yields a practical impedance of about WL=797.56 ohms at 127 kHz. The equivalent internal series resistance Rs of inductor L meets the inequality Rs = WL/Q < 13.3 ohms.

The equivalent internal series resistance of capacitor Ca at 127 kHz is not specified by the manufacturer.

Resistor R adjusts the Q of the resonance and hence the circuit bandwidth. Capacitors Cb and Cc are chosen to drop most of the voltage at 60 Hz and to give a good relative peak in transfer ratio at 127 kHz.

Resistor R was chosen to be 12K ohms to achieve an approximate match between the band pass of the Systel PLC receiver and the band pass off the noise measurement circuit. Resistor R and 100 pF 200 V capacitor Cd together form a low pass filter that assists with rejection of noise frequencies above:
F = 1/(2 Pi R Cd) = 132.6 kHz.

Capacitance Ce = Cb/2 = Cc/2 is chosen as Ca/15 to provide a good relative peak in the circuit transfer ratio at 127 kHz while minimizing 60 Hz feed through. For safety purposes the voltage ratings of Cb and Cc are important. These values are chosen to be 1000 volts to ensure that even if one unit should fail the other will safely limit the 60 Hz current.

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This web page last updated September 20, 2005

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