While debugging an X10 problem recently I paid some attention to signal levels. I have a Monterey Instruments powerline signal analyzer connected to a power strip that has an appliance module. Under some conditions, the module will not recognize the X10 command. In practice the problematic conditions are when I turn on either some fluorescent lights, or a Sony TV. Both reduce the signal level to about ..26-.29 volts, as displayed by the analyzer. I also checked the noise level using the analyzer, and it remains low (less than 20mv) all the time.
It seems to me that a quarter of a volt is plenty of signal for an X10 device to recognize. In practice, I see reliable operation with a PowerHouse AM466 and the Appliancelinc 2002SHL. An X10 Pro appliance module and some Honeywell modules do not recognize the signal. I also tried sending from a different transmitter (CM11A), lest the frequency be in error. Though the signal levels were even lower, in the low 100mV range, the appliancelinc and AM466 still responded.
What are reasonable signal levels for reliable X10 with the better switches and modules ?