It sends a constant P1 ON/OFF X-10 signal at a precise output voltage and measures the strength (voltage) of this signal. Plug the signal strength indicator into a wall outlet and send an X-10 command. The indicator will tell you the quality of the X-10 signal between the 2 locations. Has 5 signal strength levels showing the strength of the X-10 signal. High-frequency noise affecting the X-10 signal can be sensed with this device and can be neutralized by installing Filters. The proper way to trouble shoot for noise is to use an XPTT Test Transmitter and an XPTR Signal Strength Indicator. The XPTT is plugged in at the transmission point where it will transmit a constant line carrier "P1" on/off command. Taking the XPTR you go from outlet to outlet in the home and take a signal strength reading. The XPTT transmits a 2V signal and the XPTR can detect from 2V down to 25MV. The lowest possible signal that can make a module respond properly is 100MV. As you go around the building and you see a fluctuation in the signal you may have detected a noise-producing device. While leaving the XPTR plugged in you can systematically unplug a device (TV, VCR, computer or low voltage lighting power supply) and see if you detect any change in the signal amplitude. You may have to shut off a fuse if the device is hard wired without a plug. If no change is seen go on to the next device. When you see a signal increase upon unplugging a device, then you have detected an offending device. Now all that you have to do is purchase an FM10 plug in filter. The FM10 is plugged into the wall where the offending device was plugged in and then plug the device into the FM10. The X10 signal will now pass freely through the electrical system without the noise, blocked by the FM10, passing onto the power line.