/mcp0 dBm = 10 log (1/1) 0 dBm = 10 x 0 = 0 Because the power is an alternating current waveform and impedance can vary as a function of frequency, it is necessary to state what frequency the 0 dBm standard is based upon. The standard frequency is 1004 Hz. You must also know the resistance or impedance (load) of the circuit. The standard impedance is 600 Ohms. Therefore, the reference of 0 dBm is equal to 1 mW of power imposed upon an impedance of 600 Ohms of a frequency of 1004 Hz. Tests are usually performed with the use of test signals that are less powerful than 1 mW (0 dBm). If you apply a 1004 Hz test tone of ™3 dBm at A, you read ™6 dBm on the TIMS at B. The loss is still › dB. Transmission Level Point In any discussion of the performance of a circuit, it is necessary to describe the power at a particular point in a circuit with reference to the power present at other points in the circuit. This power can be signal power, noise, or test tones. The description of this power is similar to the description of the height of a mountain (or the depth of the ocean). In order to measure the height of a mountain, it is necessary to pick a reference height from which to measure. The standard reference height is sea level, which is arbitrarily assigned a height of zero. When you measure all mountains from sea level, comparisons of their height can be made even though they can be many miles apart. This figure shows test tone transmission from demarc A to demarc B.