ed as well as the the signal 10.9.8.3 Impulse noise counter and in addition the amplifier itself will introduce noise into the system, usually thermal noise and/or shot noise. Impulse noise is voltage spikes on the line which, although tolerable A term often used in connection with amplifiers is "equivalent on voice circuits, will cause errors on circuits used for data trans- noise input" which is defined as the amount of noise that would be mission. required to be present at the input of a completely noiseless ampli- To measure these noise spikes requires a weighting network, a fier in order to produce the same noise level at the output as that of rectifier, a threshold detector and a counter to record the events a practical amplifier. above the threshold. A means of measuring time is also provided SO From this definition it follows that the output noise of an amplifier that a count may be recorded for a fixed time interval. is equal to the input noise plus the gain of the amplifier in decibels. A typical impulse counter for the voice frequency band has a (See Figure 10.16 and Equation 10.39.) threshold adjustable in 1dB steps from 40dBrn to 99 dBrn, with a choice of terminating (600 ohms) or bridging (high) input imped- out = (10.39) ance. The timer can be set in 1 minute intervals up to 15 minutes, a 5 minute interval being standard for message circuits. On some instruments there are several counters in order to make counts at 10.10.3 Effect of tandem connections different threshold levels simultaneously, which allows the meas- urement of the distribution of impulse magnitudes. The doubling of noise power, by adding two identical amplifiers in tandem with equal input levels, increases the noise power by 3dB. If the input level remains the same and the noise power level is 10.10 Signal to noise ratio increased by 3dB, then the signal to noise ratio is reduced by 3dB. Thus every doubling of the number of amplifiers will show a 3dB 10.10.1 Definitions reduction in the signal to noise ratio. Figure 10.17 shows the