internally, and may have an MTBF³ of 100,000 hours. A typical microwave installation might not require the power amplifier at all, but may require transmitter power output of no more than 10 to 100 milliwatts. If the MTBF of the transmitter, including the power amplifier, is known, the MTBF Figure 50.29 Receiver noise figure trends of the transmitter without the power amplifier can be estimated using Equation 50.21 as in Equation 50.22 for MTBF XMTR W/PA equal to 26000 hours or approximately three years. output and receiver sensitivity, only 5 to 6dB increase in System Gain is anticipated. This improvement in System Gain may be used (50.21) for worthwhile improvements in system characteristics by any of XMTR W/O P.A. the means discussed earlier. It should be noted, however, that barring unforeseen technological XMTR WIPA breakthroughs, it is not and probably will not be wise to design systems for significantly higher received levels than specified for (50.22) the equipment. This is because, not only do the S/N improvements WOPA. level out at the intrinsic limitations of the transmitter and receiver, 26000 100000 but also NPR (of which S/N and distortion are components) may actually degrade due to receiver overload or saturation. Generally Thus, a low power transmitter, capable of only 10 milliwatts or SO speaking, older receivers are worse in this regard, but receiver should be preferred over a high power transmitter (1 watt or more) overload characteristics are seldom specified even today. System specification and equipment design 50/17 50.4.7 Noise Power Ratio (NPR) idle noise, receiver idle noise, receiver thermal noise, transmitter distortion, and receiver distortion. It is measured from baseband Some references to NPR were made previously; this section will input jack to baseband output jack of a transmitter-receiver pair. define the term and examine the subject in some detail. NPR is one Signal-to-noise ratio limited by idle and thermal noise could be of the most important microwave equipment specifications