. NPR is improved by increasing the baseband signal level. Normally, this a major controlling factor in system design, system performance, cannot be done without exceeding the authorised bandwidth of the and system cost. NPR is expressed in decibels and a larger number microwave transmitter, unless the entire baseband channel capacity is better. It is somewhat a function of System Gain in that it is partly of the microwave system is not going to be used. determined by the level of the received microwave signal. Improved Unfortunately, raising the baseband signal level also increases the NPR primarily allows improved S/N and lower distortion in a voice distortion level which tends to degrade NPR. Therefore, there is an multiplex channel on a microwave system. High NPR is necessary optimum baseband level which is sufficiently high to gain the for low Bit Error Rates on data channels. NPR, therefore, is a prime maximum improvement over intrinsic and thermal noise without factor in determining the desired transparency of the microwave creating an excessive amount of distortion. This situation is repre- portion of a system. sented by the 'Bucket Curve' (see Figure 50.31), and very likely the NPR stands for Noise Power Ratio. This is indicative of the test recommended level given by the equipment manufacturer is at or procedure performed in making the measurement. The basis for the near the optimum level. measurement of NPR is that microwave systems carry many inde- pendent communications channels simultaneously. All of these channels not only are affected by the noise floor of the transmitter Optimum channel level and receiver, but also create noise-like interference above this level Channel level (dB) by the creation of harmonic and intermodulation distortion due to phase and amplitude non linearities of the microwave system. I.e., harmonics of each channel are produced, and most importantly, each channel intermodulates with every other channel creating sum Increasing Increasing and difference frequencies. signal dis