A new technique for measuring mobile transmitter noise spectra

No reliable method of measuring absolute levels of noise close to the carrier of present day mobile radio transmitters has been available. The difficulty has been the large difference in levels of the carrier and noise. A new technique for measuring absolute noise power spectra near an unmodulated transmitter carrier has been developed. This technique depends on cancellation of the coherent carrier against itself without affecting nearby non-coherent transmitter noise. Equipment using this technique is shown to be adequate for measuring transmitter noise (per kc of bandwidth) more than 110db below the carrier, and for noise 10 kc or more from a 150 mc carrier. This is adequate for the best mobile transmitters we know of, but feasible methods for increasing the range by more than 30 db, and for use at higher frequency bands are suggested. It is also highly stable. This method could provide the basis for specification of maximum transmitter noise levels on an absolute power basis.