A high-performance FM receiver for audio and digital applicatons

The purpose of the slot noise generator is to evaluate the front-end performance of an FM receiver that will be operating in a metropolitan area with strong signals. Because of front-end nonlinearities, the locally strong signals may add artifacts to a frequency occupied by a weak station. A repeatable figure of merit had to be found to measure the front-end nonlinearities. Two-tone intermodulation testing does not adequately measure high-order intermodulation distortion. One measurement method is the use of over-the-air stations. However, if the antenna is moved in any way, or if atmospheric conditions change, results are no longer valid. Also, it would be difficult to find an empty space in the FM band with sufficient bandwidth to add a test signal. A second method is the combination of the output of 50 small FM trans- 30 www.rfdesign.com October 2000 mitters, all modulated with different program material. However, unless this is done with extreme care, the transmitters could intermodulate, filling the test slot with noise. In addition, this is a rather complex measurement method. A third approach is to generate noise over the entire FM band with a narrow deep slot placed in the noise at the center of the FM band. The ratio of noise in and out of the slot is defined as the noise power ratio. However, generating a slot in the noise with the required 1% bandwidth with steep skirts would be difficult. One approach might...