A high-performance FM receiver for audio and digital applicatons


Posted on Jan 19, 2013

A block diagram and a schematic of the receiver are shown in Figures 2 and 3, respectively. In Figure 3, L6 and L7 are spaced 0.6 inch for a loss of about 0.5 dB. Q4 is the first RF amplifier. L9 and L10 are spaced about 0.8 inch with a partial shield between them. The front-end gain is about 10 dB as measured at the input to the mixer. This gain is sufficient to overcome the loss of the mixer, but not to overload it. C23 and L11 provide a match between the output of Q5 (the second RF amplifier) and the mixer. The local oscillator Q3 is followed by a two-stage amplifier, Q1 and Q2. The components between the mixer output and Q8 provide termination for the mixer. CF1, CF2, and CF3 provide about 200 kHz receiver bandwidth. U5 is an IF amplifier, limiter, and detector. U4 and U5 provide phase lock for the local oscillator.


A high-performance FM receiver for audio and digital applicatons
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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 However, if the antenna is moved in any way, or if atmospheric conditions change, results are no longer valid.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...





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