audio bandpass filter


Posted on Feb 4, 2014

The audio bandpass filter described is useful for amplification and filtering weak AM TV video carriers. For example, a DFM (digital frequency audio multimeter) may have insufficient input sensitivity for measuring extremely weak SSB TV video audio signals. By using the 20 Hz filter to peak the wanted carrier, the DFM will display the carrier freq


audio bandpass filter
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uency. Another possible application for this filter is increased amplification and reduced bandwidth of weak BCB heterodyne AM carriers. The filter is also very useful for separating video carriers that are in close proximity of each other. By definition, a bandpass filter is usually a low-pass and high-pass filter in series, allowing only a certain range of frequencies through. Because the cut-off frequencies are close to one another, the effect will be similar to that of a peaking filter. The bandwidth of the filter, when peaked is approximately 20 Hz. This is much narrower than the typical 2. 4 KHz SSB bandwidth of most communications receivers. The advantage of this filter is a constant 20 Hz bandwidth, regardless of the resonant frequency, when peaked between 400-4000 Hz. The audio line-out or headphone output from a VHF/UHF scanning or communications receiver is connected to the input of the bandpass filter. The output of the bandpass filter is connected to a digital frequency meter (DFM), and/or audio monitor speaker. A monitor speaker is used when tweaking the filter`s resonant frequency. If a DFM is not used, a PC program, such as Spectrum Lab could be used for spectral display of TV video carriers on a computer screen. The tuning range of the filter is from ~ 400 Hz to 4 KHz, when using a 50 KHz potentiometer. The writer typically tunes the filter to resonate around ~ 1000-1300 Hz. This frequency range corresponds...




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