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The TV & FM Filter used in this receiver is a 30 MHz low pass design based on the article in the September/October 2000 issue of QEX called "Narrow Band-Pass Filters for HF", by William Sabin. These filters are implemented using fewer parts than other double-tuned NBPFs and they work very well.  The broadcast filter is from most of the recent Amateur Radio Handbooks ("A BC-Band Energy-Rejection Filter", Filters and Projects, The ARRL Handbook, 1999, pp 16.36). 4.5 MHz High Pass FilterAn upgrade to just building a broadcast filter is to build a 4.5MHz high pass filter. A 4.5 MHz filter keeps interference out of the IF frequencies, as well as the broadcast bands. The PCB boards currently shipping have the values for this 4.5 MHz filter. In the Handbook article, it states that the filter can be scaled for other frequencies. Multiplying the frequencies by a factor of 3, yields a 4.5 MHz filter. This means dividing all the values of the filter by 3. The following table gives the values. In actual practice, by getting the values close, but not perfect, the filter started rolling off at 6.5MHz, and was >60dB at 4.5MHz, which works very well for the receiver. 4.5 MHz High Pass Filter Values Either T50-2 or T50-6 cores can be used to build the filter.
Below is the layout of the filter showing where all the values for the 4.5 MHz Filter are placed. Notice that a wire is run from the normal Antenna Input to the output of the filter and the antenna connection has been changed to the Antenna Input at the top of the Broadcast Filter. |
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