Filters Filters and More Filters

Filters, its a huge subject with many books and millions of words devoted to trying to better understand the subject. Filters are the only real non amplifier stage in most radio designs. the covers a number of different types, Low Pass, High Pass, Band Pass, Notch, and come in a number of configurations, Chebychev, Butterworth, Cohn, Sallen Key and can be implemented either, passively with inductors and capacitors or with crystal or ceramic resonators or actively with Op Amps or CMOS Switched Capacitance.

So filters are by no means a trivial topic of learning, but one that can keep an interested person busy for quite a long time. It is also one I intend to fully explore over the next few months to try and come to a better understanding. Of all the filter types, i have only ever really built low pass filters for harmonic suppression in PA amplifiers. However, over the next few weeks I shall start experimenting with other filter types, starting with passive filters for audio, CW and SSB audio band pass filters, as these will be used in a Direct Conversion tranceiver project i have been working on for sometime on and off.

The next major filter type i wish to tackle is the crystal ladder filter, using ceramic resonators for 455khz 2nd IF of my Icom 718 radio. An Icom filter coast close to 300 bucks, a collins filter about 200 bucks, so i thought i would give the whole idea of home brewing a filter a go. Even if this is a total failure, i will have learned a lot about filter design and the motional parameters of xtals and ceramic resonators. Parts are on order from China, and as soon as they arrive I will start on the project.


To determine the motional parameters of a xtal or ceramic resonator you use a test rig like the schematic above. You insert a sine wave around the freq of the device into the input, measure the amplitude of the freq on the output with an rf meter, varying the frequency till you find the frequency that gives the maximum signal amplitude on each of the 3 sections, no capacitance, 10 and 27pf. With those numbers you can then do some math to get the series and parallel resonances of the xtals to use in designing the filter in a program like “Dishal” by DJ6EV.

More on filters to come.


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