Ok, let me just say I am no expert on these things. But this has been my experience thus far. What got me on this bandwagon was the need for a CW filter for my IC-718. As you most likely already know this is a low spec, low cost radio and a narrow CW filter for this radio costs 50% of the value of the radio itself. In local money, that is close to $300 for the crystal filter from Icom, or you can buy Collins mechanical filters for about $230. Hardly cheap at all, and for a radio I intend to upgrade very soon, it would be a waste of money. So i decided to attempt to home brew a CW filter in the mean time. I mean what is the worst thing that can happen here, the filter is crap and i wasted 10 bucks and learned a whole lot in the mean time.
So before i started on this, i needed to know some parameters the filter uses in the radio. The IC-718 uses a 2nd IF of 455khz, with an input and output impedance of 1500 ohm. Ok, so we know the basics, now i went and scoured the net for as much information as i could find about how to go about homebrewing a filter.
Next on my list of things to do, what kind of filter am i going to be building. Ceramic ladder is very common in home brew rigs, especially in the 9mhz and 10mhz ranges where crystals are plentiful and cheap. But my radio uses an IF of 455khz, and while you can get crystals for this frequency, they cost $30 US each way too expensive for a home brew experiment, but, 455khz ceramic resonators are plentiful and cheap but are low Q and Q is everything when it comes to filter building. A typical crystal might have a Q of 10,000 to 100,000, where as a ceramic might be lucky to have a Q of 3000. So, things were looking iffy at this stage.
So i started to play with some computer simulation and filter design programs, Ladder Filter Programm “Dishal” and Ladder Crystal Filter Design (easy to find on google) and I start plugging in various bits of information to see if it is even possible to make a low Q filter from ceramics and it would seem that making something usable might actually happen, in theory at least.
With a pile of cheep Chinese ceramics in hand i began the task of finding 8 closely matched resonators. Now these things have an accuracy of + or -10% ir something nuts like that. First job was to find the resonate freq of each resonator. Out of 100 i did not even get a group of 8 that were within 100hz of each other. But you can see in the picture above that i did manage a couple of groups of 7 and a few of 4 and 5. Well, enough to make a couple of groups of 8 anyway. Why 8? well i intend to make an 8th order filter, the higher the order the steeper the sides of the filter is and the greater the attenuation.
Now to design a filter you need to know some of the motional parameters of the crystals or ceramics, namely you need Fs series frequency and Ls series capacitance. There are a number of ways to do this. I started with the above schematic as described HERE and found no matter what i did, i could not get a good enough reading using my scope and signal generator to make any usable data.
Which leads me to where i am currently at with this project. The above schematic is an oscillator and buffer amp, and by using the values of capacitance in the circuit, you can determine the upper and lower frequency of the resonator and with those numbers can determine Fs and Ls for the resonators under test. I have not built this rig just yet, but plan to over the next few days to get to it, then measure my group of 8 resonators and average the Fs and Cs values and then use the averaged value to design my filter. I will update my blog once we have gotten to that stage. Anyway, failure is always an option 🙂