Homebrew 455khz Ceramic IF Filter

So today after reading some more of SolderSmoke – Global Adventures in Wireless Electronics” and feeling somewhat enthusiastic about advancing some projects i went out to my work bench and soldered up a test rig used to determine the motional parameters of crystals and in my case, ceramics.

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Circuit went together rather uneventfully, built in what i call my Ugly Manhattan style more slum lord millionaire than trump towers LOL. But it works and it does the job it was designed to do. And thats all that matters. Schematic can be seen in the above picture, but there is a better one to follow.

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So i plugged in the first ceramic and took a measurement with the switch open and as expected, the 455khz resonator gave a nice readout of 455.68khz. About what you would expect.

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Next we hit the switch, or in my case, pushed the bit of solder wire onto the groundplane with a pen and got the second reading. 448.12khz, now with these 2 frequencies in hand i could plug them into some software and get the values for Lm and Fo which are needed to design the filter.

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With the two values i plugged them into Ladder Crystal Filter Design and got the values for Lm = 128.6mh and Fo = 447.2085khz which i then plugged into Dishal to design the filter as i like its interface that little bit better.

dishal

Immediately i saw a huge issue, the centre freq is all wrong, its way to low, if this was going into a homebrew rig, that would not be so much of an issue, just change the frequency of the LO. But, dropping this into my ICYAWOOD is not going to work, the IF there is right on 455khz, and so that is also where i need my centre frequency. Not sure what to do at this point. I do have some resonators that are closer to 458khz with their resonate frequency, they might have a better chance, but im thinking the low Q of these ceramics is the real issue and even if i come up a few khz, that is still not going to get me closer to 455khz. Back to the drawing board for now.

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Determining Motional Parameters of Ceramic and Crystal Resonators

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.
ceramic-res

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.
xtaltestsetup

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.

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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 🙂

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100w Dummy Load

Not all builds are big and fancy. Sometimes you just have to build something to do the job. This 100w dummy load is 20 x 5w resistors in parallel, nothing fancy and should handle the 60w i wish to throw at it for a couple of amplifier projects i have sitting here.

The 30w dummy load i build previously, 10 x 3w resistors was starting to get warm around the 25w mark, so i think this new one should stay cool to 80 or 80w quite easily. I will add a 12v fan to the case if i need to cool it and give it more power at some stage.

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Resistors soldered to the board in a grid 4×5.

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Being 5% parts you expect some variation, but near enough to a 50 ohm load as you can get.

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RFPM1 RF Meter Kit by vk3aqz Kits.

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The RFPM1 RF Power Meter Kit by http://www.vk3aqzkits.com/   Let me preface this by saying, if this works 1/2 as good as the attention to detail that has gone into presenting this kit, it should  be nothing short of awesome. Opening the box is a WOW moment, as the level of detail and care that has gone into making this kit is evident. Lou VK3AQZ takes customer service to a whole new level, where most kits manufacturers give you a box and a bazillion parts, as well as a few photocopied pages, the RFPM1 RF Power Meter Kit is immaculately presented.

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Key to the whole device is the RF probe, small parts contained in pill boxes to keep the separate and so they don’t get lost in transit is a really nice touch. You will also notice solder in 2 different gauges, one for SMT work and one for thought holes. That is just unheard of from anyone.

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The case is well routed and drilled, so there are no expectations for you to cut and drill anything.

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Parts for the rest of the unit are all contained within the case, both the analog and digital meters have been checked before leaving.

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Build notes for both parts of the kit are in full colour, printed on laser, the kit contains 2 dvd’s showing the build process as well as many videos on how to use the unit for a number of different test scenarios. All parts have been ticked off the part list as they are packed and hand written notes about other bits not contained in that list have also been added.

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The level of care and detail are very evident and Lou has really gone out of his way to produce a wonderful kit, made with high quality parts and I am looking forward to spending some time putting this together and using it in all the filter experiments i have planned. I might not have the money to buy an expensive frequency analyser, but with this and the 12mhz function generator I have, i can still find 3db points and measure the level of harmonics after low pass filtering. And one thing im looking forward to, is being able to set the gain on buffers used on mixers without having to convert rms volts into dbm, as this RF meter gives to the power in dB’s. Awesome.

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