Switchable Crystal Filters

I am going to redesign this board. The layout is sub-optimal. But for now, I am going to assemble it and see if I can make 2 filters worthy of use.

The one thing that has always frustrated me with many SSB/CW homebrew rigs is that you get one filter and one filter only. I have seen some use varactor diodes to solve this problem, by varying a voltage on the diodes you can vary the capacitance and this the width of the filter. I thought about going down that route myself for a while, but in the end, settled on having 2 filters as I have lots of relays already.

The plan is for an SSB filter that is about 2200hz wide and a CW filter about 500hz wide. I am also using the good crystals for these also. 4.91520Mhz xtals sorted to within 10hz of each other.

Each crystal was loaded up into my you beaute xtal tester and the frequency measured to within an inch of its life.

My function gen also has a frequency counter that counts down to 1hz resolution. So I measured my crystals and sorted them into piles within 10hz of each other. If i cannot make acceptable filters with such tightly grouped crystals, then there is no hope in hell i can ever make a good filter ;

Out of this pile of crystals I have 6 groups of 8 crystals all within 10hz of each other and a few more groups all within 20hz of each other. Buying these crystals was certainly well worth it. The pile in the bottom right corner are the outliers. It seems even good quality crystals have some in each batch that should be rejected.

Got the parts all sorted, just gotta build it now. Will update this post once i have built and tested each filter.

Part 2: Well i kind of expected that to not take forever, but anyway here is the update.

The build is pretty straightforward and all went to plan

Other than the width being a little wide in both cases and a bit to much ripple, I will say that I am happy with the shape of both filters. I can narrow both up easy enough, just need to change the cap values and the ripple, well it might be a bit of work, I will need to play with transformer values. I used 4:1 impedance transform, but 3:1 might actually be closer to the mark. I will have to play there and see what i can come up with, but a better match will remove some of that ripple.

For now though, I can keep on building the rest of the project as these will be acceptable to get things working. I can make them perfect later. Next job is IF Amps and mixers.

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2 thoughts on “Switchable Crystal Filters

  1. Gład to find your blog, inspiring.
    Happy with the signal generator?
    Had thoughts about building my own, with AM modulation capabilities, but the pricing on those FY6800’s sure makes me think again.
    Keep up the good work.
    //SA2CLC

    1. Hi Tommy, very happy with the signal generator. For the price, its really hard to pass up.
      But there is nothing wrong with building even if it costs more. There is a lot of satisfaction to be gained from DIY, and you can make it perfect for your needs.

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