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Wednesday, February 19, 2020

Hi-Per-Mite Active Audio CW Filter Installed in uBITX (video)



Coming home from the Dominican Republic, this was a major item on my list of desired mods for the uBITX: a narrower passband for CW.  Using the 2.3 kHz SSB passband, you could have many CW QSOs audible in your headphones.  Narrow that passband to 200-300 Hz and life gets a lot easier. 

 Farhan had discussed adding a 12 Mhz IF filter of suitable CW bandwidth, but this seemed difficult to me. Besides, I have long wanted to use an active audio filter for this purpose. 

 The Hi-Per-Mite filter from 4 State QRP Group is just what I needed. I built the kit without problems in about an hour, then I put it in the uBITX box, with  switch on the front panel that lets me put it in the circuit or take it out -- this is very satisfying and a lot of fun.  

 The Hi-Per-Mite just goes between the uBITX audio out and the speaker/headphone connection. I built mine for "no gain" -- the uBITX provides plenty of audio. 

 The selectivity with the Hi-Per-Mite compares favorably to that of my Drake 2-B with the 500 Hz LC filter.  You can get an idea of the performance in the video. 

 Three cheers for the 4 State QRP Group and their Hi-Per-Mite!  Here is the page with all the info: 

 http://www.4sqrp.com/hipermite.php
at 3:15 PM
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3 comments:

  1. Lex PH2LBFebruary 20, 2020 at 8:30 AM

    It is indeed a very good addition for the uBitx.
    I have also implemented it in my uBitx.

    https://www.ph2lb.nl/blog/index.php?page=ubitx-mods3#ubitx-mod14

    IMHO a must have.

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  2. MichaelFebruary 21, 2020 at 1:32 PM

    They say it's easier to make a narrow ladder filter than a wide one. Many a receiver in the old days got by with a single crystal filter, which then got loaded down with resistance to broaden for voice. Of course at 455KHz the IF transformers would help more with skirt selectivity than at 12MHz.

    But then you have the SSB filter to provide skirt selectivity, so maybe a simple single crystal filter in series would be fine for CW.

    The hardest part might be adding switching of some sort to remove the CW filter when doing ssb.

    The neat thing about using a synthesizer controlled by a computer is that you can shift the frequencies as needed just by changing software. So if the filter isn't on the same frequency, just shift the local oscillator, and the BFO. So much easier than in analog days. If the IF strip is otherwise broadband, you can use that commercial crystal filter on 9MHz for SSB, and that 2MHz wider filter for AM, and some third frequency for a CW filter. No need to add mixers to make an odd filter compatible with an existing IF frequency. Of course, this may affect other things, like image rejection or where the spurs land.

    Digging into the material for this audio filter, it derives from an article in "73" for May 1994. I have no idea how much was changed, but someone could just go back a quarter century and build from scratch.

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  3. MichaelFebruary 21, 2020 at 2:19 PM

    I forgot, whatever happened to commutating filters?

    The first issue if QST I ever saw, April 1971, had an article about tgem, though called a "digital filter".

    Some capacitors of the same value, and something to switch them sequentially, and a divider to drive them. The center frequency set by the frequency of the oscillator, so easy to change.

    The Tayloe mixer derives from the concept.

    There were articoes decades ago, including one in Ham Radio that was well detailed, but not much in recent times. There are limitations, but you can't get much simpler. I've onky seen them used for audio, I don't know if they can translate up to low IF frequencies

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