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Thursday, April 7, 2016

The meaning of "CM" in the Toyo CM-455 Filter

Photo by ZS1KE
A while back I picked up (from e-bay?) a 455 kc crystal filter for use in my Lew McCoy "Mate for the Mighty Midget" receiver.  I did a quick and dirty installation.  It kind of worked, but I had it in the back of my mind that I had to work on the impedance matching to ensure minimum passband ripple.  But when I learned what the P, B, E, and G pinout designations meant (plate, B+, earth and grid), I realized that this device had been designed with tube impedances in mind, so I probably didn't have to mess around with input and output networks (as I've done with the BITX rigs).  Last week I installed it as the manufacturer intended -- it sounds great.

Today I started wondering about the passband characteristics of the device.  What do the skirts look like?  So I started Googling.  There is not much out there, but I did come across a really interesting Epson site that describes the origins of this filter, and what the CM means.  CM is for "Crystal Mechanical."   Wow, this little box combines the characteristics of  a crystal filter AND a Collins Mechanical filter:

An excerpt:  
"While at the Electrical Communication Laboratory of NTTPC, Mr. Nakazawa had had a flash of inspiration: ‘We could develop a crystal unit with a high Q factor by using the wire mount technology I'm studying now. Then, if we can achieve the idea of a mechanical filter that mechanically joins multiple units using quartz material, we should be able to develop a compact filter that achieves both excellent filter characteristics and thermal characteristics.’ Without a pause, he quickly tackled the next development issue, which resulted in the creation of the ‘crystal mechanical filter (‘CM filter’)*5). This CM filter was manufactured by processing the quartz substrate into an ‘H’-shaped filter element and functioned by using the long thin sections on the left and right sides as resonators (Figure 1). The middle portion connecting the two sides fulfilled the role of the coupler. This was precisely the ‘mechanical filter achieved using crystal (quartz)’ that Mr. Nakazawa had envisioned.
This filter was released on the market as a 455kHz intermediate frequency (IF) filter for single-sideband (SSB) modulation in radio communications. The use of quartz material meant that not only were good filter characteristics achieved, but thermal characteristics were also excellent. As this was the first filter to offer properties of this caliber, it sold extremely well throughout the world. Furthermore, this technology received the honor of being granted the Notable Invention Award from the Science and Technology Agency."

Does anyone have the specs on these filters, and perhaps a passband graph?
Three cheers for Mr. Nakazawa!


  1. Unfortunately I have no specs for your particular filters. However, if you visit www.kiwa.com , you can see some typical 455 KHz crystal filters with curves.

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  3. You could try making a mechanical filter. Someone, I think an ON5, had a construction article about 1964 or 65 for a receiver, I think solid state. I think it was in CQ. And incidentally he gave instructions on making a mechanical filter. I have no ide if enough detail was provided, or how easy it would be to make one.

    If 455KHz crystals have dried up, try making a ladder filter with 455KHz ceramic resonators. Or, I suspect not enough has been done with ceramic filters.

    Of course at this point, 455KHz IFs are old history. Not only are higher frequency filters available, but you get away from having to fuss with image rejection. "Simple" receivers still may use 455KHz IFs, yet then have to fuss about tracking the RF stage with the local oscillator. "Complicating" by going to a higher IF may simplify overall, which is what design is about.


  4. Hey, that photograph looks familiar!

    I recently started listening to Soldersmoke from the beginning on through. As you talk about things I look it up on the blog, read the other articles around the same time.

    Hoping to catch up with you before 2017.


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