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Author Topic: Substitute a ceramic filter for a mechanical filter?  (Read 467 times)

W2EAF

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Substitute a ceramic filter for a mechanical filter?
« on: June 19, 2021, 03:18:57 PM »

I'm attempting to build a transistor ham band receiver as described in a 1959 article from QST. As you might expect, finding all the vintage components has been quite a challenge, but I have managed to find most after a couple years of searching and collecting. The one part I can't find is the Collins F455A-3 mechanical filter, which has a 3 kHz bandwidth. The ones I am able to find mostly have wider bandwidths and are crazy expensive on eBay. I was wondering if I can use a readily available (and cheap) ceramic filter in its place, such as the Kyocera KBF-455P-6AS. It is the same frequency and similar bandwidth. Would it be a drop in substitute or would the circuit require extensive modification? The filter would go in between the second and third IF stage.
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WA6BJH

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Re: Substitute a ceramic filter for a mechanical filter?
« Reply #1 on: June 19, 2021, 04:42:06 PM »

At a minimum you would need to match the input and output impedances for the ceramic filter and adjust for the difference in loss in the filters. They’re probably not the same as for the mechanical filter.  But doesn’t Yaesu use Collins mechanical filters?  It’s something to look at.
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W2EAF

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Re: Substitute a ceramic filter for a mechanical filter?
« Reply #2 on: June 20, 2021, 05:55:40 AM »

Did some research, and the Collins mechanical filter specified has a 5 dB insertion loss and an input/output impedance of 2k ohms. The Kyocera ceramic has an insertion loss of 6 dB and an input/output impedance of 2k ohms. Could it actually be this easy? The frequency response curve of the ceramic isn't as flat at the top, and the sides aren't as steep, but it's close enough for me. I also checked into mechanical filters for Yaesu transceivers. DX Engineering has them at $200 a pop!
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K6BSU

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Re: Substitute a ceramic filter for a mechanical filter?
« Reply #3 on: June 20, 2021, 11:31:27 AM »

Ceramic filters were very popular in receiver design for a while.  Later, crystal filters using Cohn configuration became popular because of better bandpass characteristic.  Go ahead with the ceramic filter, for simplicity.
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HAMHOCK75

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Re: Substitute a ceramic filter for a mechanical filter?
« Reply #4 on: June 20, 2021, 04:52:07 PM »

Isn't the Kyocera KBF-455P-6AS a 6 kHz wide filter?

If you can move the IF from 455 kHz to 500 kHz, there is a Collins 3 kHz filter for $25 on eBay right now.

3 kHz is pretty wide if it is for SSB. My sdr will allow setting a 3 kHz bandwidth. SSB sounds almost hi fi with that bandwidth. I like to use it when there is little QRM or QRN but I generally have to move to a narrower bandwidth when conditions are not so great.
« Last Edit: June 20, 2021, 04:57:09 PM by HAMHOCK75 »
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W2EAF

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Re: Substitute a ceramic filter for a mechanical filter?
« Reply #5 on: June 20, 2021, 07:35:55 PM »

The Kyorcera  KBF-455P-6AS is 3kHz at 6dB down and 9 kHz at 50 dB according to the spec sheet. I am flexible when it comes to bandwidth...my Swan Cygnet has a 2.7 wide filter and it has great receive audio. I will check out that Collins filter!
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HAMHOCK75

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Re: Substitute a ceramic filter for a mechanical filter?
« Reply #6 on: June 20, 2021, 08:01:50 PM »

The Kyocera is  +/-3 kHz at 6 dB down.

The Collins on eBay at $25.

https://www.ebay.com/itm/203029050083?hash=item2f457972e3:g:LMEAAOSwyyZe77~s
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W2EAF

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Re: Substitute a ceramic filter for a mechanical filter?
« Reply #7 on: June 21, 2021, 09:12:59 AM »

Just bought the Collins filter from Ebay.  The output from the first mixer is 2090kHz, and the second mixer uses a 2545 crystal to produce the 455kHz IF frequency. I can change that to a 2590 crystal to produce the 500kHz frequency. Hopefully I can tweak the IF cans up to it. Worth a shot.
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HAMHOCK75

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Re: Substitute a ceramic filter for a mechanical filter?
« Reply #8 on: June 21, 2021, 10:03:09 AM »

Hope it works out for you. I seldom see a Collins filter at that kind of price and in my opinion, SSB sounds really good with that bandwidth.
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W2EAF

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Re: Substitute a ceramic filter for a mechanical filter?
« Reply #9 on: June 21, 2021, 11:54:51 AM »

HAMHOCK75
Hope it works out for you. I seldom see a Collins filter at that kind of price and in my opinion, SSB sounds really good with that bandwidth
Thanks for finding it! Fingers crossed!
Rob W2EAF
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W2EAF

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Re: Substitute a ceramic filter for a mechanical filter?
« Reply #10 on: June 21, 2021, 11:58:18 AM »

Also going to get the ceramic filter as a plan B as K6BSU suggested
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KM1H

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Re: Substitute a ceramic filter for a mechanical filter?
« Reply #11 on: June 21, 2021, 03:45:34 PM »

The Collins filter adapter was an option in the National HRO-60 and NC-400.

I did add CB 455 kHz filters to my HRO-50 to get rid of the poor skirt selectivity on AM. There was a complete kit being sold by ???? Murata??
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W1BR

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Re: Substitute a ceramic filter for a mechanical filter?
« Reply #12 on: June 30, 2021, 02:02:21 PM »

I think you need to refer to a Collins filter reference booklet.

Early magnetostrictive filters had termination impedances in the hundreds of thousands of ohms in parallel resonance mode, or under 100 ohms in series resonance.  I suspect any filter dating back to 1959 were   magnetostrictive wire/ferrite filters with typically high impedances.

Later filters used piezo electric transducers and had much lower impedances.  You could use a capacitive divider to both resonate and match an early filter to a lower impedance--I assume that is what the QST article did?  I'd be curious to look up the circuit if you can pass along the year and month.

As an aside, I see quite a few cheap being offered on eBay, while ceramic filters with a  decent number of poles and narrow BW (3kHz or less) seem to command steep prices.  Impedance matching is typically for most filters to control BP ripple and losses.
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KM1H

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Re: Substitute a ceramic filter for a mechanical filter?
« Reply #13 on: June 30, 2021, 05:00:23 PM »

I see a fair number of 500 kHz Collins filters on Fleabay over several years as both the 51J4 and the 51J3/R-388A upgrade used it along with a few high end European sets.

Carl
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W2EAF

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Re: Substitute a ceramic filter for a mechanical filter?
« Reply #14 on: July 01, 2021, 02:41:56 PM »

To W1BR: (And everyone)
The circuit is from the Feb 1959 issue of QST..."All-Transistor Communications Receiver" By H.F. Priebe W2TGP. The IF section calls for a Collins F455A-3 filter with a 3 kHz bandwidth. The IF cans specified are 25k ohms to 600 ohms impedance and the filter is between the secondary of the second IF transformer and the base of the transistor in the next stage through a .01uf cap, so it seems like the filter would work into a fairly low impedance. (Not an electronics expert here by any means!) I would be very interested to see what you think.

To Carl:
Got a 500 kHz Collins on Ebay just the other day.

Rob
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