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Author Topic: Homebrew Crystal Filters  (Read 10259 times)
VK2TIL
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Posts: 317




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« Reply #15 on: March 28, 2013, 09:36:11 PM »

I accept that ",,, quite difficult ..." is too strong; I regretted using this term when I re-read my post.

A warning about the difficulty of making simple ladder filters with a high bandwidth/centre-frequency ratio would be a better way of putting the point.

The sentence that followed my quoted comment reads;

"The difference (between series-resonance and parallel resonance) increases with crystal frequency which is one reason for using 9 or 10MHz crystals instead of lower-frequency ones".

The principle remains; the parallel-resonance causes a lop-sided response; this asymmetry increases as the filter bandwidth increases.

W7ZOI described the effect in QEX (Jun 1995) and, abbreviated, in EMRFD (3.20 & 21); W7AAZ (unpublished paper) advises us to "... be aware that filters that are a large percentage of the crystal frequency may not be possible in simple ladder filters ...".

In pers.com to me W7AAZ has also commented that "... In technical terms, you cannot make a filter wider than the difference between series resonance of the crystal and parallel resonance with the "holder" capacitance. In fact you cannot approach that width very closely. Thus a 4.434 MHz xtal with a spacing of 5 KHz makes it very difficult. SSB width ladder filters are easy to make at 9 MHz where the two resonances are separated by 15 KHz".



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WB6BYU
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Posts: 13125




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« Reply #16 on: March 28, 2013, 09:40:36 PM »

Quote from: WA0VPR

...I am currently using four 9 MHz crystals in the filters...



4 crystals makes a good casual filter, but not one with sharp skirts.  Most commercial
rigs (at least before DSP) use at least 8 poles to get the desired selectivity.  My old
Argonaut 505 had a 4-pole crystal filter and got great audio reports, but it was about
2.7 kHz wide and suffered in a crowded band.  Since you have separate filters for TX
and RX you can optimize them for different characteristics if you want, or build several
filters to switch among.  My Argosy came stock with a similar 4-pole filter and I quickly
decided that the optional 8-pole filter was a good investment.


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KM3K
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Posts: 285




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« Reply #17 on: March 29, 2013, 08:04:59 PM »

In pers.com to me W7AAZ has also commented that "... In technical terms, you cannot make a filter wider than the difference between series resonance of the crystal and parallel resonance with the "holder" capacitance. In fact you cannot approach that width very closely. Thus a 4.434 MHz xtal with a spacing of 5 KHz makes it very difficult. SSB width ladder filters are easy to make at 9 MHz where the two resonances are separated by 15 KHz".
The first sentence of W7AAZ's personal-correspondence, if correctly quoted, is not true.
Perhaps, W7AAZ means to limit discussion to "ladder-filters", in which case, the ending part of that sentence about "holder" capacitance can give wrong impressions to a reader.
73 Jerry KM3K
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VK2TIL
Member

Posts: 317




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« Reply #18 on: March 29, 2013, 08:58:04 PM »

The greater proportion of filters described in amateur literature and built by amateurs are of the ladder variety; I have assumed that it is that kind of filter that we are discussing.

I previously referred to " ... the difficulty of making simple ladder filters with a high bandwidth/centre-frequency ratio ...".

Zverev (chapter 8, p.422 in my edition) covers the matter mathematically and observes that "... the obtainable bandwidth of the filter, as is easy to recognise, is dependent on (the ratio of Cp to Cs)..."; since Lm is constant, Cp determines the parallel resonance frequency and Cs determines the series resonance frequency, so that Cp/Cs is equivalent to Fp/Fs.

To clarify; when designing simple ladder filters for SSB-type bandwidths, choose a high IF, perhaps 9 or 10 MHz or even higher.
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AJ4SN
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Posts: 21




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« Reply #19 on: March 31, 2013, 07:45:26 AM »

I'm grateful for the great discussion on homebrew crystal filter issues. I have concluded (per WB6BYU and KB1GMX) that the performance I'm seeking will only be achieved if I add more poles to the filter. I am also going to look at the AD8307 meter (per VK2TIL) in EMRFD to improve my ability to evaluate homebrew filters. Currently, I am using a VXO, frequency counter,  and the scope for a test setup. Thanks for all your helpful suggestions.

Stan
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KB8E
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Posts: 24




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« Reply #20 on: April 01, 2013, 06:24:58 AM »

Stan,

You should have your filter today. Enjoy, it's a good one.

Sam
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G3RZP
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Posts: 4445




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« Reply #21 on: April 02, 2013, 01:00:43 AM »

The XF9B is a very good filter. To get the best out of it, you want a shield so the input and output pins can't 'see' each other.

The 3rd order input intercept point for signals at 5 and 10 kHz removed is about +16dBm.
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AJ4SN
Member

Posts: 21




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« Reply #22 on: April 03, 2013, 06:17:17 PM »

Thanks, Sam, for the filter! I won't get it installed until this weekend, but I'm really looking forward to it. I'll let you know hwo it works out. Very good of you, my friend.

Stan
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VK2TIL
Member

Posts: 317




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« Reply #23 on: April 03, 2013, 07:20:43 PM »

A nice filter; you can find out how it works here;

www.axtal.com/data/publ/mcf.pdf

(The page numbers are muddled-up which is confusing).

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