RE: Bandwidth of crystal ladder filters CW and SSB

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From: Nick Kennedy (nkennedy@tcainternet.com)
Date: Sat Sep 28 2002 - 15:37:37 CDT


Hello Prof. Arnaldo-

Some good questions. I'm not able address most of them, but I hope to be
able to as I learn more about crystal filter design.

I think close matching in frequency is important if that's your only method
of imposing control over parts quality. Then you use a design technique
that assumes identical crystals. But Glen Leinweber (VE3DNL) and I had a
mini-thread on the subject the other day about how you might use crystals
that aren't exactly matched.

All meshes of the ladder should be at the same frequency. The shunt
capacitors at the ends of each crystal create a series resonant loop with
the crystal that is higher in frequency than the crystal's natural
frequency. The normal technique for correction is to place additional
capacitors in series with the crystals with the lowest mesh frequencies,
bringing them up to match the highest.

But another method might be to exploit the fact that your pile of crystals
will have some frequency differences among them. Put those with the lowest
frequencies in the higher frequency meshes and vice versa--maybe even
avoiding the need for the extra capacitors. Of course, this method doesn't
lend itself well to mass production, or maybe not even to "kitting", but
it's great for a ham custom building a filter or two. I don't know how
often this method would work out in practice, but it's interesting to
consider.

I'm not really up on all the ways that SSB BW filters are different from CW
BW ones. But I do think I've heard that crystals should be matched to
within 10% of so of the BW, so it seems you would have more leeway with
phone width filters.

Regarding costs--yes, I was amazed to find that good quality crystals are
available from Mouser and similar places for 50 cents or so apiece in a
pretty good selection of standard frequencies. Hope you're able to take
advantage of something similar in your QTH. And now we have a "gas war"
type situation going on here on QRP-L to boot! It's a great time for
homebrewing crystal filters. (And little QRP RXs and TXs.) I'm going to
grab some of those 9 MHz rocks and some of the 3.579's too. Of course, if
someone has gone to the trouble of measuring and matching the crystals for
filter use, surely he's entitled to ask for some compensation for that
effort.

I would think that even a three crystal unit would do OK for an economy
class SSB rig. I'd be interested to hear if folks with more experience
agree with that.

72--Nick, WA5BDU

-----Original Message-----
 OK, but does
the accuracy of the match influence the wider
bandwidth SSB filter less than the CW much narrower filters ??

4. What would be the simplest and lowest cost
design for a SSB filter... and by this I mean using
the least number of crystals and other parts ?

5. Which are the more popular and lower cost
frequencies available from suppliers around the world ???

I
HOPE this is accepted as a QRP related topic
and that it may generate a LOOOONNNG and interesting thread...

Me too.


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