Re: Question About Crystals (Round 2)

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From: Dave Fifield (dave@redhotradio.com)
Date: Sun Jul 14 2002 - 02:15:01 CDT


Thanks for digging that email out Conrad - it will save me the
trouble of typing it again!

Jeff, if you like, I can sell you a set of 9MHz crystals and
an AFA chip if you like for you Manhattan NC20 effort.
The crystals are nicely matched to make about a 230Hz
wide filter (on average) on receive. I have a gazillion of
them - several hundred already matched into nice sets
of 7 ready for when (eventually!) we ship some more
Red Hot Radio kits.

Please reply to me privately to sort out the details if you
would like to take me up on this. Anyone else who would
like a set too can do the same.

Cheers es72,
Dave F.
AD6A

----- Original Message -----
From: "Conrad Weiss" <radman@best.com>
To: "Low Power Amateur Radio Discussion" <qrp-l@Lehigh.EDU>
Sent: Saturday, July 13, 2002 10:40 PM
Subject: RE: Question About Crystals (Round 2)

> Jeff, George et al... hi again...
>
> As I read George/W5YR's post in response to the series/parallel question,
> it sparked a memory! I recalled that we, on QRP-L, went thru a "wrangle" on
> this topic earlier this year. I searched my archives and found Dave
> Fifield's post of March 14 on this very topic in which he demystifies the
> series/parallel quandary. Dave Fifield's post, following, is in response to
> a fellow pondering series/parallel crystals for an HB SST... but the theory
> obviously applies to either filter circuit.
>
> I recalled that **either** series or parallel crystals may be used, but
> group matching is critical. Jeff, this still doesn't explain the phenomenon
> you've noted (IMO) - which I can only explain as mismatched crystals; or a
> bad crystal in the original batch. At any rate here's Dave's original post,
> and hope it helps in some way :)!
>
> (OK, I'm now betting on a bad rock in the original batch... :) )
>
> Enjoy... & g'nite all,
>
> Conrad
> NN6CW
>
> *********
> From: Dave Fifield
>
> The only difference between series and parallel resonant crystals
> is the frequency for which they are cut. A series resonant crystal
> is cut to the specified frequency such that it will oscillate at that
> frequency with no parallel loading in circuit. A parallel resonant
> crystal is cut such that, when it is loaded with the specified parallel
> capacitance, it will work at the specified frequency. Thus, a parallel
> crystal can be used in a series resonant application just fine, but it
> will oscillate higher than the specified frequency since there will be
> no parallel loading.
>
> How does this help you?
>
> Well, for crystal filters, the only thing that you need to worry about
> is that the crystals you use are matched to each other very closely.
> Therefore, for your scratch built SST, you can order *any* crystal
> type you like (as long as it's somewhere near the intended IF
> frequency of course!). You will have to order more crystals than you
> need and then select the 4 or so (not sure how many are in the SST
> crystal filter, sorry) that are the closest matched. You measure
> them by making them oscillate in a simple circuit and measure
> the frequency of oscillation of each crystal, then you select the
> ones that are closest together.
>
> A rough rule of thumb is that the crystals need to be within 1/10th
> of the width of the crystal filter of each other. So, for example,
> if you wanted a 500Hz CW filter, you would need to use crystals
> that match to within 50Hz of each other.
>
> Let us know if you need more help on this.
>
> Cheers es 72,
> Dave F.
> AD6A
>
> **********
> **********
>
> ----------
> I'm building a Manhattan version of the Red Hot 20, and just got the
> receiver portion working, and very well I might add. Anyway, I ran into
> a problem that I can attribute to the crystals I originally ordered.
> The ones I was using were 9MHz series crystals. They didn't work very
> well, as I lost nearly all of my signal from the front end in the
> crystal filter, and the magnitude of the oscillator on the SA612 mixer
> was very low. I replaced the series crystal with ones I got from a
> local surplus place. Everything is fine now. I'm assuming that these
> crystals have a capacitance associated with them (20 to 30 pF seems to
> be common in catalogs).
>
> So here is the questions. What is the difference? I've looked through
> all the QRP books that I have and looked on the internet, and I haven't
> found a discussion on this topic.
>
> In case you would like to follow my progress on the Manhattan RH20 I'm
> building... check out the web site.
>
> http://www.qsl.net/k8gd
>
> Thanks, and best 72.
>
> Jeff - K8GD
>
>


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