[EMAIL PROTECTED] wrote:
>
> I can't really answer these questions, but I can tell you that Hitec
> crystals seem to work fine in Futaba receivers, despite what Futaba says.
> It would be interesting to know exactly who is mfg these things.
So as I start doing some initial research, I have more questions than answers,
but a few theories. I think I remember sticking a crystal from
one brand of Rx into another once and it just didn't work. There was no
grey area, just <works fine>/<don't work at all>. I haven't seen that
I know of, and really doubt the <works fine but intermittently fails>
scenario, unless someone knows for a fact otherwise.
I'll talk to some electronic engineers I know and see if they know the
scoop. I know that crystals can be driven at their base frequency or
at an odd multiple or resonant frequency. I would think that a receiver
would have to be committed to driving the crystal in one particular way
though, and that could explain the <works fine>/<don't work at all>
compatibility levels between manufacturers (guessing).
DigiKey (http://www.digikey.com) is a worthwhile electronic component
manufacturer to know about. They have a big selection of neat stuff,
at premium prices though... Looking in their catalog, I see several,
types of crystals listed. They have SMT (Surface Mount) versions which
are smaller (not what we use) and are more delicate, as well as some
in more similar larger metal cases.
Here are some characteritic variations I see among them:
Frequency tolerance (in parts-per-million):
+/- 20 ppm
+/- 30 ppm
+/- 50 ppm
+/- 100 ppm
Aging (degradation of the crystal [first year]):
+/- 2 ppm
+/- 3 ppm
+/- 5 ppm
Maximum Drive level
1 uW (SMT package)
50 uW (SMT package)
100 uW (metal case)
.5 mW (metal case)
Load capacitance
15 pf
16 pf
18 pf (most common)
Shunt capacitance
.8pf - 1.7 pf (SMT package)
7.0 pf (metal case)
Series Resistance:
12K - 8K > 100Khz and < 200Khz
I found a nice (brief) online FAQ which defines some terms pertaining
to cyrstal characteristics. It shows a test circuit [which requires
both a scope and a frequency counter].
http://www.timingtechnology.com/crystal_techinfo.htm
>From what I can see at that page, load capacitance is the most
significant variable in keeping a crystal running at its target
frequency, and most crystals I saw (esp. in the metal cases)
have a load capacitance of 18pf.
It would be great if someone could use that test circuit to compare
manufacturer's crystals. I currently only have a scope, so I can't do
it at this time. (budget). It would be interesting to learn how
uniform the Rx crystal playing field is and how many 'black sheep'
are out there.
I'm sure the receivers are designed around the availability of crystals
(and NOT the other way around). Further, I would guess that an Rx
manufacturer's options in crystal selection for a given case-type and
frequency is somewhat limited to one or two types (crucial question).
If so, I would imagine that, since R/C aircraft Rx crystals have the
virtually identical metal case type and share the same frequency
requirements... (not counting resonant frequencies), that some
or several RX manufacturers almost certainly using the same kind
of crystal.
I also believe that receiver vendors are probably committed to long-term
adherance to particular crystal characteristics, lest a crystal that works
with one of their units one year fails with another unit the next.
Without suggesting anyone acts on this idea, I have this notion that,
if a crystal seems to actually work in a receiver and handles a range-check
real well, maybe left turned on for long enough to see if it gets "burnt out,"
that its characteristics are probably compatible with the receiver, and
wouldn't just arbitrarily change [this speculation is unverified in
a controlled experiment or by hard data].
However, the idea that crystals actually age and drift from their base
frequence is news to me though!
Anyway, I'm flying in the dark here with this, and would love to get
the facts.
The manufacturers say "Use OUR crystals ONLY!!" Is that a generic
liability disclaimer, or to help them make a profit, or just
because there really are one or two incompatibilities
(and then the question becomes how how black-and-white is compatibility?
Is it your flying happily for days and then suddenly, due to the crystal
you run into problems, or does the incompatible crystal just not
work and its really obvious from the get-go).
I don't recommend that anyone try anything risky or illegal with
this. I'm just trying to seed some research and collect some data.
-Paul
>
> At 04:23 AM 4/8/01 -0700, Paul Klissner wrote:
> >It is fairly common knowledge that manufacturers of
> >RC receivers strongly recommend that only their brand
> >of crystals be used with their receivers.
> >
> >I'm wondering if that is a question of manufacturers
> >conservatively and arbitrarily limiting their liability
> >(on the off chance of incompatibility) or if the risk of
> >a serious problem is genuinely high? [Not that I'm
> >advocating taking chances!].
> >
> >Is this an issue of FCC rule that manufacturers are following?
> >
> >If the risk of crystal/Rx incompatibility is high, can it be
> >reduced or eliminated by having just a little more information
> >than the typical uninformed consumer has?
> >
> >
> >More specifically:
> >
> > How different *are* the Rx crystals really?
> >
> > How many varieties of crystal can there really be in
> > the common high-profile crystal case? One? Two? Three?
> > Or something as seriously evil as significant tuning
> > anomalies which vary from lot to lot?
> >
> > Is there a simple test circuit anyone knows about that
> > can detect differences among crystal batches in the same
> > case that would affect Rx performance?
> >
> > Is there a good way to analyze and deterministically
> > contrast crystals in an oscilloscope to determine their
> > compatibility or incompatibility with one another?
> >
> >-Paul
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> ____________________________________________________________________________
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>
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