Changing Frequencies.....danger??

[Bill Glaze]

Ed:
Good explanation.  To those folks that are asking why the crystal is so 
easily available if it's not to be changed, all I can offer is an 
explanation that was given me by Tony Stillman several years ago.
He said that, as the transmitters and receivers travel down the assembly 
line, they are, at one point, without crystals.  As orders develop, and 
as different frequencies are needed, the production workers insert the 
proper crystals into the (otherwise) unfinished radios, to satisfy 
orders which they have received.  (Presumably from wholesalers, etc.)  
Obviously, they want this production step to be as quick and easy as 
possible.  I assume that then, (and this is a large assumption) the 
transmitters and receivers are scoped and checked for proper operation.  
Tony didn't mention this, so I have no verification of that statement.
Of course, that doesn't satisfy the question:  why are  spare crystals 
available for public sale?  As I understand it, even if you have a 
"lazy" crystal, or a non-working crystal, it is still not within the 
owners purview to change it.  Apparently, however, the FCC considers 
these infractions not serious enough to spend their enforcement time 
going after the "offenders."  At least, so far.

Bill Glaze
KI4JQW


Ed Alt wrote:

> This is quite possibly incorrect.  Depending on how it has been 
> designed, it may not legal or advisable to change crystals in a module 
> unless you are FCC certified to do so and have the equipment to assure 
> that the transmitted energy is within spec.  Think of it: the reason 
> we have modules is because you're suppose to swap the module, not the 
> crystal.  Ofcourse it "works" to swap a crystal.  All this proves is 
> that  you didn't break anything important when you broke the potting 
> seal that you usually find on the crystal.  This potting seal is there 
> to tell the bench technician whether you've been tampering with the 
> module.
>
> When you swap crystals, you run the risk of transmitting with less 
> than peak power on your channel, thus wasting energy that would 
> normally radiate out of the antenna and instead heating the module's 
> components more than normal. You also run the risk of transmittnig 
> above legal levels of RF energy elsewhere, in other words, you may 
> contribute to interference on other channels.  You have to realize 
> that you never get a completely clean signal just on your channel when 
> you operate your transmitter.  Part of the procedure of producing an 
> RF module includes tuning, or more correctly, aligning the tuned 
> circuits in the module to the center frequency you want to transmit on 
> and bringing the unwanted byproducts down to acceptable levels.  When 
> you swap crystals on your own, you have undone this work, at least a 
> little bit, maybe by an unacceptable amount. Depends on how broadly 
> tuned the module is designed to be in the first place.
>
> Why does a synthesised RF module work?  It's designed with a tightly 
> controlled feedback loop to lock onto the desired frequency and 
> maintain other unwanted RF energy well below specified levels.  Why 
> does it work to swap crystals in receivers?  Because the front ends 
> are broadly enough tuned to allow energy to be received efficiently 
> within a specified pass band, in our case usually much of the 72 mhz 
> band.  The fine tuning, i.e. what makes our receivers highly selective 
> to the channel we want is mostly accomplished by circuitry that is 
> past the stage of the front end.
>
> Ed
>
>
>
>
> ----- Original Message ----- From: "Ed White" <edvwhite at yahoo.com>
> To: <discussion at nsrca.org>
> Sent: Wednesday, June 29, 2005 11:16 PM
> Subject: RE: Changing Frequencies.....danger??
>
>
>> I've done this (flown on chan 16 and 29 on 72 MHz and
>> chan 00 on 50 MHz) and done this with both my Futaba
>> Super 7 and 8UA transmitters with no problem.  The 9C
>> uses the same module.  You can do this freely and
>> legally.  This is not the same as changing a crystal
>> in the transmitter.  To the FCC what we call the
>> module they consider the entire transmitter, except
>> for antenna.
>>
>> But Steve is absolutely correct, the receiver is a
>> completely different matter.
>>
>> --- RC Steve Sterling <rcsteve at tcrcm.org> wrote:
>>
>>> No problem in a 9C. That module in the back that you
>>> swap to change
>>> frequency is a self contained module, no RF parts in
>>> the rest of the
>>> transmitter (except the antenna I guess). You can
>>> even switch to 6 meters
>>> which is 20 mhz away.
>>>
>>> Receiver is a completely different story.
>>>
>>> -----Original Message-----
>>> From: discussion-request at nsrca.org
>>> [mailto:discussion-request at nsrca.org]On Behalf Of
>>> F3AFlyer7 at comcast.net
>>> Sent: Wednesday, June 29, 2005 5:41 PM
>>> To: discussion at nsrca.org
>>> Subject: Changing Frequencies.....danger??
>>>
>>>
>>> Well I've heard a couple of opinions on this so I
>>> figure I'd get some more.
>>> I need to get a new frequency for the NATS but don't
>>> want to send my
>>> transmitter in. I have a Futaba 9C. Some have told
>>> me that I can change as
>>> many frequencies as I want without mishap, while
>>> others have told me that
>>> you can only go 6 up or 6 down or else your transmit
>>> range gets shorter. Is
>>> this true for transmitters with crystals in them
>>> only, or for both crystal
>>> and module transmitters? If this is also true for
>>> module transmitters, then
>>> why are there synthesized modules for the 9C?
>>>
>>> Thanks!
>>>
>>> Scott
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>>
>>
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