Changing Frequencies.....danger??

[dwaynenancy]

Thanks, Ed.  Dwayne
----- Original Message ----- 
From: "Ed Alt" <ed_alt at hotmail.com>
To: <discussion at nsrca.org>
Sent: Wednesday, June 29, 2005 10:51 PM
Subject: Re: Changing Frequencies.....danger??


> 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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