Changing Frequencies.....danger??

[Ed Alt]

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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