Changing Frequencies.....danger??

[John Pavlick]

Scott,
 Yeah, what Ed said... Just change the RF module in the transmitter and get
a new crystal for the receiver. I think the RF module for a 9c is about
$50.00. Not too bad. You are not supposed to change just the crystal in the
transmitter - that's why we have modules. Changing just a crystal (in the
transmitter) requires re-tuning otherwise you might interfere with someone
else. The module is tuned to the chosen frequency and sealed up. Nothing
wrong with having a few modules either. The synthesized RF module is another
alternative. Like Ed said it is tuned to a stable frequency, then a PLL
(Phase Locked loop) circuit generates the chosen carrier frequency. Also
acceptable and legal. Bottom line: change the transmitter module and the
receiver crystal and you'll be within FCC regulations and safe from any
Homeland Security issues too.

John Pavlick
http://www.idseng.com


> -----Original Message-----
> From: discussion-request at nsrca.org
> [mailto:discussion-request at nsrca.org]On Behalf Of Ed Alt
> Sent: Wednesday, June 29, 2005 11:52 PM
> To: discussion at nsrca.org
> 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
>
>
>
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