Nate Duehr wrote:
>> Thanks for your replies,
>> The actual problem is ,  i am modifying a rf transceiver board (rf
>> id) , for
>> more power. There is low power transceiver ic (TI S67 00) on the
>> board, which
>> do all the modulation and demodulation (only for low power output).
>> I need to modify it to get more power output and more range for rf
>> id tag.
>> I added a power amplifier on transmitter path.
>>
>> For the receiver path, the rx pin of IC is limited to 5V, so i need to
>> attenuate the rx signal, without losing the data on its envelope.
>> direct
>> resistance attenuation will affect the signal strength (envelope).
>> so i
>> thought of  shifting down the signal and feeding it to the IC.
>
> Most RFID systems I've seen were already running at their maximum
> legally allowable power limits.   Are you sure your modification is
> legal where you live?  Are you going to end up turning the RFID
> system into an RF noise generator that someone will have to come hunt
> down to keep it from interfering other radio services?

To add to that, you've already got a 7.1VRMS carrier, which results in 1W if
dumped into 50ohms.  Maybe your antenna isn't 50ohms, but that's still a lot
of power for an active RFID tag.  Messing with the RF output may not be
legal.  What application do you have that requires such long range that you
need a watt or more of power?

In contrast, one of my customers makes an active RFID and location system
where the tags run for over a year on two coin cells.  They transmit for
about 8mS every 10 seconds at 434MHz.  They are specified for 30 foot range,
but can in reality do about 50 feet with the new receiver I'm working on.  1
Watt is a great deal of power compared to that, and can go a long way if
treated right.

The FCC limits power on most bands specifically to limit range and reduce
interference.  Increasing power output defeats that and may require a
special license if you can even get it, or land you in jail.  So again, what
is your application?


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