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[Clansgian AT wmconnect.com]

> >As you can tell Don, although I've done a lot of reading in the scientific 
> journals, I don't know how to explain what I've read correctly.  But I 
> could 
> have sworn that the verb broadcast was used in one of the articles--- but 
> you are correct.

Don's a radio man and knows what he's talking about.  It is not possible for 
the same thing to be passive and also broadcasting continuously.  That's like 
saying "this joint is concave but it's also a mortise".  

Lots of people who write articles are quite ignorant of electronics.

RFID tags are viewed as coming in three flavors:

Active - have a built in power source, usually a battery.  The battery powers 
the passive circuitry and powers the oscillator for transmission.

Passive - has no built in power source.  RF energy is received (detected) and 
charges a small capacitive or inductive circuit. Sometimes only the junction 
of a CMOS is charged.  This is enough power to transmit a brief signal in 
response.

Have you ever had the experice of turning off a radio and unplugging it and 
when you switch it on again without plugging it up, it plays for about a 
second 
then fades off (and you discover it is unplugged)?  That's because the rather 
large capacitors in the power supply (that convert the AC to lower voltage 
DC) hold a charge even unplugged.  This is enough power in fact to opearate 
the 
radio for a second or two.  That's how passive RFID circuits work.  Radio 
frequency energy of a specific frequency causes and induced current which is 
stored (for a microsecond) as an inductance or a capacitance and powers the 
response transmission.

The process is not so much like a "transciever" as I am making it sound.  It 
is not like the PTT button on a CB radio - listen thent talk.  It functions 
more like a 'backscatter' of the original RF signal that excited it.

A third type is the semi-passive RFID in which a battery powers the passive 
circuitry but the trurn signal is just a backscatter.



Obviously a device without a power source can be made much, much smaller than 
one with.  For example, the working guts of your cell phone is no bigger than 
a the nail on your little finger, the rest of the bulk and weight of the 
phone is the battery.  

But the signal that provokes the active RFID or the semi-passive RFID into 
action can be a very tiny (weak, in microwatts) signal.  To get the same 
results, the RF signal to activate a totally passive RFID must be more than 
100 times 
stronger.  It is a massive heathen transmitter that is activating a passive 
chip more than a very few feet.


James, WA4CYU   </HTML>