Simpler than RFID?!?

[Brian]

OK, experimenting with this...

Would like an RFID-like system that can read a passive as small as possible
tag. However, it only needs to know its prescence, not an id or any data. It
must now it arrives, that is is still there, and that it has left.

Reader could read directly up, some side lobing perhaps, and know the object
with tag is with approx 30 inches.

I thought metal detection type circuitry, but how would I discriminate other
objects except tag?

Any thoughts?

 

[Jim Thompson]

OK, experimenting with this...

Would like an RFID-like system that can read a passive as small as possible
tag. However, it only needs to know its prescence, not an id or any data. It
must now it arrives, that is is still there, and that it has left.

Reader could read directly up, some side lobing perhaps, and know the object
with tag is with approx 30 inches.

I thought metal detection type circuitry, but how would I discriminate other
objects except tag?

Any thoughts?

The equivalent of a tank brought near a grid-dip meter?

...Jim Thompson

 

[John Larkin]

The equivalent of a tank brought near a grid-dip meter?

...Jim Thompson

How about resonant ringdown?

John

 

[Jim Thompson]

How about resonant ringdown?

John

Pardon my ignorance... what's "resonant ringdown"?

...Jim Thompson

 

[Brian]

Yes, please give info on resonant ringdown.

Mr Thompson, wouldn't Grid Dip type idea require large inductor to work?
Wouldn't other objects in the field create a dip also, perhaps causing a
false trigger?

 

[Wim Ton]

Pardon my ignorance... what's "resonant ringdown"?Maybe look at anti-shoplifting devices?

Wim

 

[Wim Ton]

Maybe some ideas here:

http://www.beer.org/~tpark/strips.html

Wim

 

[Jim Thompson]

Yes, please give info on resonant ringdown.

Mr Thompson, wouldn't Grid Dip type idea require large inductor to work?
Wouldn't other objects in the field create a dip also, perhaps causing a
false trigger?

I suppose that lossy elements could possibly do that.

What I had in mind:

"Reader" is an oscillator with AGC, i.e. just enough power to maintain
oscillation.

Bring a lossy tank, tuned to the same frequency as Reader.

AGC level should register a difference.

======

Anti-shoplifting devices are usually "tanks" or a stripline loop tuned
to a transmitter frequency, BUT with a diode so they re-radiate at a
multiple of the original frequency which a receiver is tuned to.

...Jim Thompson

 

[Brian]

Don't take this as a challenge, just my own ignorance, trying to learn....

How could you do this and read at 2 feet? Wouldn't say a persons arm in the
field create a dip similar if it was very close to it?

 

[Brian]

I have, but these seem to require an antenna on each side.

This would be a single antenna.

 

[Jim Thompson]

Don't take this as a challenge, just my own ignorance, trying to learn....

How could you do this and read at 2 feet? Wouldn't say a persons arm in the
field create a dip similar if it was very close to it?

[snip]

Why don't you try it. Learning is best done from failed experiments
;-)

...Jim Thompson

 

[Brian]

Oh don't think I won't. However, best to try to understand it first.

Wouldn't the coil part need to be large?

Don't take this as a challenge, just my own ignorance, trying to learn....

How could you do this and read at 2 feet? Wouldn't say a persons arm in the
field create a dip similar if it was very close to it?

[snip]

Why don't you try it. Learning is best done from failed experiments
;-)

...Jim Thompson

 

[John Larkin]

Pardon my ignorance... what's "resonant ringdown"?

...Jim Thompson

Oh, sorry, maybe obscure terminology.

If the ID tag is a resonant circuit (possibly printed L-C, or even a
dipole at very HF) shoot a burst of RF at it to get it going, then
shut off the transmitter and listen. The tag will produce an
exponentially decaying RF thingie (a ringdown) that can be detected
with high sensitivity.

The other thing that tags sometimes have is a small printed antenna
feeding a schottky diode. Send CW RF, listen for 2nd or 3rd harmonic.

John

 

[Jim Thompson]

Oh don't think I won't. However, best to try to understand it first.

Wouldn't the coil part need to be large?

I've worked on tag chips for frequencies from 135KHz to 2.8GHz, none
had large antennas.

...Jim Thompson

 

[Brian]

ive read alot of it. with simple antennas and PASSIVE tags, read range is
short. large being bigger than 1"square.

 

[Brian]

I assume that you must totally shut the souce down fast, listen for harmonic
then?

 

[Jim Thompson]

I assume that you must totally shut the souce down fast, listen for harmonic
then?

[snip]

Only if you're listening for fundamental. A harmonic receiver would
allow for continuous transmission, although it might be easier to
decode an event if the transmitter were pulsed, and that pulsing acted
as system clock.

...Jim Thompson

 

[Brian]

I would assume under operating conditions, the source itself would throw
harmonics.

I assume that you must totally shut the souce down fast, listen for harmonic
then?

[snip]

Only if you're listening for fundamental. A harmonic receiver would
allow for continuous transmission, although it might be easier to
decode an event if the transmitter were pulsed, and that pulsing acted
as system clock.

...Jim Thompson

 

[Brian]

Lets redo some parameters....

OK, one antenna only on "reader". Must detect up and perhaps lobes, maximum
30", but reliable there within 6".

No data needed, just presence.

Antenna of reader max size 48" wide by 24", non-polarized.

Tag must be passive.

Tag must not be bigger than 1" by 1.5" by 0.5"

AC Voltage on reader antenna small, perhaps 24Vpp

Frequency needs to be on 100% perhaps, so 125kHz to meet FCC.

I assume that you must totally shut the souce down fast, listen for harmonic
then?

[snip]

Only if you're listening for fundamental. A harmonic receiver would
allow for continuous transmission, although it might be easier to
decode an event if the transmitter were pulsed, and that pulsing acted
as system clock.

...Jim Thompson

 

[John Larkin]

I assume that you must totally shut the souce down fast, listen for harmonic
then?

Fundamental resonant ringdown, yes. Harmonic mode can be done CW.

Please don't top post.

John