Parallax O-scope and RFID Antenna

John R.

I separated this from the other thread because it is a specific question, not meant as part of the onging discussion on applying RFID to model railroad cars.

Does anyone know if·the Parallax USB Oscilloscope is fast enough to get a meaningful trace on the signal on the RFID reader antenna?

The scope says 200 kHz, but also says sine waves to 60 kHz and square waves to 120 kHz.

I believe the RFID is roughly 150-170- kHz.

I'd like to see basic wave form, and amplitude.· In concept, I'd like to be able to look and see what affects any "shielding" or "reflectors" might have, as well as possibly (maybe) validate any alternative antennas.

I plan on measuring the antenna on the board, and possibly also making an "induction coil" to try and measure field strength.

If I'm way off base here, feel free to smack me upside the head and pound some reality in!

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John R.

8 + 8 = 10

Paul Baker

The Parallax RFID operates at 170kHz, according to the Nyquist theorem, to properly sample a 170kHz square wave requires a 340kHz sample rate. To sample a sine wave would require at least 566 kHz sample rate. Sampling at frequencies less than this will result in aliasing of the signal (aka beating).

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·1+1=10

steve_b

TWEAKY!!!! "beedeebeedeebeedee...hey BUCK!"
sorry...couldn't resist!! haha

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Steve

"Inside each and every one of us is our one, true authentic swing. Something we was born with. Something that's ours and ours alone. Something that can't be learned... something that's got to be remembered."

Paul Baker

Hehe your the first one to comment on my "costume" of the day, I thought of holding a contest to see who could name the character and movie/show of all the costumes, but I don't have anything of universal appeal to give away. I remembered this guy as Twiggy or Tweaky, but its spelled Twiki.

<ethereal voice from the rafters> "We now return to our regularly scheduled programming" </voice>

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·1+1=10

Post Edited (Paul Baker) : 10/28/2005 4:54:42 PM GMT

Newzed

I didn't comment on it because I thught something had happened to you and I didn't want to embarass you.

Sid

John R.

Paul;

I recognize Twiki by appearance, but can't remember the show. What was it?

On the O-scope, I'm familiar with the concept of Nyquest limits, etc., but don't have the experience to know how bad things would get distorted. Can you take a stab at how meaningless/meaningful the traces would be? If I wasn't after "quantifyiable" results, but just "relative" results looking at gross cause/effect, would I be likely to see anything? (assuming I get up to speed on actually measuring the signal to start with).

Really, I'm looking for an excuse to buy the scope, but if I can't use it, and it starts to get dusty, my wife will start to remove body parts (from me). These would be some of the parts I find most useful.

I realize I have a lot to explore learn, but I want to get up to speed as quickly as possible. Inbetween my applied experimentation, I am taking some time to refresh, and build up my basic knowledge and understanding. I was involved with industrial control circuits and some logic and analog stuff at work long ago, so I'm not a total newbie, but by no means proficient yet.

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John R.

8 + 8 = 10

Paul Baker

Twiki is from Buck Rodgers, yesterday I was Muffit the Cyber Daggit from Battlestar Galactica, and a Star Wars cantina band member the day before (70's sci-fi theme running here), haven't decided whether to stick with the theme until Oct 31st or change it up a bit.

The amount of beating in the signal is dependent upon the ratio of the frequency of the signal to the sampling frequency. RF signals typically start with a sinusoid then modulate the signal to encode information on it. So we'll assume a 566 kHz sample rate is required, this is 2.83 times faster than provided by the oscope. When violating Nyquist, the FFT peak wraps around in the frequency domain. Because the sampled frequency isn't a integer multiple of the signal, eventually the sampled signal's peak value will be sampled by the oscope. Therefore you can still use it to get the amplitude of the sampled signal and it should work for what you are trying to do, you just won't be able to see the data modulated onto the carrier.

Here's a good synopsis of the concept of aliasing, the second picture is what will happen.


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Post Edited (Paul Baker) : 10/28/2005 7:07:51 PM GMT

John R.

Perfect! I follow what you're saying. On the premise of "in for a penny, in for a pound", can you (or someone) point me an economical scope with a little higher frequency? PC based would be ideal, but I'd considder a stand alone. I'd _like_ a multi-channel storage scope, but I did mention "economical".

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John R.

8 + 8 = 10

Paul Baker

This question came up around the time the oscope came to market (via members griping about it's low sample rate), There were a couple relatively affordable solutions mentioned, but I cannot remember what they were.

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Paul Baker

Ok picoscope's (believe this was one bantered around) cheapest scope with enough bandwidth is the 2202 for $355. USB Instruments sells a few models, but lack of listed price ussually means $$$.

Heres a list of links, your milage may vary:

http://news.thomasnet.com/fullstory/465621/rss

http://www.linkinstruments.com/oscilloscope.htm

http://www.marketwire.com/mw/frame_multimedia?prid=86603&attachid=185167

http://www.dontronics.com/swordfish.html

http://www.dontronics.com/stingray.html

http://www.geotestinc.com/products/index.asp?sct=5&OVRAW=usb%20oscilloscope&OVKEY=usb%20oscilloscope&OVMTC=standard



Or make your own:

http://www.eix.co.uk/Ethernet/USB/

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John R.

I was searching the forums here (marginal results), but did run accross a reference to an updated scope from Parallax. There wasn't much word on it at the time, and the focus of the particular discussion I ran across was geared more toward the abiltity to measure up to 24 Volts.

Any more word on this scope? Still "vapor ware"? Need a beta tester?

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John R.

8 + 8 = 10

Paul Baker

Here's an Idea, how about making a header board to contain a flash ADC set to·free run (may require some additional circuitry) then plug it into the BSLA? Since it samples at 2Ms/s, you can get a 2Msps oscilloscope, you can even incorporate a second ADC for a second channel.

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John R.

Paul;

Thanks for the insight and the links. I'll be playing with what I have and doing more research on the scopes you mention and see if I can find any others over the weekend.

The "roll your own" idea looks interesting, but I think I'll pass on that for now.

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John R.

8 + 8 = 10