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Analayzing RF Signal using an Oscilloscope

Posted on 2009-04-07
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Last Modified: 2012-05-06
I have a wireless device that transmits in the 900 - 928 MHz range. I would like to observe Voltage v. Time waveform of the transmitted RF signal on an Oscilloscope. I tried using a 8cm long wire as an antenna and connected it to the oscilloscope probe. But the oscilloscope did not display anything. Next I tried using an antenna from a wireless router but that did not work too. Now I do not know if the antenna that i am using is too trivial or if the signal strength is too low to be displayed on the oscilloscope without an amplifier.

I have no experience with RF, so do not know if it is possible to measure RF Signals using oscilloscope.  If it is possible what should i do to achieve this?  If an oscilloscope cannot be used, are there other instruments that could be used to observe Voltage v. Time waveform of an RF Signal? I really appreciate any help. Thanks
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Question by:chakrapanir
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Expert Comment

by:d-glitch
ID: 24092328
You need a very fast (and very expensive) oscilloscope to look at a 900 MHz signal.
The bandwidth of the scope is usually written somewhere on the front panel.

Or let us know what model scope you have?

And also let us know in more detail what you are trying to do.
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by:d-glitch
ID: 24092365
Even with a slower scope, you might be able to see the transmitted signal with an RF detector or an RF receiver.

A small loop of wire (1 ft diameter) with a diode and capacitor can convert the RF
signal to DC.  I will try to find a link to an appropriate circuit.

What sort of device gets the signal from this transmitter.  You might be able to do your probing there.
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by:d-glitch
ID: 24092397
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Author Comment

by:chakrapanir
ID: 24092821
Thanks d-glitch.
I am using a HP/Agilent 16500B Logic Analyzer. The Oscilloscope in it has a sampling rate of 1 Gsamples/second and the time scale can go as low as 1ns/division. Is this scope good enough to observe a 900 MHz signal?

I do not have the device that receives the transmitted signal. So i cannot probe the receiver.

The objective of my experiment is to determine the modulation scheme used by the transmitting device. Initially i wanted to probing the RF I/O Pin of the transceiver chip but the pins of the  transceiver chip were not accessible for probing. The only other method i know is to capture the RF transmissions around 900 MHz using an antenna, connect the antenna to an oscilloscope and observe the waveforms. As mentioned in my post above, i first tried connecting a  8 cm wire directly to the probe, then i tried connecting antenna from a wireless to the probe. I am guessing that i not using a proper ground for the antenna. Do you think the above set up would work? Do you have any suggestions for an antenna? Is there anything else i could do to achieve my objective? I read some where that using a coaxial cable with an Inline amplifier would act as a good antenna. What is your opinion about such an antenna?

Thanks again.
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d-glitch earned 500 total points
ID: 24092956
A logic analyzer is not the same as an oscilloscope.  
It can't display analog levels, only binary information.
Typically more than 2 Volts is HIGH, less than 1 Volt is LOW.

What you are getting out of your marginal antenna is probably on
the order of microvolts, so I would expect the screen to display
solid zero.

The bandwidth is not really adequate either.  Generally you want
the scope bandwidth to be at least 2x the signal bandwidth to
satisfy Nyquist criteria.  

Direct measurements without the proper equipment are going to be
difficult.  

 don't know what device you're looking at, but most commercial units
follow some industry standard protocol and need FCC approval.
If all you want is the modulation scheme, you might try some research
along those lines.

You could also look at chips designed for this band, like this one from NXP:

      http://www.nxp.com/acrobat_download/literature/9397/75015689.pdf


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by:compfixer101
ID: 34358936
This question has been classified as abandoned and is being closed as part of the Cleanup Program.  See my comment at the end of the question for more details.
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