Author Topic: Near-Field Probes  (Read 21425 times)

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Offline olsenn

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Near-Field Probes
« on: April 23, 2012, 12:52:33 pm »
Lately I have been hearing a lot about near-field (EMC) probes, more precisely, these: http://beehive-electronics.com/probes.html.

I would like to ask for those with experience with magnetic field probes like these (doesn't have to be these same ones); how good are they? I have no interest in doing EMC work, but as an alternative to an active probe for my spectrum analyzer, I'm interested in knowing if these can serve as a fully functional probe without fear of it loading down a DUT?

The datasheet provides some info on their responsiveness, but they refer to the field strength, flux density, teslas... and I have no idea what these are or how to put them in perspective for the work I'd like to do. Mainly, I would like to be able to be able to measure the frequency response of filters, but my worry is that the field that a simple RLC filter would produce with say a 1V excitation voltage wouldn't be large enough to be picked up by this thing??? The probe I'm looking at right now is the 100A model on the link provided above.
 

Offline Rufus

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Re: Near-Field Probes
« Reply #1 on: April 23, 2012, 04:58:05 pm »
IMO you are still barking up the wrong tree with this active probe and now magnetic probe stuff.

You said you wanted to measure the response of 'filters and things'. The response of filters and things depends on the source impedance driving them and the load impedance. At RF all impedances are generally low.

Your SA + TG has 50 ohm source and load impedances. If your 'filters and things' are not designed for 50 ohms then you need matching networks.

The only reason for wanting an active probe is to try to measure something in a complete circuit without additional loading and you are basically on a looser, 0.6pF from your active probe is a substantial load at high frequencies, likewise poking one of these magnetic probes close to the circuit is also a potentially substantial load. The power to provide a measurable signal into 50 ohms has to come from somewhere.

RF design is full of matching networks. Relatively broadband with transformers, narrowband with L and C networks or even bits of coax cut to the right length. Resistive PI or T networks are very broadband paid for with substantial attenuation. The 450 ohm and 50 ohm coax resistive probe in the other thread is a 500 to 50 ohm matching network with 20db loss.

As a general observation when you have difficulty buying something that does what you want or have difficulty getting someone to tell you how to do what you want it is often because no one else ever wanted to do what you want and there is a good chance that no one else wanted to do it because it is dumb.

$0.02 from someone who doesn't claim to have more than a passing knowledge of RF matters. 
 

Offline Mechatrommer

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Re: Near-Field Probes
« Reply #2 on: April 23, 2012, 06:03:24 pm »
+1 to rufus. and also be careful about advices your getting. since you dont provide your problem or your circuit, people may come up with something you dont want, you need to filter that, because you are the one who know exactly what the problem is. i wish i dont post here, but its a pain esp buying the wrong thing, and more pain if it is an advice from somebody else. who to blame? yourself.
if something can select, how cant it be intelligent? if something is intelligent, how cant it exist?
 

Offline olsenn

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Re: Near-Field Probes
« Reply #3 on: April 23, 2012, 06:31:33 pm »
I'm not buying it; I just want to learn what they are and what they are used for... that is why I put up this post. A simple google search doesn't provide much good information on it.

I don't have any one circuit I would like to analyze; I just want to see what is available for viewing the frequency responce of ANY circuit or any impedance (within reason). This seems to be something that is easy to do in the time domain (oscilloscope) but not so much so in the frequency domain. I don't really care for transmitters and radios; I just occasioanlly want to see how steep a drop off I am getting on a low/high/band pass filter. In many cases i can simply take voltage readings from my oscilloscope and plot the thing in excel; I just figured it would be nice to have a device do that for me automatically, not to mention being able to go to a higher frequency than my DS1102E and not needing to use a function generator (god bless tracking generators)
 

Offline nctnico

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Re: Near-Field Probes
« Reply #4 on: April 23, 2012, 08:57:25 pm »
Most filter frequency responses are specified in dB. An oscilloscope to not very accurate. A difference of 24dB means a signal is 8 times larger or smaller. For audio range work I use an AC voltmeter with two inputs (and two needles). It shows what goes into a circuit and what comes out.
There are small lies, big lies and then there is what is on the screen of your oscilloscope.
 

Offline dfnr2

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Re: Near-Field Probes
« Reply #5 on: April 24, 2012, 07:06:10 pm »
I have those probes, and they work fine for the intended purpose.  However, one could probably make homemade near field probes that would work equally well, or--if tailored to an application--even better.

However, they are not useful for what you appear to want.

You should start by perhaps being a little more specific about the kinds of filters you want to characterize.  What frequency range?  What are they for?  How are they arranged in the circuit?  What do they look like when separated? 

Dave
 

Offline Rufus

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Re: Near-Field Probes
« Reply #6 on: April 26, 2012, 07:25:53 pm »
However, one could probably make homemade near field probes that would work equally well

One I made today from a rigid SMA coax cable which as part of an assortment cost about $1 from ebay.
 

Offline dfnr2

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Re: Near-Field Probes
« Reply #7 on: April 27, 2012, 08:48:25 pm »
Very nice.
 

Offline Mark

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Re: Near-Field Probes
« Reply #8 on: April 28, 2012, 12:23:26 pm »
I made one out of flexible coax, still with insulation on which worked well trying to find the source of emissions on a mixed signal board.  Someone had placed a 40MHz oscillator on the digital part and routed the ground to the analog plane, so a radiating loop was created. 

I'd feel much more comfortable with an insulated probe, remember you will be waving these probes very close to powered circuits. 
 

Offline Rufus

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Re: Near-Field Probes
« Reply #9 on: April 28, 2012, 02:24:06 pm »
I'd feel much more comfortable with an insulated probe, remember you will be waving these probes very close to powered circuits.

The one in the photo is insulated, coated with hot melt glue. With hindsight it would have been better to use coloured hot melt to make coverage easier to see.
 

Offline steve_w

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Re: Near-Field Probes
« Reply #10 on: April 29, 2012, 04:32:53 am »
Probes like that are used to make field strength measurements of e and h fields for antenna field strength.

Normally the field strength is measured in power flux density mW/cm^2 (or plain old power).  However a power flux density measurement is only accurate in the far field of the antenna, where the distance from the antenna is 2d^2 / lambda.  If you want to make power flux density measurements in the near field then you make e and h field measurements individually and then manually calculate the power level. 

What you are proposing doesn't make sense to me, I concur with Rufus.  For accurate filter measurements at RF I would use a network analyser, If you can't afford that have a go using a vector network analyser, If you don't know what these instruments are or how to use them; then you must research these topics.  RF measurements are not easy to make accurately unless you have a good test equipment (read HP or reputable manufacturers with HP like performance) and a good understanding on how to use it.

regards

Steve W
So long and thanks for all the fish
 

Offline dfnr2

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Re: Near-Field Probes
« Reply #11 on: April 30, 2012, 02:16:00 pm »
For accurate filter measurements at RF I would use a network analyser, If you can't afford that have a go using a vector network analyser, If you don't know what these instruments are or how to use them; then you must research these topics. 
I agree on the VNA.  But what "network analyzer" do you have in mind that is so much better and more expensive than a VNA that the VNA is the poor-man's solution?  I doubt I'd be able to afford whatever it is, but I'd love to know.

BTW, I agree fully that the OP needs to read up on these instruments and measurements.  HP has some good application notes on SA and VNA/SNA measurements, calibration, and error correction.  It seems he's wanting to use the SA as a poor-man's scalar network analyzer, and now that he's spent the money, is also trying to investigate other ways to measure "stuff"--and trying to figure out what that "stuff" might be, and if it could be useful.  To be honest, it warms my heart. 

Dave
 

Offline olsenn

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Re: Near-Field Probes
« Reply #12 on: May 01, 2012, 01:13:44 pm »
Quote
It seems he's wanting to use the SA as a poor-man's scalar network analyzer, and now that he's spent the money, is also trying to investigate other ways to measure "stuff"--and trying to figure out what that "stuff" might be, and if it could be useful.  To be honest, it warms my heart. 

That's precisely it. I am just trying to figure out what all is available, what it does and how well, and if it could prove useful to me. I'm not buying a network analyzer, and from what I gather, it is no more suited to what I need than my cheap SA (I don't need phase information, s-parameters and so forth, I just want to pass my tracking generator into a DUT and measure the -3db cutoff of that device. Simple????? If it matters greatly, the frequencies I want to measure are 1MHz to 1.5GHz
 

Offline steve_w

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Re: Near-Field Probes
« Reply #13 on: May 08, 2012, 11:38:11 am »
I stuffed up as happens when you have craft syndrome, I meant vector voltmeter,

regards

SW
So long and thanks for all the fish
 

Offline hehe

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Re: Near-Field Probes
« Reply #14 on: July 27, 2016, 09:27:00 pm »
Hello,

I have made a rectangular resonant cavity that resonates around 1GHz and I have made hexagons cuts on the top so the magnetic field can escape from the top of the cavity. What do you recommend for measuring the magnetic field on the surface of the cavity?
The ultimate goal is to use very small coil (around 1mm diameter and 3 turns) and turn on an small LED using the field coming out of the box.

Thanks!
 

Offline MilkmanCDN

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Re: Near-Field Probes
« Reply #15 on: March 10, 2019, 06:16:38 pm »
I too am looking for some EMI probes as well.

I've seen some low-cost PCB material probes on ali express.
     - https://www.aliexpress.com/item/1PC-EMC-EMI-Near-Field-Probe-Conducted-Radiation-Correction-Simple-Magnetic-Field-Probe-9KHz-6GHz-Type/32966266773.html

I'm wondering how they compare to the slightly more expensive alternatives:

- Rigol - http://www.saelig.com/product/nfp-3.htm

- Siglent
     - http://www.saelig.com/product/srf5030-1.htm
     - http://www.saelig.com/product/srf5030t.htm
 

Offline 2N3055

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Re: Near-Field Probes
« Reply #16 on: March 10, 2019, 07:11:23 pm »
Tekbox or Beehive. Medium price , good quality
 

Offline nctnico

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Re: Near-Field Probes
« Reply #17 on: March 10, 2019, 08:07:28 pm »
There are small lies, big lies and then there is what is on the screen of your oscilloscope.
 

Online tautech

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Re: Near-Field Probes
« Reply #18 on: March 10, 2019, 08:15:06 pm »
I too am looking for some EMI probes as well.
Don't do anything until you've watched these:

https://youtu.be/2xy3Hm1_ZqI

https://youtu.be/nImoQcoqkuQ

Quote
I've seen some low-cost PCB material probes on ali express.
     - https://www.aliexpress.com/item/1PC-EMC-EMI-Near-Field-Probe-Conducted-Radiation-Correction-Simple-Magnetic-Field-Probe-9KHz-6GHz-Type/32966266773.html

I'm wondering how they compare to the slightly more expensive alternatives:
Nice, $40 for the set, at that money how could you go wrong.


Avid Rabid Hobbyist
 
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Offline MilkmanCDN

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Re: Near-Field Probes
« Reply #19 on: March 14, 2019, 09:48:42 pm »
Bought the $40 from Ali-Express.    Now just need to wait a month or two for them to show up.  :)
 

Online tautech

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Re: Near-Field Probes
« Reply #20 on: March 14, 2019, 10:40:50 pm »
Bought the $40 from Ali-Express.    Now just need to wait a month or two for them to show up.  :)
Cool, I'm gunna get some too when I have another Aliexpress session.
At that price they're certainly worth a punt.  ;)

Just need get SMA to N or BNC flexible cables to go with them too.
Avid Rabid Hobbyist
 

Offline EEVblog

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Re: Near-Field Probes
« Reply #21 on: April 04, 2019, 11:52:55 pm »
I'm wondering how they compare to the slightly more expensive alternatives:
Nice, $40 for the set, at that money how could you go wrong.
[/quote]

No point making your own at that price.
 

Offline wilfred

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Re: Near-Field Probes
« Reply #22 on: April 05, 2019, 02:55:22 am »
No point making your own at that price.

That's probably as good a comment on the decline of hobby electronics as "Now just need to wait a month or two for them to show up."

Sigh.

 


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