Author Topic: Response of cheap RF Active Probe from Ebay.  (Read 12149 times)

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Offline 2N3055

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Re: Response of cheap RF Active Probe from Ebay.
« Reply #25 on: March 07, 2018, 03:58:52 pm »
I use it quite often... It is actually quite good up to some 500ish MHz, and has 10 Mohm input resistance combined with 0.5- 0.6 pF input capacitance..
I terminate it to 50 Ohm on scope side, and have a custom probe defined for it.

So, as a 1 GHz FET high impedance probe is not very good. As 500MHz active FET probe is great bargain for money...

P.S. Putting it into metallic case and good grounding will flatten frequency response... If one would want to play with it, compensation network could   be made to flatten it more... But up to 500 MHz it's not really necessary .. Using passive probe would be much worse...
 
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Offline InsatmanTopic starter

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Re: Response of cheap RF Active Probe from Ebay.
« Reply #26 on: March 08, 2018, 03:20:27 am »
Hello Everyone,
I wrote the Ebay vendor and told him I tested his probe and it didn't meet the promised specifications (see attached pic from his ad).  His response speaks for itself
===============
New message from: 60dbmcom (764Purple Star)
Hi

i tested many times too
I got about 7dB attenuation in the range from 0.1 to 500 MHz
20 db attenuation i got at 1.5 Ghz. My result nearly the same as yours
I think it is very good result for usd8 board
Anyway, if you don't like my probe and according ebay rules, you can send me back and i give you money back


Best regards
Paul
==================

So he flat-out knew it wasn't even remotely close to the specifications claimed.   Plus he also knows that with the cost of international postage returning the probe is pointless.   Note I paid $12 plus $6 shipping, not $8.   

I'm not wildly upset because I'm out less than $20 and I have a probe that has some uses...but this just goes to show you how some vendors will claim anything to get a sale.


FYI....After a further round of emails, Paul claims the specification of +/- 2.5dB was a mistake.  He says he has removed it from his ad.    He offered to refund me if I returned his probe.  I do not intend to do so as shipping it back to Ukraine is more than it's worth.   
Retired Pulsed Power Engineer/Physicist...now I just dabble in electronics
 

Offline InsatmanTopic starter

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Re: Response of cheap RF Active Probe from Ebay.
« Reply #27 on: March 08, 2018, 03:22:13 am »
Copper tubes are good for this..

Do you have plans for this? Thanks

I experimented with covering my probe with copper foil and it made zero difference in performance or noise level.   Maybe if you are trying to measure down near the noise floor it might help.  Personally I don't think it's worth the effort on this probe.
Retired Pulsed Power Engineer/Physicist...now I just dabble in electronics
 

Offline rhb

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Re: Response of cheap RF Active Probe from Ebay.
« Reply #28 on: March 08, 2018, 02:29:40 pm »
It might be worth trying it in a copper tube or at least  calculating the resonances of a copper tube.  I mention this because the cavity dimensions for a wide band diode noise source are quite important.  There may be a convenient dimension that would flatten out the response a bit.  A quarter wavelength at 1 GHz is ~3".  And we *are* talking about RF at  frequencies where it starts becoming black magic.
 

Offline Wolfgang

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Re: Response of cheap RF Active Probe from Ebay.
« Reply #29 on: August 08, 2018, 12:07:38 am »
Hi,

I made one myself. It is a lot better than the chinese one you tried, because it is flat within +/- 1dB from some kHz to 1.5GHz.

Look here:

https://electronicprojectsforfun.wordpress.com/

Its AC only, and it works using a "gimmick" capacitor.
 
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