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From: Klaus Vestergaard Kragelund <klauskvik@hotmail.com>
Newsgroups: sci.electronics.design
Subject: Re: Scope Probes off Ebay
Date: Mon, 8 Apr 2024 15:30:57 +0200
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On 08-04-2024 06:40, Chris Jones wrote:
> On 8/04/2024 11:36 am, John Larkin wrote:
>> On Mon, 8 Apr 2024 11:06:52 +1000, Chris Jones
>> <lugnut808@spam.yahoo.com> wrote:
>>
>>> On 8/04/2024 2:57 am, John Larkin wrote:
>>>> On Sun, 07 Apr 2024 14:51:29 +0100, Cursitor Doom <cd@notformail.com>
>>>> wrote:
>>>>
>>>>> I never learn. I bought a used "Agilent 500Mhz probe" off Ebay for 50
>>>>> quid as the highest bandwidth probe I thought I had was a 150Mhz Tek
>>>>> one. Anyway, I have a large selection of old probes lying around so
>>>>> decided to check to make sure the "Agilent" one was genuine. Not
>>>>> surprisingly it turns out it isn't. I haven't calculated what it's
>>>>> real bandwidth is. I've established it's not as sensitive as the
>>>>> 150Mhz one and that's all I need to know. Whilst I was going through
>>>>> this palarva, I tested a old probe I came across that I've never used
>>>>> before and was amazed at the improvement in signal I got with it. I've
>>>>> just Googled its part number and it turns out it's a 3.5Ghz passive
>>>>> probe! I never even knew I had one so fast. I would never have ordered
>>>>> the "500Mhz" one if I'd known I had this forgotten-about one already.
>>>>> So the fake's going back for a refund and I won't be ordering  any
>>>>> more probes from anywhere in the forseeable future.
>>>>
>>>> The HP54006 is a 6 GHz probe, into a 50 ohm scope.
>>>>
>>>> https://www.dropbox.com/s/lxq4ujmkvo755uy/HP54006_probe.zip?dl=0
>>>>
>>>> They show up on ebay now and then. There's one now, asking $150.
>>>>
>>>> You can make your own with some of those Caddock resistors. The
>>>> Caddocks have some magical recipe. And unlike a fet probe, they are
>>>> hard to damage. I use them to probe 7 ns 1400 volt spikes in my
>>>> Pockels Cell driver.
>>>>
>>>> And you can do a GHz at least with a 1-cent axial or mini-MELF or 0805
>>>> resistor on the end of a coax. 450 ohms makes a 10:1 probe. Fast
>>>> circuits are often low impedance circuits and don't mind a 500r or 1K
>>>> load.
>>>>
>>>
>>>
>>> Which Caddock part number?
>>
>> In the zip file.
>>
> 
> 
> Thanks, I only looked at the pdf before. For usenet posterity the zip 
> file contains Agilent document 54006-90002 and a photo of a probe made 
> from a SMA connector with a resistor marked MD1248 950 1%, and a packet 
> of resistors marked CADDOCK Part No. 0699-2371 (MD1247) Rev B Model 
> MG680 Resistance 450 Ohm Date 12-7-88 Date Code 8845 amongst other things.
This is comparison with sub pF probes vs just a resistor/coax. Seems the 
performance from the coax is very similar to high end active probe. From:

http://www.sigcon.com/Pubs/straight/probes.htm

Notice, that you can
have a rather long pigtail on the coax probe and it won't affect the 
signal.