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From: Klaus Vestergaard Kragelund <klauskvik@hotmail.com>
Newsgroups: sci.electronics.design
Subject: Re: Scope Probes off Ebay
Date: Tue, 9 Apr 2024 01:42:22 +0200
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On 08-04-2024 16:24, John Larkin wrote:
> On Mon, 8 Apr 2024 15:30:57 +0200, Klaus Vestergaard Kragelund
> <klauskvik@hotmail.com> wrote:
> 
>> 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.
> 
> We often probe without a ground connection at all, with fet and Z0
> probes. You get a bit more noise but you see what's going on.
> 
> Critical signals get their own "probe" built into the board, with a
> coax connector.
> 
> https://www.dropbox.com/scl/fi/jpy8onicfd1yrlzyp8qc2/T660_Phemt_Oscillator.jpg?rlkey=f5uzzmpmc3z6edqz48c47o31q&raw=1
> 
> That's the clock from a triggered LC oscillator.
> 
> https://www.dropbox.com/s/674b2w56c0q9orn/Burst_1.jpg?raw=1
> 
> 
> SMB, when there's enough room.
> 
> 
I do that too, also for all PSU rails. And add resistor in series with 
the feedback path of PSUs, then it real easy to do gain/phase measurements

I add 900ohm resistor on the board before the UFL, I guess you do the same?