DIY 10:1 Oscilloscope Probe

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roglok
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DIY 10:1 Oscilloscope Probe

Post by roglok » Wed May 16, 2012 11:04 am

Hey wigglers!

I have recently bought an oscilloscope for the Game Boy. It has 1MΩ 3.5 jack inputs, which are a perfect match for my modular. But in order to measure 10V signals I need to attenuate the input signals using a 10:1 probe.

I'd like to either build some kind of dongle, or, if possible, fit everything inside the jack/cable. Has anyone here rolled their own 10:1 probes?

I found several resources with different approaches, but I'm not sure which way to go...

Cheers!

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daverj
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Post by daverj » Wed May 16, 2012 1:43 pm

The normal method is to place a 9 Meg resistor in series with the input (assuming that the input has 1 Meg impedance). That gives a 10:1 reduction of the signal by basically making a resistor divider with the external resistor and the input impedance of the circuit.

Sometimes tiny capacitors are added to increase frequency response.

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Post by roglok » Wed May 16, 2012 2:48 pm

thanks, dave.
Sometimes tiny capacitors are added to increase frequency response
yeah, that was my main concern - how much does cable capacitance manipulate the measured signal? does this matter for audio frequencies at all?

will this do the job?

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daverj
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Post by daverj » Wed May 16, 2012 7:48 pm

With that much input capacitance and cable capacitance, yes it will effect the frequencies of interest for modulars. That 11pf parallel cap is about perfect for those other capacitances. Though if they turn out to be different, then that cap needs to change to match. That's why probes usually make that capacitance variable, or make it a bit higher and add a variable one in parallel with the input. Either way you have a variable one to trim and flatten the response of the probe.

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Post by roglok » Thu May 17, 2012 4:25 am

That's why probes usually make that capacitance variable, or make it a bit higher and add a variable one in parallel with the input. Either way you have a variable one to trim and flatten the response of the probe.
I see. What would the "equalizer box" contain? Could I fit the resistor and trimmer capacitor in there, or would it have to be near the "tip"? I assume the "parasitic capacitor" is supposed to smooth out the cable capacitance?

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daverj
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Post by daverj » Thu May 17, 2012 1:56 pm

The simplest way is if you can make the capacitor in the tip be a variable capacitor, or a fixed in parallel with a variable, so you can adjust it's value to get a good square wave on the scope.

The resistor and compensation cap need to be at the tip. They separate the circuit being probed from the scope cable capacitance and the input capacitance of the scope.

Or you could go buy a 10:1 probe and one of these.


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roglok
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Post by roglok » Thu May 17, 2012 2:28 pm

Cheers, Dave.

I actually just took apart an old, broken Hameg probe. It's a very simple assembly, pretty much like your suggestion: a variable cap and 390Ω resistor in parallel to a 9MΩ resistor inside the tip and a 100Ω resistor on the other side.

The 9MΩ resistor looks very interesting; it's a flat rectangle:

Image

The coax cable has a single, very thin solid wire - it's impossible to reuse it without some special crimping gear. It would break immediately when soldered straight to a jack, so i'll have to find some replacement. Also, the variable cap won't fit into the Rean jack housings I have here...
Or you could go buy a 10:1 probe and one of these.
That was my first thought, but I want jacks on both ends, so the scope can be plugged directly into the modular outputs...

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roglok
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Post by roglok » Sat May 19, 2012 6:37 pm

FTR:

I got some flexible 75Ω coax cable and put the resistor and variable cap inside a little box that was filled up with epoxy. It works pretty well. Thanks again, Dave! :yay:

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daverj
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Post by daverj » Sun May 20, 2012 3:01 pm

Looks great

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AST
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Post by AST » Tue Mar 20, 2018 2:34 am

Darn, they dont sell that gameboy oscilloscope kit anymore. Must be some other kit one can use.
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