EEVblog Electronics Community Forum
Electronics => Projects, Designs, and Technical Stuff => Topic started by: Christopher on April 09, 2015, 05:42:42 pm
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All,
I am looking to create my own crude-ish 10:1 probe to embed inside some test gear.
The story is: I have a 0-60V square wave which is coming out of a product into a scope. Originally I thought No problem, just connect some coax straight into a scope. It works! But the scope can only handle 0-20V on the display. So we have to divide it down by 10:1. The impedance of the output is fairly low.
Just putting a resistive 10:1 attenuator got rid of all of the HF content, obviously.
The scope input impedance is 1M || 15pF. Can I put a series resistor and compensation cap across the series resistor? My maximum frequency is about 1Mhz.
I have drawn an example of my circuit. What do you think about my idea?
(http://i.imgur.com/Lz2S00k.png)
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thats how they do it in many 10:1 probes, the most work is in compensating it to the right BW and attenuation. alas the higher quality probes dont use the normal joe coax cable like that.
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It all depends on the frequency content and the length of the coax.
With the design as you have shown it, the 50ohm coax is improperly terminated, and this will manifest itself as a ripple in the amplitude vs frequency response of your probe+scope. Professional scope probes avoid that by replacing the "good" transmission line with a very lossy transmission line of very special coax (>100ohms/metre DC resistance, centre conductor not being straight, etc). To see those effects, you can quickly model the system using a lossless transmission element. Fundamentally, it is difficult to make a good homebrew high impedance passive probe.
An alternative, since you have a low impedance source, you can easily homebrew a "low impedance Z0" passive probe - and these have remarkably good frequency response; 40 year old professional probes had 1.5GHz bandwidth, and modern ones have 6GHz bandwidth.
FFI, have a look at the references at https://entertaininghacks.wordpress.com/library-2/scope-probe-reference-material/
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Since you have a low-impedance driver, you could simply use a 200-450 ohms attenuator in your product and a proper 50 ohms cable termination at the scope. That way, you get either 5:1 or 10:1 attenuation, solve your improper cable termination issue and won't need to worry about compensation - at least not until you start looking at 100+MHz.