Products > Test Equipment
making an 20:1 coax probe
<< < (5/10) > >>
tggzzz:

--- Quote from: Zeyneb on July 09, 2022, 05:55:41 pm ---I didn't thought I would get so many responses on this topic.

Ok, my 200MHz scope input is 1Mohm and 13pF. I do have two channels.
Still, some parts of the discussions go over my head.

Maybe G0HZU is right and if RG-316 is easier to solder I might go for that. And I also agree with what Bud said and just buy a coax that has the BNC connectors.

--- End quote ---

But consider not cutting the cable at all. From Gyro's post https://www.eevblog.com/forum/projects/lo-z-probe/msg801772/#msg801772



Or don't modify the cable but solder your resistor and ground stake to one of these, and attach the standard cable to that in the usual way. That gives you easily changeable tips.


gf:

--- Quote from: G0HZU on July 09, 2022, 03:38:54 pm ---The input VSWR of my Infinium scope is very low when the internal 50R mode is selected but it does degrade up towards V/UHF. I would recommend placing an attenuator ahead of the scope input for critical measurements. This does demand a scope with a low noise input. Maybe try 3dB, 6dB or 10dB to see if it helps. I found that using an SMA connector on the end of the cable, then an SMA attenuator followed by a decent SMA to BNC adaptor and then selecting the internal 50R termination works the best for me.

--- End quote ---

If the termination on the scope side of the cable isn't perfect (e.g. freed-through terminator on a capacitive high-Z input), then an additional 50 ohm source termination on the cable also helps to keep reflections from bouncing back and forth (i.e. using a resistive divider at the probe tip, instead of a series resistor only).
G0HZU:
I do think it's worth it to try and minimise the VSWR at the scope end of the cable. I've also used the SMA end launch system shown by tggzzz and it means you can use a good quality RF cable. Minicircuits make an inline SMA DC block when using it with a spectrum analyser. It does become quite stiff to use if a really good cable is used.

Even with a 200MHz scope I think there can be issues with the integrity of the displayed waveform because the reflections that occur with a poorly terminated system happen within the cable and this is independent of the scope bandwidth. It's a function of the mismatch at the scope input. So a fast pulse edge will hit the discontinuity at the scope input and this will cause energy to slosh back and forth through the cable. This can cause a blip in the scope trace several nanoseconds after the first edge.

A lot depends on the rise-time of the pulse being measured. A 200MHz scope could have a rise-time of just under 2ns so when probing digital signal with a similar rise-time I think the above blip issue could occur. It probably won't be that significant but I'd expect it to spoil the look of the 'flat' section just after the pulse edge.
If measuring much slower rise-time waveforms (eg 10ns rise-time) then there probably won't be a problem. If using the probe for general sniffing of fast logic then I think even a 100MHz scope can be affected to some degree. If a really short cable is used (eg 30cm) then the blip reflection could begin to eat into the rising edge displayed on the scope but a lot depends on the rise-time of the waveform being measured.
tggzzz:
This topic is a great use for spice, since it enables a quick sanity-check of your understanding of theory.

All simulations below are with a 6ns 50ohm delay line and a 450ohm resistor at the source end of the cable and a 50ohm terminator by the scope. The variants are time domain vs frequency domain, scope input capacitance 0.1pF (proper 50ohm scope input) 12pF ("high" impedance scope input), with or without a 50ohm termination at the source end of the cable, and source end termination but note scope end termination. Look at the schematic to see which is which.

Note the very significant differences in the frequency domain Y axis! (Click to embiggen :) )

What is and isn't acceptable is left as an exercise for the student.


















sicco:
If your scope does not have a 50 ohms input option (selected) then you do need to add something like this on the scope BNC connector:

https://www.distrelec.nl/en/feed-through-termination-50-ohm-2w-rohde-schwarz-hz22/p/11085041?ext_cid=shgooaqnlnl-p-shopping-fallback&gclsrc=aw.ds&?pi=11085041&gclid=EAIaIQobChMIiK7X08Tu-AIVy513Ch2y8wCQEAQYASABEgJO2_D_BwE

A 50 ohms coax cable connected to just a 1 Mohm scope input is a pure reflection point for sharp fast pulses. The coax cable is a transmission line, a waveguide. Waveguides shall be terminated, on both ends, with a resistor that is as many ohms as the waveguide’s characteristic impedance. 50 ohms that is for the coax you’re considering to use. For a Z0 probe, one can omit the 50 ohms resistor in the probe because there is no need to cancel echoes from what might travel as pulse/edge in the cable back from scope to probe. But then we must first guarantee that nothing could have bounced at the scope BNC input. The 50 ohms scope input resistor does give that guarantee. If it is there…

The spice models posted just above confirm it all…
Navigation
Message Index
Next page
Previous page
There was an error while thanking
Thanking...

Go to full version
Powered by SMFPacks Advanced Attachments Uploader Mod