[TriEmbed] Scopes, probulators, and matchmaking

[Brian]

Hi Mauricio!

My advice would be to go with an off-the-shelf probe and just not worry 
about the impedance mismatch; you can do a few calibration tests with 
known signal sources if you want to find the correction curves.  That 
said, read on...

The very best cable for the job would be small coax such as RG-159. 
However, you'd be hard-pressed to find that locally, I think, and it's 
probably not too cheap in the lengths you'd want.

You could try microphone cable, at a cost of having a fairly bulky lead 
on your probe and possibly fairly low bandwidth.  Mic cable is typically 
two-conductor twisted pair plus a shield, and generally designed to 
perform best with audio frequencies.  I'd run signal and ground along 
the pair, then connect the shield to chassis ground only on the scope end.

You might consider flexible Cat-5 cable.  You'll have a bunch of unused 
conductors, but better bandwidth than your typical mic cable.

As far as impedance matching, that can be done as simply as a 
resistor-divider network, or with a transformer, or with an active 
op-amp circuit.  Again, the determining factors will be the signal 
quality and bandwidth you require.  A simple resistor network is cheap, 
but will likely be an entrypoint for environmental noise (EMI) and will 
divide the signal you see (with the impedances you've named, it would 
make your 1X probe effectively 2X).  A transformer would be expensive 
and useless for measuring DC.  Your best bet would probably be to find 
an active impedance-matching servo circuit, which you could tune for the 
bandwidth and noise-rejection you need, at the cost of, well, cost!

On the other hand, unless you're measuring quite high frequencies, the 
reflections caused by the impedance mismatch are not likely to be a 
problem; you'll only observe a lower-than-expected signal level, and 
there are equations (that do not live in my head) to help determine what 
the difference would be.

In any case, good luck!

Cheers,
-Brian

On 07/27/2018 09:00 AM, Mauricio Tavares via TriEmbed wrote:
>        So I found something I have not seen in ages: my DSO Nano V1
> scope. Yes, v1 as in out of production many moons ago. Here is a link
> to admire its prehistoricness:
> 
> https://www.seeedstudio.com/DSO-nano-Pocket-size-digital-storage-oscilloscope-p-512.html
> 
> The probes are rather noisy; if you look how they were make you can
> see why. I was thinking on making new ones using some kind of flexible
> shielded wire (gnd can be the shield), but what would be flexible
> enough? Is there some kind of, say, audio wire that is easily found
> locally and would fit the bill?
> 
> Second, what if I want to use a proper scope probe? If we forget about
> the connector, the main issue is impedance: AFAIK your normal
> garden-variety 1X probe is 1MOhm, but the little scope guy was
> designed for whatever reason to have 500KOhm impedance. How to match
> it and where would be the cleanest place to do it?
> 
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