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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