extending scope probe leads

[Simon]

We need to put a device in a blind test chamber but need to see waveforms on a scope, what is best practice for extending the probe cables, can it be done with an off the shelf 50 ohm coax cable with bnc connectors ? loooks like those available are female to female so will need a male-male adapter as well.

Quality of signal integrity is not a huge concern as we just need to measure the drive frequency to a motor to determine it's speed.

[Andy Watson]

Quality of signal integrity is not a huge concern as we just need to measure the drive frequency to a motor to determine it's speed.

What's driving the signal? If it has enough capability I would simply use a length of 50 coax. Crimp a male BNC on one end to match the scope and leave other end open, or insert a 100 R in series to isolate the capacitance. If you want a bit more quality, make a resistive divider for the measuring end - 510R and 56R should match the line reasonably well.

[tszaboo]

You can test your theory by connecting the extended lead to the probe compensation 1 Khz output.
I dont see a reason why it would not work up to some 50-100 Mhz.

[jahonen]

Ordinary coax can't be generally used to extend scope probe cables, because scope probe cable is special with high resistance inner conductor to force it to use RC-mode propagation instead of LC propagation. Using RC mode dampens the unterminated reflections so they are not a problem.

It might be better to use "Z0 probe"-like construction (suitable series resistance right at the point of the measurement, ordinary 50 ohm coax and 50 ohm termination at the scope end) than try to extend the cable of an existing probe.

But if the signal integrity is not a problem, you might still try if that extension works for you or not.

Regards,
Janne

[Simon]

I'm looking at running the scope inside the chamber and controlling/viewing it from a laptop. I think we are looking at up to 100 KHz

[tggzzz]

We need to put a device in a blind test chamber but need to see waveforms on a scope, what is best practice for extending the probe cables, can it be done with an off the shelf 50 ohm coax cable with bnc connectors ? loooks like those available are female to female so will need a male-male adapter as well.

Quality of signal integrity is not a huge concern as we just need to measure the drive frequency to a motor to determine it's speed.

You can't simply extend a "normal" Mohm *10 scope probe's cable.

Consider:

• making your own *10 or *20 low impedance scope probe using a 450ohm or 950ohm resistor at the measurement end of a 50ohm coax cable, plus 50 ohm termination at the scope
• if you are only measuring frequency, could you avoid the problem by having a trivial digitiser, and then transmitting a digital signal by whatever means is convenient (e.g. ribbon cable probably with a 100ohm termination).

The first is quick, simple, effective, with high frequency response dictated by the standard coax losses.

[tszaboo]

Ordinary coax can't be generally used to extend scope probe cables, because scope probe cable is special with high resistance inner conductor to force it to use RC-mode propagation instead of LC propagation. Using RC mode dampens the unterminated reflections so they are not a problem.

It might be better to use "Z0 probe"-like construction (suitable series resistance right at the point of the measurement, ordinary 50 ohm coax and 50 ohm termination at the scope end) than try to extend the cable of an existing probe.

But if the signal integrity is not a problem, you might still try if that extension works for you or not.

Regards,
Janne

Interesting... I've tried my theory with a 1.5m 50 Ohm coax and a TEK P2200 probe, TDS2014B.

1x termination looks OK, rise and fall times are OK.

If you use 10x termiantion, it becomes a mess. No other setting was changed.
The probe shielding is around 0 Ohm (DC) while the inner part, without the tip is ~300 Ohm.

I might have been half wrong, in this case I would bring the measured signals outside and use the scope as is.
I guess I've learned something today.

[Simon]

We will just use a couple of wires, it's only 1 KHz anyway.

[G7PSK]

You can get long lead scope probes, I have  6 foot and a 9 foot probes by tektronix and they list a 12 foot one in the manual that came with the probes.

[Neilm]

I take it you can't put the scope in the chamber and use a remote control function on it?

[krivx]

I'm looking at running the scope inside the chamber and controlling/viewing it from a laptop. I think we are looking at up to 100 KHz

At 100 kHz you'll probably be fine with a twisted pair.

It should be pretty easy to verify this anyway, just measure the same signal with and without an extension.

[Christopher]

Whats wrong with a bit of twisted pair/RG178 coax tacked onto the board and straight into the scope at 1x ?

Surely it's only low frequency so the transmission line effect shouldn't be too great.

You may also experience ground loops. I have gotten over this by putting a small resistance in the coax return line so the scope isn't used as a return line for ground current. Or twisted pair with a diff probe.

[Simon]

I'll just use a pair of wires and clip the scope probe to them, I did suggest remote controlling the scope in the chamber but my boss was unsure of the idea.