Oscilloscope probe question & bypassing the probe.

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zebra50

Well-known member
Joined
Jun 4, 2004
Messages
2,943
Location
York, UK
Hi! I have a quick question.

I have a 'box' that outputs two audio frequency signals, which I have semi-permanently connected to a dual channel scope. Output of the box is sine wave of the order of 10 mV to 1V, and about 20Hz to 20KHz. The outputs share a common ground with the box.

Rather than use the scope's probes, are there any problems with just adding a couple of BNC connectors to the 'box' and connecting those directly to the scope inputs with a meter of 50 ohm coax? I don't need the system to be ultra accurate so long as the two channels are comparable, but it would be nice to be flat over the audio region. I know there are cable capacitance issues at MHz frequencies, but my rough calcs show that I ought to be OK (1 meg input, about 100pF capacitance).

Thanks!

Stewart
 
I have a similar arrangement tapped off the soak load that I use for testing power amps, never had a problem - except that I can't attenuate the signal into the scope if I want to, which is sometimes a bother.

 
Sounds fine to me; you'd only need the trim / X10 for more critical work at higher frequencies. Audio is easy-peasy (well, sometimes!).
 
Thanks guys. That's what I thought, or at least that's what I thought that I thought....

(I have actually done this in the past at MHz frequencies, but I was young and enthusiastic and didn't know how little I knew.)

rodabod said:
Audio is easy-peasy (well, sometimes!).

... If it wasn't for those pesky musicians!  :D


 
> 1 meg input, about 100pF capacitance

Do math. 100pFd against 1Meg is 1,592 Hz.

What you ALSO need to know is the box's OUTput impedance.

If true Zero, response into 100pFd is infinite.

If 1K, response is -3dB at 159KHz, generally ample for audio.

Personaly I like clip-leads, speaker-wire, whatever. No reason to use hi-spec coax for 3 feet of audio. The only issue is that 'scopes often come with BNC, and some BNC plugs mate easily with coax, not an old power cord. Then be aware that there are two "BNC"s, 50 ohm and 75 ohm. 'Scopes are usually 50 ohm. If you try a 75 plug you won't get good contact.
 
Thanks. I did that calculation first off, but as I understand it (and read), the 1meg at the input of the scope is in parallel with the capacitance. So the 'filter' is the output Z against the (cable + input) capacitances in parallel with the input resistor. Am I wrong?

(http://en.wikipedia.org/wiki/Oscilloscope#Inputs)
(http://en.wikipedia.org/wiki/RC_circuit#Parallel_circuit)

Output impedance is variable - but generally in the range 30R to a few K.

There's a useful little calculator here, although it seems to use Farads as the unit, so you need to add lots of zeros.

http://www.digitalfilter.com/products/jvcr/enjvcr.html

Cheers!
 
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