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Hi Andreas,
Yahoo doesn't like to transfer the complete messages today even though the delay isn't too great. I completed the message history from the picture and hope it gets through.
To make your own BNC feed through get one male and one female 50 Ohm connector and an inductance free 50 Ohm resistor. Then connect them all in parallel. Instead of one 50 Ohm resistor perhaps use two100 Ohm resistors in parallel. The resistors must be able to dissipate the power at the max voltage across them. With a low impedance current shunt this voltage won't be great. The 50 Ohm feed through must be placed as close as possible to the input of the scope's input amplifier which means the BNC input terminal of the scope.
For high speed oscilloscope probes the inner conductor of the coax cables are made of resistive wire to dissipate energy along the wire. This reduces reflections when the signal reaches the ends of the cable.
I know of a compensation device like used in oscilloscope probes (i.e. like 1:10, 1:20, 1:50, 1:100 etc.). But this device is for audio signals. He tells that when it's correct adjusted the sound picture has depth end everything placed correct. He got the idea from oscilloscope probes where you want to see the signal on the screen like how it is at the probe tip. Never mind how it looks along the signal path as long as the end result is correct. That's what the compensation is for. I.e. when the compensation is correct adjusted an input square wave looks like a square wave on the screen. Without the compensation the corners of the square wave are round. With too much compensation the corners becomes pin shaped.
Regards
Ole
---In [email protected], <freeen2012@...> wrote :
Hi Ole,
On 22 Nov 2018, at 18:51, onielsen@... mailto:onielsen@... [EVGRAY] <[email protected] mailto:[email protected]> wrote:
Both ways can be used. For low inductance of a current shunt let the return wire go close to and in parallel with the shunt resistor to minimize any area for the magnetic field.
Here the return wire is twisted around the current shunt.
Also the 50 Ohm feed through is necessary. Without it a lot of reflections will happen unless the scope itself has a 50 Ohm input. As scope inputs usually are 1M Ohm the signal w ill be reflected due to the great impedance mismatch from 50m Ohm to 1M Ohm and the signal will drown in noise. 50m Ohm to 50 Ohm reduces the reflections a lot.
Thanks for the pic (illustrates it well). I already have such a 50 Ohm feed through but never really understood what exactly it does and therefore have never used it. I understand it has an impedance of 50 Ohm like the coax cable to prevent reflexions, yes, but how does it transfer the potential difference to be measured (over the shunt) to the 1M Ohm impedance input of the scope. If we had to build such a 50 Ohm feed through what would we have to do (what is it composed of)? This might help me to understand how it works.
Thanks a lot,
Andreas