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Do it yourself 50 ohm BNC to alligator clip cable for function generator
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tautech:

--- Quote from: robsims on July 11, 2019, 03:05:26 am ---One thing i still don't understand is the 50 ohm. What do they mean by 50 ohm on the coax cable. The resistance of the cables is not 50 ohms. I read on the net that it is for better transfer of electronic signals.

--- End quote ---
It's not about resistance but impedance.
So a 50 ohm impedance source >50 ohm impedance cable (and connectors)>50 ohm load is an impedance matched system. It really matters for fast frequencies and fast edges.

Have a study here:
https://www.eevblog.com/forum/testgear/show-us-your-square-wave/
robsims:
Thanks Tautech, i get it now
MarkF:

--- Quote from: robsims on July 11, 2019, 02:55:16 am ---My scope has 2 cables, for the two outputs. Can i use one of the scope cables, because i think the scope cables are of higher quality? If not, than i will make the cable for the function generator myself. I will use the 50 ohm coax cable, with the BNC connector. I will make sure the cables to the alligator clips will be short.

--- End quote ---

If you are talking about 1x/10x scope probes, no you can not use them.
See a video Dave did on scope probes (time 17:00)

   
radiolistener:

--- Quote from: robsims on July 11, 2019, 03:05:26 am ---One thing i still don't understand is the 50 ohm. What do they mean by 50 ohm on the coax cable. The resistance of the cables is not 50 ohms. I read on the net that it is for better transfer of electronic signals.

--- End quote ---

this is impedance, it depends on relation of distributed inductance to distributed capacitance of the cable.

In simple words, it means that cable will have 50 Ohm for just very short period of time, this short period of time is the time of electromagnetic wave propagation forth and back in the cable. As you know, it is moving with a speed of light, this is very-very fast. So, you can measure such 50 Ohm for very-very short period of time. When the wave come back to the input of cable it will change it's input resistance. The new resistance will depends on frequency, cable length and load resistance on the second end of the cable.

So, you cannot measure impedance with multimeter, because it's too slow for such measurements. There are exists special equipment for such measurements. For example, vector analyzers. You can also measure it with LC meter.

Actually the things is a little more complicated, because wave propagation speed also depends on distributed inductance and distributed capacitance of the cable. What you're needs to know is relation between impedance and distributed inductance and distributed capacitance in transmission line:

Z = sqrt( L / C )

and wave propagation speed:

v = 1 / sqrt( L * C )

Where:
L - distributed inductance [H/m]
C - distributed capacitance [F/m]

You can use LC meter in order to measure L and C values. Use long piece of cable for better precision (for example 5-10 meters), because short piece of cable has too small capacitance and inductance and it will be hard to measure.

Then measure L of the cable shortened on the second end. And measure C of the cable open on the second end. Then divide these values with cable length. For impedance you can just ignore cable length and use total L and C of the cable. For wave propagation speed you're need to divide it with cable length.

Usual RG58 coax cable has about the following L and C:
C = 100 [pF/m]
L  = 250 [nH/m].

It may vary for different manufacturer. Let's calculate it's impedance:

Z = sqrt( 0.000000250 / 0.000000000100 ) = sqrt( 2500 ) = 50 [Ohm]

v = 1 / sqrt( 0.000000250 * 0.000000000100 ) = 1 / sqrt( 0.000000000000000025 ) = 1 / 0.000000005 = 200'000'000 [m/s]

As you know, speed of light in the vacuum is 299'792'458 [m/s], so the wave propagation speed in RG58 cable will be slower than  speed of light in the vacuum. The relation of this speed to the speed of light in the vacuum is known as velocity factor of the cable:

RG58 velocity factor = v / c = 200'000'000 / 299'792'458 = 0.667

In the same way you can calculate impedance and wave propagation speed in the vaccum. Distributed inductance and capacitance of the vacuum are known as physics constants - magnetic permeability and electric permittivity of the vaccuum:

L = μ0 = 4 * pi * 1e-7  [H/m]
C = ε0 = 8.8541878128e-12 [F/m]

Z = sqrt( (4 * pi * 1e-7) / 8.8541878128e-12 ) = 376.73 [Ohm]

v = 1 / sqrt( (4 * pi * 1e-7) * 8.8541878128e-12 ) = 299'792'458 [m/s]

This 376 [Ohm] is known as impedance of free space.
And this 299'792'458 [m/s] is known as speed of light in the vacuum. :)

Now you're ready to understand - simple wires with alligators will have random and variable inductance and capacitance through it's length. This is why it's impedance will not be constant through it's length. And this is why it so much affects measurements for high frequencies.
robsims:
Many thanks for the info radiolistener. Okay, i will make the cable. The 2 meter 50 ohm coax cable i bought is good. I have already attached the BNC connector. Tomorrow i will attach the alligator clips.
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