Do it yourself 50 ohm BNC to alligator clip cable for function generator - Page 1
Forum section: electronics / beginners

author: robsims
author_rank: Frequent Contributor
timestamp: July 10, 2019, 08:38:46 pm
content: Hi. i need a BNC to alligator cable for my function generator. I'm new to electronics and this is my first generator. I want to make the cable myself, because it will take up to 4 weeks to get a cable to my country. The local electronic shop doesn't have cables. I want to know if i'm on the right track. I bought a BNC connector, 2 meters of standard 50 ohm coax cable and a black and red alligator clip. What is the maximum length the cable can have? What do i need to do to make the cable perfect or what do i need to do to prevent making a poor quality cable?

author: tggzzz
author_rank: Super Contributor
timestamp: July 10, 2019, 09:05:01 pm
content: What frequencies and voltages do you want to use?The frequency determines the length, the voltage the cable type. As you are a beginner, I hope the latter is unimportant!

author: robsims
author_rank: Frequent Contributor
timestamp: July 10, 2019, 09:07:43 pm
content: I see on the net that a 50 ohm cable (coax cable) is the impedance of the cable itself. Do i need that cable, or a cable pair (normal cable) with a total resistance of 50 ohm?

author: robsims
author_rank: Frequent Contributor
timestamp: July 10, 2019, 09:13:10 pm
content: The generator is a siglent SDG 810. Max frequency is 10 mhz and max voltage is 20 volt pp. So i want to be able to do tests at max frequency and voltage

author: tautech
author_rank: Super Contributor
timestamp: July 10, 2019, 09:22:51 pm
content: Quote from: robsims on July 10, 2019, 09:13:10 pmThings to note, 20Vp-p is only available into a HiZ load.Whatever load setting you select, the source impedance is always 50 ohms.In the menus check that output protection is turned ON so that for a AWG beginner there is less chance of damaging the output stages if connection is to a powered circuit.The datasheet and manual has more info about this.For general LF and normal amplitude levels nearly any type of connection will work but for fast edges ringing can be a problem if the output is not terminated into 50 ohms.

author: thm_w
author_rank: Super Contributor
timestamp: July 10, 2019, 10:27:05 pm
content: Quote from: robsims on July 10, 2019, 09:07:43 pmThe cable will say "RG58" or something like that on the side, 50 ohm impedance, sounds like you have the right cable.Attenuation is 10dB at 50MHz and 100m length, so any sort of length you'd want to use will be fine (<5m).Heatshrink and some short wires to extend the leads out 5cm or so would be recommended.

author: robsims
author_rank: Frequent Contributor
timestamp: July 10, 2019, 10:44:57 pm
content: Hi Tautech, many thanks for the advice

author: robsims
author_rank: Frequent Contributor
timestamp: July 10, 2019, 10:55:35 pm
content: Yes thm_w, like "Coax cable 50 ohm", "RG58" is also is imprinted on the piece of coax cable. I bought 2 meters of the coax cable. Can i just connect/solder the BNC connector on one end and the alligator clips on the other end of the cable or do i have to take other things into consideration?

author: robsims
author_rank: Frequent Contributor
timestamp: July 10, 2019, 11:01:35 pm
content: Did not read you're post fully thm_w. Cable is fine and i need to extend it with short wires for the alligator clips.

author: tautech
author_rank: Super Contributor
timestamp: July 10, 2019, 11:03:23 pm
content: Quote from: robsims on July 10, 2019, 11:01:35 pmPay particular attention to strain relief.

author: robsims
author_rank: Frequent Contributor
timestamp: July 10, 2019, 11:29:00 pm
content: Many thanks for you're help Tautech and thm_w. I'm going to make the cable right away.

author: radiolistener
author_rank: Super Contributor
timestamp: July 11, 2019, 01:37:50 am
content: alligator clips will be unusable for frequencies more than 1-2 MHz. High frequency should be delivered through transmission line with constant impedance. Coax line has constant impedance and it can be used, BNC connector also can be used. But usual wires with alligators cannot have constant impedance it will leads to random reflections, interference and radiation loss. You can use it with no issues for DC voltage or at frequencies below 100 kHz.You can buy such cable on aliexpress:https://www.aliexpress.com/item/32816571566.htmlIt's price is just 1-2 usd.if you decide to build your own cable, then try to make wires from coax cable to alligators as short as possible, it will reduce negative effect on high frequency.

author: MarkF
author_rank: Super Contributor
timestamp: July 11, 2019, 01:55:47 am
content: F.Y.I.You might like a BNC to Minigrabber Test Lead instead of the alligator clips.Or one of each:BNC Test Leads Set

author: robsims
author_rank: Frequent Contributor
timestamp: July 11, 2019, 02:55:16 am
content: 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.

author: robsims
author_rank: Frequent Contributor
timestamp: July 11, 2019, 03:05:26 am
content: 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.

author: tautech
author_rank: Super Contributor
timestamp: July 11, 2019, 03:10:48 am
content: Quote from: robsims on July 11, 2019, 03:05:26 amIt'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/

author: robsims
author_rank: Frequent Contributor
timestamp: July 11, 2019, 03:48:24 am
content: Thanks Tautech, i get it now

author: MarkF
author_rank: Super Contributor
timestamp: July 11, 2019, 04:25:09 am
content: Quote from: robsims on July 11, 2019, 02:55:16 amIf 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)

author: radiolistener
author_rank: Super Contributor
timestamp: July 11, 2019, 04:28:43 am
content: Quote from: robsims on July 11, 2019, 03:05:26 amthis 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 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 = 0.667In 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 permeabilityandelectric permittivityof 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 v = 1 / sqrt( (4 * pi * 1e-7) * 8.8541878128e-12 ) = 299'792'458 [m/s]This 376  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.

author: robsims
author_rank: Frequent Contributor
timestamp: July 11, 2019, 04:30:20 am
content: 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.

author: tggzzz
author_rank: Super Contributor
timestamp: July 11, 2019, 07:20:30 am
content: Quote from: robsims on July 11, 2019, 02:55:16 amYou need to understand the basic theory and practice of different types of scope probes. I suggest you have a look at the links athttps://entertaininghacks.wordpress.com/library-2/scope-probe-reference-material/I also recommend you have a look at some of those safety links, so that you don't accidentally cause loud noises and clouds of smoke.

author: bsfeechannel
author_rank: Super Contributor
timestamp: July 11, 2019, 06:49:03 pm
content: There is a nice tutorial embedded in the video below on how to solder alligator clip leads to coaxial cable.It starts at 22min and goes up to 30min.

author: robsims
author_rank: Frequent Contributor
timestamp: July 11, 2019, 10:07:57 pm
content: Very informative vid . Nice to see how to attach alligator clips the proper way.

author: Gyro
author_rank: Super Contributor
timestamp: July 12, 2019, 08:53:05 am
content: Quote from: robsims on July 10, 2019, 09:13:10 pmIf you want to preserve maximum signal integrity at the top end of the frequency range (say, a 10MHz squarewave), then it's useful to buy or make a BNC to BNC lead too and buy a BNC socket with a ground solder tag.Tack the socket onto the edge of your PCB prototype or whatever with the solder tag and solder a 50 ohm resistor (47 or 51 will do) between the centre pin and the tag. You can the take the signal straight to the input point of your circuit using a very short wire.As others have mentioned, you won't get the full 20V p-p at 50 ohms (the resistor would fry for one thing) but you will get the cleanest possible high frequency signal.Having both lead options will cover you for all practical circumstances.You can also hook your scope probe to the resistor leads if you just want to look at the signal.