Author Topic: 50 ohm terminators  (Read 9172 times)

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Offline BillWojo

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50 ohm terminators
« on: October 09, 2014, 06:55:01 pm »
So after watching Daves video on connecting a frequency generator to your scope I went on the hunt for some terminators and some decent cables. Came across 2 HP 50ohm cables on Ebay, a 24" and a 48" for 28.00 for the set, both tested to 400Mhz with a scope says the seller. Found a Tektronix 2W 50ohm feedthrough for 18.00. That came in today, looks new. The regular HP or Tektronix 2W 50ohm terminators are pretty expensive used. Instead I bought a few Mini-Circuits Karn-50CH N 2W 50ohm terminators, new for 3.00 each and a few N to BNC adapters for 3.89 each.
Will using the N style terminators with the adapters cause any problems? Hope I didn't waist my money.
Anything else I should keep my eye out for?
Thank you

BillWojo
 

Offline radhaz

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Re: 50 ohm terminators
« Reply #1 on: October 09, 2014, 11:16:32 pm »
I'm interested in this as well. Is there info online regarding equipment impedance?
 

Offline T3sl4co1l

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Re: 50 ohm terminators
« Reply #2 on: October 10, 2014, 12:57:45 am »
Brand name cables aren't necessary (HP just sounds ridiculous, as if Pomona weren't bad enough!), as long as they meet the specs for the type of cable and connectors used (i.e., 50 ohm RG-whatever or equivalent; normal BNCs).

Beware with terminators: the distance between stub-to-terminate, terminator, and tee must be smaller than the smallest event you can measure or generate.  Obviously, a terminator stacked on a bunch of tees and gender changers and cables isn't going to work as well as one direct on the load; likewise, you can't just terminate a line in the middle, it needs to be as close to the reflection (stub end) as possible to do its job.

I recently noticed that my scope's internal terminator does a substantially better job than an external tee and terminator; the difference seems to be much more than the stub lengths can account for.  My terminators are probably just crap.

Tim
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Offline BillWojo

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Re: 50 ohm terminators
« Reply #3 on: October 10, 2014, 01:53:30 am »
Well since I don't have the tools to make my own cables and the only ones I could find on Ebay are either respected name brand cables or 5 dollar cables of dubious quality, I purchased the best I could at what I thought was a reasonable price. I'm just learning and one thing I don't want is a defective product causing me problems.
 At this stage I wouldn't know that the bad signal is a cable problem or something I'm really looking at. As an industrial mechanic I know the value of purchasing good tools and have learned that the bargain tools are rarely a bargain. And I buy a lot of my tools second hand at flea markets, craigs list or Ebay. Servicing industrial machine tools is my bread and butter, I'm trying to learn more of the control end of the machines.

BillWojo
 

Offline radhaz

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Re: 50 ohm terminators
« Reply #4 on: October 10, 2014, 03:49:35 am »
Probe Master makes what looks to be a series of good products. Pretty good prices for American made stuff.
http://www.probemaster.com
 

Offline DanielS

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Re: 50 ohm terminators
« Reply #5 on: October 10, 2014, 03:52:09 am »
Servicing industrial machine tools is my bread and butter, I'm trying to learn more of the control end of the machines.
For most electromechanical industrial stuff, you will rarely need to look at signals with frequencies much beyond 10MHz. At such low frequencies, almost any reasonable piece of wire will be good enough. If you plan to probe around mains voltages, a 100X high-voltage probe or two might be a good idea. If you need to measure floating voltages in a circuit, differential probes could be useful too.
 

Offline nixfu

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Re: 50 ohm terminators
« Reply #6 on: October 10, 2014, 03:57:56 am »
I'm interested in this as well. Is there info online regarding equipment impedance?

Watch this.  Forum member Alan/w2aew has a good video about it.  50ohm impedance is very important for those of us that deal with radio stuff.

« Last Edit: October 10, 2014, 03:59:57 am by nixfu »
 

Offline XFDDesign

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Re: 50 ohm terminators
« Reply #7 on: October 10, 2014, 02:11:49 pm »
For most electromechanical industrial stuff, you will rarely need to look at signals with frequencies much beyond 10MHz. At such low frequencies, almost any reasonable piece of wire will be good enough.

I beg to differ and load it with, "it depends."

 In characterizing an OXCO product I've been working on, I built up some prototypes of the 10.000MHz output driver. Un-terminated, the following is the 10MHz output on 1m of Tenoslite coax. At 10MHz, the effective electrical length is 20.4m so 1/20th of a wavelength shouldn't matter... right?

This is pretty ugly, and the chatter right in the crossover zone may lead to glitching. This is the result with not even "any old wire" but relatively good quality coax. Let's see what happens when that same output is put to the scope directly, with essentially a BNC barrel stub into the scope, still unterminated:


Similar issues, and while the waveform is "usable" it has no capacity for transmission length. How about the primary use-case when it's properly terminated, and using the 1m of Tensolite?


Perfect.

What's the catch? "10MHz" is relative. If it's a sine-wave, then a stray piece of wire may be sufficient. Start moving to square waves, and you get very rich harmonic content depending on your driver's capabilities. In this application, I needed razor sharp edges and minimal jitter as primary requirements. A wish-list item was to have something that was viable for standard logic level operation in an unterminated condition. Once you're talking about square waves, especially ones with very fast edges, you have a lot more than just 10MHz, but 30, 50, 70, 90, 110... etc. With some degradation, you can pick a coax which has minimal losses for at least 5x your fundamental.

OP mentioned "400MHz" which is actually kind of meaningless. These things don't just "brick-wall" stop operating, but rather have a spec on losses vs. frequency. It is these losses which screw with the signal seen on the other end of the coax.
 

Offline T3sl4co1l

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Re: 50 ohm terminators
« Reply #8 on: October 10, 2014, 08:07:08 pm »
Well, yeah, when you put a signal with significant harmonics up to ~200MHz, of course that should happen. Protip: know your signals and Fourier transforms offhand (quantitatively, inside and out, is not necessary, but being aware of the time-frequency duality is), and know how to estimate bandwidth from risetime and vice versa. :)

Tim
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Offline Marco

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Re: 50 ohm terminators
« Reply #9 on: October 10, 2014, 08:34:37 pm »
I recently noticed that my scope's internal terminator does a substantially better job than an external tee and terminator; the difference seems to be much more than the stub lengths can account for.  My terminators are probably just crap.

Don't the more expensive scopes have a impedance matched circuitry in 50 Ohm mode? You obviously can do better than throwing 50 Ohm in parallel to ~10 pf.
« Last Edit: October 10, 2014, 08:40:56 pm by Marco »
 

Offline pascal_sweden

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Re: 50 ohm terminators
« Reply #10 on: October 10, 2014, 09:09:04 pm »
Can somebody here give a brief, but complete, explanation of terminators.
Basic purpose? When to use them? And when NOT to use them?
Things to look out for? Use case examples, etc.

« Last Edit: October 10, 2014, 09:13:16 pm by pascal_sweden »
 

Offline w2aew

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Re: 50 ohm terminators
« Reply #11 on: October 10, 2014, 09:40:20 pm »
Can somebody here give a brief, but complete, explanation of terminators.
Basic purpose? When to use them? And when NOT to use them?
Things to look out for? Use case examples, etc.

My video covers most of this


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Offline XFDDesign

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Re: 50 ohm terminators
« Reply #12 on: October 10, 2014, 09:40:41 pm »
Well, yeah, when you put a signal with significant harmonics up to ~200MHz, of course that should happen. Protip: know your signals and Fourier transforms offhand (quantitatively, inside and out, is not necessary, but being aware of the time-frequency duality is), and know how to estimate bandwidth from risetime and vice versa. :)

Tim

Right, that was kind of the point of my post?
 

Offline pascal_sweden

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Re: 50 ohm terminators
« Reply #13 on: October 10, 2014, 09:56:20 pm »
My video covers most of this
Nice video! Good job!

Here is a nice document about this subject matter =)
http://www.ultracad.com/mentor/transmission%20line%20terminations.pdf
« Last Edit: October 10, 2014, 10:09:12 pm by pascal_sweden »
 

Offline T3sl4co1l

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Re: 50 ohm terminators
« Reply #14 on: October 10, 2014, 10:12:15 pm »
Well, yeah, when you put a signal with significant harmonics up to ~200MHz, of course that should happen. Protip: know your signals and Fourier transforms offhand (quantitatively, inside and out, is not necessary, but being aware of the time-frequency duality is), and know how to estimate bandwidth from risetime and vice versa. :)

Tim

Right, that was kind of the point of my post?

Yep, so, newbies, don't look at the fundamental alone :)
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Offline G0HZU

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Re: 50 ohm terminators
« Reply #15 on: October 10, 2014, 10:37:04 pm »
Brand name cables aren't necessary (HP just sounds ridiculous, as if Pomona weren't bad enough!), as long as they meet the specs for the type of cable and connectors used (i.e., 50 ohm RG-whatever or equivalent; normal BNCs).

Tim

I don't think it's ridiculous to use branded BNC cables if you want to use the cables at RF frequencies or if you want to get reliable connections.

My advice to anyone is to avoid unbranded cables unless you are happy to make imprecise and unreliable measurements. Cheap unbranded cables are often unreliable and they just get worse with age because the connectors are made cheaply with poor quality materials and plating and the strain relief is poor quality and leads to unreliability in the long term.

Because of bitter experience with using crap unbranded cables or handmade cables I try and only use branded cables or adaptors here.  Eg most of mine are HP or Suhner or Radiall or MiniCircuits brands. I do have a few crappy unbranded cables but I'm very careful how and where I use them. However, if anyone can point out some cheap unbranded cables that are good quality then I'll try them out.




 

Offline G0HZU

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Re: 50 ohm terminators
« Reply #16 on: October 10, 2014, 10:41:56 pm »
One of the best ways to see how crap unbranded BNC cables are is to put them on a decent network analyser and wiggle the cable ends. Even brand new unworn ones can fail badly here in terms of resonances, poor (and wobbly) VSWR and higher (and unsteady) insertion loss.
« Last Edit: October 10, 2014, 10:54:35 pm by G0HZU »
 

Offline DanielS

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Re: 50 ohm terminators
« Reply #17 on: October 11, 2014, 12:47:14 am »
For most electromechanical industrial stuff, you will rarely need to look at signals with frequencies much beyond 10MHz. At such low frequencies, almost any reasonable piece of wire will be good enough.

I beg to differ and load it with, "it depends."
It does not "depend" that much when OP says he is an electromechanic who wants to expand somewhat beyond what he currently does. That tells me the bulk of what he will be dealing with assuming he sticks mostly to the machine aspect will be sensors, position encoders, switches, motor drives, PLCs, etc., most of which operating in the (k)Hz range.

As for your nice 10MHz waveforn, try the same thing with the 20MHz LPF turned on - OP works on industrial machinery and would need the 20MHz filter for most of the measurements I suspect he is likely to make.
 

Offline BillWojo

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Re: 50 ohm terminators
« Reply #18 on: October 11, 2014, 05:51:00 pm »
Hi DanielS, yes, most of what I have in mind is work related but I also want to keep myself open to other projects as well.
Today I received the Mini-Circuits KARN-50CN+ terminators, still waiting on the adapters. I have to say the quality of these is awesome as far as the build is concerned. Very well machined indeed. The HP cables came in yesterday and they look very nice to.
T3sl4co1l warned about stacking parts on the termination end, I guess it has to do with the wave length of what your looking at. I'm looking at using a "T" into the scope with the cable at one end, the adapter and the terminator at the other end of the "T". Any problem with that? I'm using a 100MHz Tek 465M scope.
Thanks

BillWojo
 

Offline pascal_sweden

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Re: 50 ohm terminators
« Reply #19 on: October 25, 2014, 10:14:28 pm »
Can somebody here give a brief, but complete, explanation of terminators.
Basic purpose? When to use them? And when NOT to use them?
Things to look out for? Use case examples, etc.

If you connect a signal generator directly to the scope, you can still use the proper impedance in the scope to properly terminate without an extra terminator, right?
 

Offline BillWojo

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Re: 50 ohm terminators
« Reply #20 on: October 26, 2014, 04:19:57 am »
Only if the scope gives you that option. Thats why I purchased some terminators for my scope. Dave has a great video on doing just that and shows the problems with improper termination.

BillWojo
 


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