Author Topic: 50 Ohm feed-through terminator  (Read 5216 times)

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Offline jgalak

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50 Ohm feed-through terminator
« on: December 16, 2017, 02:57:56 pm »
Newbie here.  My scope (Rigol DS1054Z) doesn't have 50 Ohm termination, so I need some external feed-through terminators.  As it's only a 100MHz scope (unlocked), they don't need super high bandwidth, but I do want something that won't create distortion or high SWR.  Can folks recommend something for this application?

Pasternack sells some (https://www.pasternack.com/2-watts-bnc-male-feed-thru-load-1000-mhz-pe6tr000-p.aspx) for about $35 each, Rigol itself has some for $19 each.

And then there's these: https://www.aliexpress.com/item/P7001-2PCS-High-quality-BNC-to-BNC-Female-seat-50KY-50-ohm-matching-device-Q9-adapter/32661228102.html for $4.55 each....

Since I want to get 4, the lower price is tempting, but will they be crap?  Or does it not matter at this bandwidth?

Mostly planning on looking at digital circuits, audio signals, and RF signals below 100MHz.

(note: Yes, I know that I can build them.  I'd prefer not to do so right now.  I need working test gear, not more projects)
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Offline Hydron

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Re: 50 Ohm feed-through terminator
« Reply #1 on: December 16, 2017, 03:35:21 pm »
I think there's already a thread about the cheap ones.
 

Online tggzzz

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Re: 50 Ohm feed-through terminator
« Reply #2 on: December 16, 2017, 04:15:41 pm »
Audio signals won't need 50ohms.
Digital signals will only need 50ohms if you are using a "low impedance Z0" "resistive divider" probe. (Which are much better than "high impedance" passive probes, and can be homemade).
RF signals will still be affected by the 15pF in parallel with a 50ohm terminator (unless you "isolate" it with an attenuator).
And for RF signals, what do you expect to measure with an 8-bit ADC?

W.r.t. cost. Consider what else you need to buy, and whether a cheap purchase might be a sufficient learning tool. One way or another you will pay for your experience!
« Last Edit: December 16, 2017, 04:17:23 pm by tggzzz »
There are lies, damned lies, statistics - and ADC/DAC specs.
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Offline tooki

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Re: 50 Ohm feed-through terminator
« Reply #3 on: December 18, 2017, 01:12:45 am »
I think there's already a thread about the cheap ones.
That’s nice. Do you have a link? I once looked for this topic and found nothing...
 

Offline SkyMaster

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Re: 50 Ohm feed-through terminator
« Reply #4 on: December 18, 2017, 01:15:55 am »
I think there's already a thread about the cheap ones.
That’s nice. Do you have a link? I once looked for this topic and found nothing...

Could be that one: https://www.eevblog.com/forum/testgear/banggood-50-ohm-bnc-feed-through-terminator-a-quick-review/msg1250379/

 :)
 

Offline xrunner

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Re: 50 Ohm feed-through terminator
« Reply #5 on: December 18, 2017, 01:47:19 am »
I ordered two pair of these from Hong Kong (Ebay)- $4.79 a pair. I haven't got them quite yet, been 3 weeks ... but they are probably the same as has been reviewed elsewhere here.

50 ohm feed through
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Offline xrunner

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Re: 50 Ohm feed-through terminator
« Reply #6 on: December 18, 2017, 10:35:36 pm »
They had delayed the order because their supplier had "bad quality control". I guess they didn't solve that problem. One of the four seems to be a little off.  ???
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Offline Hydron

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Re: 50 Ohm feed-through terminator
« Reply #7 on: December 18, 2017, 10:49:26 pm »
Open it up and resolder/replace the smd resistor which has cracked off. You get what you pay for, but mine are doing the job for what I need at the moment.
 


Offline Octane

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Re: 50 Ohm feed-through terminator
« Reply #9 on: December 18, 2017, 11:05:42 pm »
They had delayed the order because their supplier had "bad quality control". I guess they didn't solve that problem. One of the four seems to be a little off.  ???

I also order four of those from this ebay seller and three out of the four where 75 Ohm instead of 50 Ohm. So yes, they seem to have quality control problems.

Michael
 

Offline xrunner

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Re: 50 Ohm feed-through terminator
« Reply #10 on: December 19, 2017, 12:02:50 am »
Open it up and resolder/replace the smd resistor which has cracked off. You get what you pay for, but mine are doing the job for what I need at the moment.

That's what I thought too. When I took it apart to observe the failure it read 50 ohms. I looked at the SMD 100 ohm resistors and they didn't seem to be cracked. I put it together again, and it again read 100 ohms, and apart it read 50. I then made the assumption that the solder work on one of them was poor. I tried to re-flow the solder and on one it didn't take to the brass, so I put on a dab of flux and that indeed fixed the connection. I guess I got what I paid for, but since I was able to repair it I guess the price was OK.  :phew:
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Offline Mr. Scram

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Re: 50 Ohm feed-through terminator
« Reply #11 on: December 19, 2017, 12:06:15 am »
I ordered these cheap ones too. The price difference with "real" ones is considerable.

However, I have proper Molex 50 ohm end terminators and Molex T pieces, which gets me fairly close to where I need to be, so these feed through terminators are just a nice to have. Using T pieces is just a bit more cumbersome at the scope end.
 

Online VK5RC

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Re: 50 Ohm feed-through terminator
« Reply #12 on: May 26, 2019, 11:54:44 am »
Bit of a necro-bump - but this post inspired me to have-a-go

Needed a 50 feedthru to test an old HP 3400A AC voltmeter, the below pictures are self explanatory -
- a pair of 100 ohm 0805 1/8W SMD resistors and two BNC connectors that very conveniently screw together  :-+

Results at DC, 140MHz and 425MHz, the VNA is a home brew using EU1KY's design PCB and an STM32 demo touch screen , my Cal kit is only good to 200MHz so the cal of the VNA at 400 is unknown but looks OK with a Good 50 Ohm end termination - so I don't think it's too far off.
Rob
Whoah! Watch where that landed we might need it later.
 


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