50ohm Oscilloscope Straight Through Termination  BNC to BNC - Page 1
Forum section: electronics / projects

author: dazz1
author_rank: Frequent Contributor
timestamp: August 06, 2020, 10:38:28 pm
content: Hi50ohm through terminations are most commonly used to terminate a 50ohm cable at the input of an oscilloscope. They stop signals being reflected back down the cable. They can be expensive.I found a source on Aliexpress that is fine for general electronic workshop use.https://www.aliexpress.com/item/33027400993.html?spm=a2g0s.9042311.0.0.27424c4ddHDFlcIf the link doesn't work in the future, search Aliexpress for "DANIU 1Pcs Y1078 50 BNC Male To Female Plug High Frequency Test Socket for CCTV System Connection High Quality"You can see that the title doesn't really say what it is. I was only certain when I dismantled it.The termination consists of 2x 100ohm leaded resistors that are accessible by unscrewing the adapter. I am considering replacing those with 4x 200ohm SMD resistors to raise the frequency response to a few GHz to plug into my 60MHz analog scope. Why would I do that?? Because I can !!Dazz

author: thm_w
author_rank: Super Contributor
timestamp: August 06, 2020, 10:58:02 pm
content: So we are sure its easy to unscrew?https://www.eevblog.com/forum/rf-microwave/inexpensive-50-ohm-attenuator-tests/https://www.eevblog.com/forum/testgear/50-ohm-feed-through-terminator/https://www.eevblog.com/forum/testgear/banggood-50-ohm-bnc-feed-through-terminator-a-quick-review/

author: ChristofferB
author_rank: Frequent Contributor
timestamp: August 06, 2020, 11:12:01 pm
content: What is the advantage of this, at 60 MHz over a BNC Tee with one arm terminated into 50 ohm? Aside from the obvious style points

author: David Hess
author_rank: Super Contributor
timestamp: August 07, 2020, 01:05:56 am
content: The stub length of the high impedance input after the termination ultimately limits performance so even a 1 GHz termination is sufficient. Better performance requires moving the switchable termination to a point inside of the oscilloscope or using a coaxial relay to switch between high and low impedance inputs.Another option to reduce return loss is to insert an attenuating pad before the feed-through terminator.

author: magic
author_rank: Super Contributor
timestamp: August 07, 2020, 06:45:17 am
content: Quote from: ChristofferB on August 06, 2020, 11:12:01 pmAnother thread:https://www.eevblog.com/forum/projects/home-made-feed-through-terminator-for-bnc/I built this and couldn't or could barely tell any difference in rising edge on a 200MHz scope.Both looked pretty much spot on, though the generator also was an uncalibrated DIY job, I just tried my best.

author: dazz1
author_rank: Frequent Contributor
timestamp: August 08, 2020, 10:17:04 am
content: Quote from: ChristofferB on August 06, 2020, 11:12:01 pmIt was cheaper to buy a pair of through-terminators than to buy Tees and 50ohm loads.Dazz

author: dazz1
author_rank: Frequent Contributor
timestamp: August 08, 2020, 10:45:24 am
content: Quote from: David Hess on August 07, 2020, 01:05:56 amThe two 100ohm leaded resistors used in the Aliexpress through terminator are only 1/8W.Using smd resistors to improve the frequency response wouldn't make the slightest difference on my 60Mhz scope, and my Spec-An already has a 50ohm input. If done the right way, it would increase the power rating. As the links above in other posts show, it would be cheap and easy to replace the leaded resistors with SMD resistors. If care is taken with the physical layout, then the frequency response should be improved.Dazz

author: tggzzz
author_rank: Super Contributor
timestamp: August 08, 2020, 10:58:08 am
content: Quote from: dazz1 on August 08, 2020, 10:45:24 amDon't forget the effect of the scope's 20pF (or 15pF) input capacitance.As you probably realise, in RF circuits a pad is often used to reduce such effects, on the grounds that gain is cheap, but reflection coefficients are a pain. A 20dB pad has the advantage of making the mental arithmetic simple.