50 ohm inputs for Siglent SDS1104X-E

[skander36]

On a Micsig scope (TO 3004) I have obtained even a slightly a better rise time with this load resistor(P57) than with it's internal 50 ohm path.
The result was 1,060 ns for internal 50 ohm and 0,993 ns with P57. With another source the result was  1.606 ns external 50 ohm(P57) and 1,879 ns for internal 50 ohm.

I would assume that the difference is caused by peaking because of inductance in the resistor or layout.

Very likely, because the signal looks quite peakey 

[joeqsmith]

If you followed along in that video where I went over some of the basic math and simulate the termination,  I am suggesting it is the scopes input that screws up the response, which I assume is why graybeard asked my to look at it.

I enjoyed watching the video a lot, great presentation of measurements and their evaluation.

My takeaway was that for best performance, a custom termination is adjusted to the individual scope on which the termination is going to be used. I especially liked the use of an inductance to linearize the response. Felt like adusting a speaker crossover.

Thanks for the recording,
Christian

I was planning to make a terminator that I could trim.   Ran some simulations for it but didn't have parts on-hand to construct it.   OP who started the last thread had lost interest, so I saw no point taking it any further.   

I summed it up in the end.  For best performance, buy tools capable of making the measurements you need to make.   You may be able to gain some performance by hacking up a custom terminator like I show but if you are working with say high speed digital,  it is going to be difficult to match the performance of a better scope and probes.   Of course, that can be six figures which is why you see old vintage equipment in my home lab.   

[David Hess]

I was planning to make a terminator that I could trim.

When Tektronix first started making 1 GHz oscilloscopes, they discovered that all of the 50 ohm BNCs they could buy were not 50 ohms, and ended up making their own.

A trimmed terminator is not going to matter unless the cables, connectors, and adapters are consistent with it, and suitable precision resistors are available.

[joeqsmith]

I was planning to make a terminator that I could trim.

When Tektronix first started making 1 GHz oscilloscopes, they discovered that all of the 50 ohm BNCs they could buy were not 50 ohms, and ended up making their own.

A trimmed terminator is not going to matter unless the cables, connectors, and adapters are consistent with it, and suitable precision resistors are available.

That's the idea of using a trimmer, we are compensating for these small errors.  Similar to why you need to compensate all your 10X probes for the best performance. 

I don't know what these precision resistors would be for that you mention. 

We can get some idea of what Tektronix was up to in the 1950s, looking at the release of the 519.  It appears to use 125 ohms with N125 connectors. 
https://7vmc31.p3cdn1.secureserver.net/wp-content/uploads/2020/07/HighSpeedScopeBrochure_507_517_519_1961.pdf

Looks like you can still find some of these parts at salvage yards.  125ohm thru terminators.   Nice. 
https://www.surplussales.com/EQUIPMENT/TEKHPLISTED.HTML

[David Hess]

I was planning to make a terminator that I could trim.

When Tektronix first started making 1 GHz oscilloscopes, they discovered that all of the 50 ohm BNCs they could buy were not 50 ohms, and ended up making their own.

A trimmed terminator is not going to matter unless the cables, connectors, and adapters are consistent with it, and suitable precision resistors are available.

That's the idea of using a trimmer, we are compensating for these small errors.  Similar to why you need to compensate all your 10X probes for the best performance. 

I don't know what these precision resistors would be for that you mention.

I mean 50, 100, and 200 ohm 0.1% surface mount resistors are available, and 50 and 100 ohm 0.1% RF surface mount resistors.

We can get some idea of what Tektronix was up to in the 1950s, looking at the release of the 519.  It appears to use 125 ohms with N125 connectors. 
https://7vmc31.p3cdn1.secureserver.net/wp-content/uploads/2020/07/HighSpeedScopeBrochure_507_517_519_1961.pdf

Looks like you can still find some of these parts at salvage yards.  125ohm thru terminators.   Nice. 
https://www.surplussales.com/EQUIPMENT/TEKHPLISTED.HTML

The 519 was a special case where the oscilloscope deflection plates were driven directly without a vertical amplifier, and Tektronix even had a variation of the GR-874 connector with a smaller center connector which was 125 ohms for it.

The Tektronix 011-0129-00 was a 0.1% 50 ohm feedthrough termination but only good up to 100 kHz.  As I recall it was for calibration of something.

The Tektronix 017-0084-00 was a 50 ohm GR-874 air line used as an absolute impedance reference of 50 ohms +/- 0.4%.

[joeqsmith]

...
The 519 was a special case where the oscilloscope deflection plates were driven directly without a vertical amplifier, and Tektronix even had a variation of the GR-874 connector with a smaller center connector which was 125 ohms for it.
...

Indeed it was special as it appears to be Tektronix's first GHz BW product.  I take it your first comment wasn't actually about their first GHz products but something much later.

When Tektronix first started making 1 GHz oscilloscopes, ...

No matter.  It all very old tech and not anything relevant today.   Even my 4GHz 80's LeCroy is in that category but it does use a standard 50 ohm input w/ SMA. 

[David Hess]

When Tektronix first started making 1 GHz oscilloscopes, ...

No matter.  It all very old tech and not anything relevant today.   Even my 4GHz 80's LeCroy is in that category but it does use a standard 50 ohm input w/ SMA.

It came up with the 7A29 1 GHz vertical amplifier in the 7104 1 GHz mainframe.  The amplifier was designed with a BNC input and Tektronix found out that commercially available BNCs were not actually 50 ohms, so they made their own which was.

I doubt SMA connectors had this problem, but I wonder if TNC or N connectors did.

It is relevant today if you are expecting BNC connectors to be 50 ohms.

[joeqsmith]

..
It is relevant today if you are expecting BNC connectors to be 50 ohms.
..

I thought the discussion was about low end scopes with no internal 50 ohm termination.  The dominant error will be the scope's front end, not the connector.  Still, for the interconnect, while my 80's LeCroy used SMAs, the one from 2000 uses a their own interface.  No BNCs.   

But I am sure people who work in this are understand that even bent coax will cause a mismatch. 
https://www.eevblog.com/forum/rf-microwave/is-is-possible-to-repair-a-hairline-crack-in-semi-rigid-coax/msg4896009/#msg4896009

***

... I doubt SMA connectors had this problem, but I wonder if TNC or N connectors did. ...

All connectors have their limitations, including SMAs.  I have a few 3.5mm connectors at home.  These are similar to an SMA but are an airline.  The following article talks about the history.  It includes a plot comparing the SWR for combinations for 3.5mm and SMA interconnects. 
 
https://www.gore.com/sites/default/files/2016-06/Intermateability-SMA-Connectors.pdf

I wouldn't be at all surprised to find some of these connectors costing more than the scopes we are talking about in this thread.   

[backyard_cnc]

The main focus of my need for these is 50ohm work under 50mhz and besides with a 200mhz BW on the scope I’m hardly concerned about how this device might perform at such high frequencies!