BNC 50ohm Feed Thru

[Randy222]

I ran VSWR test again, this time fronting the 50 ohm feedthru with a 6db 50 ohm attentuator.

Very flat and below 1.1 from 10kHz to 300MHz (no 1M terminator)
Adding my homebrew 1M terminator the VSWR improved very slightly, like 1.082 to 1.079

Getting in 500MHz+ areas the VSWR curve starts to look like full rectified AC (humps) and in some areas of bandwidth the peaks reach up near 2. I suspect this from the feedthru items since they are only rated to 1GHz, whereas my attenuators are ratsed to 6GHz.

So at least for now, I think I am good with terminating 50 ohm on my 1M scope.

[joeqsmith]

I ran VSWR test again, this time fronting the 50 ohm feedthru with a 6db 50 ohm attentuator.

Very flat and below 1.1 from 10kHz to 300MHz (no 1M terminator)
Adding my homebrew 1M terminator the VSWR improved very slightly, like 1.082 to 1.079

Guessing you did not watch my video where I show that the capacitor is the dominant factor.  You need to measure with your scope, or improve your load to show anything meaningful.   

Attached simulation using your 6dB attenuator attached to a 50 ohm thru terminator, then to your scope.  For the scope, I kept the 1M constant and then swept the capacitor from 0 to 20pF.    With 0 capacitance, the VSWR is perfect (which is what you measure with your 1M load).   At 20pF, it's 1.4 at 300MHz.   

[joeqsmith]

If we fix the scope's capacitance at 16pf then sweep the inductance,  VSWR is below 1.5 to 300MHz for all inductance values.   But then look at what the scope sees.   

[joeqsmith]

What I was hinting at with the 6dB loss was not to add an attenuator like previously shown.  Rather I was thinking to make a uni-directional divider with a bit of inductance added to grounded leg.   Note the VSWR is a wash but what the scope sees can now be fairly flat.

Obvious question may be, why didn't I demo this.   Problem was to cut the pin to add the series R and then somehow add the other parts without unsoldering the first, then get the thing back together without damaging it....   Seemed like a lot of effort for something that I considered a bad idea to start with.  So I took the lazy way out and showed a simple inductor.   

[G0HZU]

I think that is the way some low cost scopes may do the internal 50R termination. There's usually already something like a 20R series resistor just inside the BNC input. This is there all the time.

Then, to provide the 50R termination mode they exploit the series 20R and switch in a 30R resistor in shunt. This sets up a 50 ohm potential divider and it masks some of the input capacitance from the input. The penalty is some loss in the divider but the scope will apply a fudge factor for this to keep the calibration correct.

Earlier this year, I mentioned this stuff about adding some series inductance and the resistive divider in this thread here in posts #4 and #14:

https://www.eevblog.com/forum/beginners/whats-the-input-capacitance-of-an-oscilloscopes-50-ohm-input/

[Randy222]

I ran VSWR test again, this time fronting the 50 ohm feedthru with a 6db 50 ohm attentuator.

Very flat and below 1.1 from 10kHz to 300MHz (no 1M terminator)
Adding my homebrew 1M terminator the VSWR improved very slightly, like 1.082 to 1.079

Guessing you did not watch my video where I show that the capacitor is the dominant factor.  You need to measure with your scope, or improve your load to show anything meaningful.   

Attached simulation using your 6dB attenuator attached to a 50 ohm thru terminator, then to your scope.  For the scope, I kept the 1M constant and then swept the capacitor from 0 to 20pF.    With 0 capacitance, the VSWR is perfect (which is what you measure with your 1M load).   At 20pF, it's 1.4 at 300MHz.

But my 1meg terminator itself does show it has some pF, it does not appear to be 0F. In general, poorly constructed terminators, thus I do expect some pF to be there.

I guess my next step is, connect VNA through attentuator+feed-thru to scope.

[joeqsmith]

....But my 1meg terminator itself does show it has some pF, it does not appear to be 0F. ...

If you do watch that video, you may notice that the two terminators I made back in 2016 exhibited a worse return loss than the OEM part when connected  to my scope.  That was a big hint to what was coming.  I posted how the OEM part was nothing more than an axial resistor.  That added parasitic inductance improved the performance.   Swapping that axial part for the 4, then the 2 X 1206's lowered that inductance.   

[joeqsmith]

All scopes are different.  The two GHz scopes I have are fixed at 50 only.   That LeCroy 7200 I showed has SMA connectors on the ICs themselves.  I wouldn't be surprised if the waveblunder doesn't have a different front for the two modes.     

[joeqsmith]

Looks like KeySight has a whole series on oscilloscope front end designs.  Here showing two separate paths:

https://youtu.be/1sVEWzMjkBo?t=258

[tszaboo]

We can achieve higher BW using a homemade resistive probe and better scope.   
https://www.eevblog.com/forum/testgear/12-ghz-active-probe-project/msg5006290/#msg5006290

I've been experimenting with Z0 probes myself. I have a design with multilayer PCB, and pogo pin to pick up signals, or 100 mil pin header at the end. I think I got quite good results, below is a comparison to a N2795A active probe. This is probably the sharpest square wave I could do without designing something, it's for sure is limited by the source. It shows faster rise time than the N2795A so I'm hoping it will be above a GHz, but I'm yet to make a test setup to measure it.
This is at work. The reason I'm interested in the through terminators is because I cannot really afford a new scope with 50 Ohm inputs at the home lab.

Note that a typical scope input isn't as simple as 1Meg in parallel with 18pF. There will be some series resistance right at the scope input and this is typically in the range 20R to about 50R. This resistance often varies a bit depending on the attenuation setting. A typical scope input can be modelled reasonably well with the network given in the first image below, as long as the correct value for R1 and C1 are used.

That's a very good point. I'm going to guess it depends on the attenuator they have in front of the input buffer.
For realistic measurements with a 50Ohm input, we can probably guess the input signal is going to be in the 5VRMS range though.

So what can we do? Connect a VNA on the scope input, and see whatever it measures?

[G0HZU]

Here's an old plot showing the simple scope model against a VNA measurement of an old Tek 465 scope and a Tek TDS2012 input. I can't remember what values I used for R1 and C1 for the TDS2012 but it was probably about 40 ohms and 20pF.

The agreement for Rp and Cp out to about 150MHz was very good as you can see in the plot below. This model is much better than the basic 1Meg in parallel with 20pF model that many people continue to use.

There's also a plot showing a classic old Tek 465 input being compared to the basic model. Again, the agreement is very good. In this case, the values for the model are given next to the plot.

[G0HZU]

I've got quite a few scopes here, all made by Tek or HP or Hameg, and I've measured the inputs of all of them with a VNA at some point.

For example, I have an old HP Infinium scope (500MHz) and this simply switches in a shunt 50R resistor at the input when 50 ohm mode is selected. There is some obvious printed inductance in series with the 50R load and HP will have added this deliberately to help improve the input VSWR at higher frequencies. However, the VSWR of the Infinium scope is only low up to about 250MHz. It reaches 1.6:1 up at 500MHz for example. This is despite the extra inductance in series with the 50 ohm resistor.

The most modern scope I've got here is a Tek MSO4104 and this has a dedicated 50 ohm signal path when the 50 ohm input is selected. A relay is used to swap across to the 50 ohm path. In the 50 ohm mode, the input VSWR of this scope is very good to well past 500MHz. It's still quite an old scope though...

[Randy222]

....But my 1meg terminator itself does show it has some pF, it does not appear to be 0F. ...

If you do watch that video, you may notice that the two terminators I made back in 2016 exhibited a worse return loss than the OEM part when connected  to my scope.  That was a big hint to what was coming.  I posted how the OEM part was nothing more than an axial resistor.  That added parasitic inductance improved the performance.   Swapping that axial part for the 4, then the 2 X 1206's lowered that inductance.

I did not watch vid yet, but will.

My two items are just axial metal film 1% 1/4w.

One is a single 1meg.
The 2nd is two 2meg in parallel, but they loop away from each other.

Both poorly made items have the resistors soldered to center pin with about zero space between center pin and resistor, then the other axial lead bends around on about 1/16" radius on 180deg turn to reach back to ground.

Thanks for all the info.

[G0HZU]

Here's a VSWR plot of the Tek MSO4104 when set to the internal 50R path. I've measured it out to 1GHz.

The VSWR is quite low up to about 850MHz. If I switch to the 1Meg input and try and fit an external 50R through termination (a commercially made BNC model) the VSWR is then <1.3:1 up to about 100MHz. Above this frequency, the VSWR rises quite fast. By 200MHz it hits 1.9:1 for example.

I can swap this through termination for an inductively compensated 50R termination and this does improve the VSWR, but it will cause some rising and falling edge issues when trying to look at a very fast pulse.

[joeqsmith]

A better model to me would as a minimum include the scopes transfer function for the different ranges.   

Looking at the manual for my LeCroy 7200, they spec the VSWR at 1.2, BW is 4GHz with a 125pSec transition for 50mV and up.  Otherwise, 3GHz / 150ps.  Isolation is 60dB min at 1GHz. 

Normally, I'm not using the scopes inputs directly.  More useful for me is how the probe+scope effect the circuit being measured.  See attached:

[joeqsmith]

...
I've been experimenting with Z0 probes myself. I have a design with multilayer PCB, and pogo pin to pick up signals, or 100 mil pin header at the end. I think I got quite good results, below is a comparison to a N2795A active probe. This is probably the sharpest square wave I could do without designing something, it's for sure is limited by the source. It shows faster rise time than the N2795A so I'm hoping it will be above a GHz, but I'm yet to make a test setup to measure it.
This is at work. The reason I'm interested in the through terminators is because I cannot really afford a new scope with 50 Ohm inputs at the home lab.
...

2.2ns edge rates are fairly slow.  As a comparison, back in 2015 a member was posting some data for a ring oscillator they had made using an LS part.  I still had several old 7400 DIP parts and joined the fun.   Shown is an F with fall times around 600ps.

https://www.eevblog.com/forum/projects/waveforms-in-a-74ls04-ring-oscillator/msg645166/#msg645166
I had made a cooler for this test and tried a National military F but could not achieve those edge rates.
https://www.eevblog.com/forum/projects/waveforms-in-a-74ls04-ring-oscillator/msg1033429/#msg1033429

[G0HZU]

I've got a little pulse gen board here that uses AC logic gates and it is also quite fast for rise time. I'll dig it out tomorrow.

Usually, the basic scope model does quite well on all attenuator ranges. Often the changes in input Rs are quite minor, especially with the newer scopes that use cheaper technology in the front end.

By contrast, my old 500MHz HP Infinium scope does show a fairly significant change in the Rs series resistance according to the attenuation selected. This scope uses old school relays and a very exotic looking front end PCB with printed components. There are different sized printed resistors ahead of each attenuator so I assume this corresponds to the differences I see on the VNA.

Normally, it will be this series Rs that will get hot if the scope is driven with large AC waveforms up at V/UHF. Some scopes have to be used with caution in this respect or they can easily be damaged. The 1Meg input might only be rated at about 3Vrms up at UHF for example.

[joeqsmith]

I have that PECL driver I show in the video that I put together but even that is fairly slow.    That early 1970's Tektronix tunnel diode I showed is the fasted pulser I have.   That old WaveMaster isn't near fast enough to show anything.  I did recently have it and smacked a filter with it for fun to look at the resonance.  Signals are far beyond the limits of that old scope. 

https://www.eevblog.com/forum/testgear/show-us-your-square-wave/475/

[tszaboo]

...
I've been experimenting with Z0 probes myself. I have a design with multilayer PCB, and pogo pin to pick up signals, or 100 mil pin header at the end. I think I got quite good results, below is a comparison to a N2795A active probe. This is probably the sharpest square wave I could do without designing something, it's for sure is limited by the source. It shows faster rise time than the N2795A so I'm hoping it will be above a GHz, but I'm yet to make a test setup to measure it.
This is at work. The reason I'm interested in the through terminators is because I cannot really afford a new scope with 50 Ohm inputs at the home lab.
...

2.2ns edge rates are fairly slow.  As a comparison, back in 2015 a member was posting some data for a ring oscillator they had made using an LS part.  I still had several old 7400 DIP parts and joined the fun.   Shown is an F with fall times around 600ps.

https://www.eevblog.com/forum/projects/waveforms-in-a-74ls04-ring-oscillator/msg645166/#msg645166
I had made a cooler for this test and tried a National military F but could not achieve those edge rates.
https://www.eevblog.com/forum/projects/waveforms-in-a-74ls04-ring-oscillator/msg1033429/#msg1033429

I know 2.2ns is nothing to write home about, It was one of the whatever MCU board I had lying around. I just wanted to see if it's comparable to a GHz active probe. It was already outperforming a 500MHz passive probe that Keysight packs with the scope, so I was about to make a test setup with a high speed clock buffer, but then it's holidays time now.

[G0HZU]

I managed to find my old logic gate boards. These have HCT and AC gates and I drive them from an old Datapulse pulse generator. By passing the pulse through several gates, it does achieve a reasonably fast rise time at the output. According to the 4104 scope, the rise time is about 600ps. The original goal for these boards was to try and find the source impedance of the gates, or at least to find the optimal series resistor to put at the output when driving 50R coax.

The plot below shows a straight 50R measurement via a series matching resistor and some 50R coax into the 50R input of the 4104.

The channel 2 trace is my old Zo probe. This probe has lots of bandwidth, probably flat to about 3GHz. It's a 30dB probe that I normally use with a VNA or a spectrum analyser.

You can see that the probe did OK here. It does add a few artefacts to the pulse shape, but I'm not sure what to expect from this setup in terms of the 'perfect' pulse shape.

It really is quite important to have a scope with a low VSWR over a huge bandwidth in order to get a result like this when using a Zo probe. In the past, I've used an attenuator at the scope input with the 500MHz Infinium scope. The plot below is with the Zo probe directly fed into the 50R input of the Tek 4104.

[tszaboo]

I managed to find my old logic gate boards. These have HCT and AC gates and I drive them from an old Datapulse pulse generator. By passing the pulse through several gates, it does achieve a reasonably fast rise time at the output. According to the 4104 scope, the rise time is about 600ps. The original goal for these boards was to try and find the source impedance of the gates, or at least to find the optimal series resistor to put at the output when driving 50R coax.

You got this square wave from a bunch of generic inverters? Damn, I haven't even considered to try that, I was expecting much worse results. I have to try this when I get back to the lab.

[G0HZU]

Looking at the board, it is just a couple of AC gates in series and a 39R series output resistor. The HCT version used four gates each with a series resistor and the outputs are then summed together into a single output. The rise-time of the HCT version is a lot slower.

Both chips are SMD and are mounted upside down on some bare copper board. I think the HCT chip is a 74HCT244 and the AC gate version is probably just a hex inverter. I can just about make out 74AC04 on the bottom side of the board where I once labelled it with a marker pen. The input to the AC series chip is to pin 1 and pins 2 and 3 are connected together and the output is from pin 4. It's tempting to try going through three gates in series to see if there is any improvement.

The pulse gen I use to drive it is nothing special. It's an old Datapulse 101 with a ~5ns risetime. The AC gates speed this up to a sub 1ns rise time.

The -30dB Zo probe is a simple homebrew design that works to about 3GHz. I've got several homebrew Zo probes here and one of them is good to about 6GHz.

[G0HZU]

Here's what happens if I put the Zo probe on channel 3 set to 1Meg input and I fit a commercial 50R BNC through termination at the input of channel 3. You can see lots of artefacts appear now and these are caused by (re)reflections in the cable of the Zo probe. It's possible to estimate the cable length by looking at the delay time of the artefacts as they appear on the waveform.

[Randy222]

Here's what happens if I put the Zo probe on channel 3 set to 1Meg input and I fit a commercial 50R BNC through termination at the input of channel 3. You can see lots of artefacts appear now and these are caused by (re)reflections in the cable of the Zo probe. It's possible to estimate the cable length by looking at the delay time of the artefacts as they appear on the waveform.

Now put a ~3db 50ohm attenuator on front side of feed thru. Does the trace look better?

[joeqsmith]

I know 2.2ns is nothing to write home about, It was one of the whatever MCU board I had lying around. I just wanted to see if it's comparable to a GHz active probe. It was already outperforming a 500MHz passive probe that Keysight packs with the scope, so I was about to make a test setup with a high speed clock buffer, but then it's holidays time now.

Point being for lower speed signals like this, it may not make enough difference. 

Shown is the fast edge from my old Tektronix scope demo board.  C1 is a standard 10x probe, M1 is a LeCroy resistive probe attached using that Pasternak thru terminator.  M2 is the same resistive probe, using the scopes internal 50 ohm termination.    Sorry, I didn't comp the 10x probe but point being there is little difference when working with these slower signals.   

***
Just in case there is some confusion, I have attached a link showing the board.  There is no terminator.  The resistive probe presents more of a load than the 10X at DC.  Again, crossover for these two probes was around 60-70MHz where the 10X probe starts to load things down.   Best to know what you want to measure and buy the equipment for the job. 

https://www.eevblog.com/forum/testgear/show-us-your-square-wave/msg5164857/#msg5164857