Author Topic: Breadboard probing made easy  (Read 1997 times)

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Online tggzzz

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Re: Breadboard probing made easy
« Reply #25 on: January 22, 2019, 05:54:22 am »
Below are some screenshots. Because signal was very repetitive I used averaging mode.
Rise and fall changed with vertical range a bit, prop. because of the resolution.

The second trace shows the effect of the inductive ground leads. If you measure the period and know the probe tip capacitance, you can estimate that inductance. I'll bet it is around 0.8nH to 1nH per mm of lead length.

From the last two traces we can see that your cables are unterminated, and the driver cannot supply the necessary current to drive the cable. The 6ns and 12ns numbers aren't helpful and should be ignored.
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Offline HendriXML

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Re: Breadboard probing made easy
« Reply #26 on: January 22, 2019, 06:28:35 am »

From the last two traces we can see that your cables are unterminated, and the driver cannot supply the necessary current to drive the cable. The 6ns and 12ns numbers aren't helpful and should be ignored.
The first 2 measurements probed with the setting on 10x. The BNC cables lack such a mode so would’nt those pull more current?
Some scopes have a 50 ohm input capability would that be in this artificial situation better? Is it possible to add a normal 50 ohm resistor, or does it need to be combined with a capacitor/inductor, or is a good old fashioned bnc terminator on a T connector a possibility?

What would be the verdict on the probing method which started this thread? Any good?
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Online tggzzz

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Re: Breadboard probing made easy
« Reply #27 on: January 22, 2019, 06:40:54 am »

From the last two traces we can see that your cables are unterminated, and the driver cannot supply the necessary current to drive the cable. The 6ns and 12ns numbers aren't helpful and should be ignored.
The first 2 measurements probed with the setting on 10x. The BNC cables lack such a mode so would’nt those pull more current?

You need to look up the theory of digital signals and transmission lines.

Your "high impedance *10" scope probe's input impedance is lower than a "low impedance Z0 resistive divider" scope probe. Work out the impedance of a 15pF capacitor at 100MHz.

Quote
Some scopes have a 50 ohm input capability would that be in this artificial situation better? Is it possible to add a normal 50 ohm resistor, or does it need to be combined with a capacitor/inductor, or is a good old fashioned bnc terminator on a T connector a possibility?

A proper 50ohm input is ideal, but they are only found in high end scopes (and Tek 485s). In other cases the 50ohms is simply a resistor added in parallel with the scope's high impedance input (i.e. 1Mohm//20pF).

Any L/C would only be 50ohms at one frequency; at all other frequencies there would be a mismatch.

The BNC terminator plus T connector will be 50ohms//20pF, where 20pF is the scope's input capacitance. It isn't too bad at these risetimes, but becomes more problematic at higher speeds.

Quote
What would be the verdict on the probing method which started this thread? Any good?

Look at your second graph.
There are lies, damned lies, statistics - and ADC/DAC specs.
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Offline tautech

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Re: Breadboard probing made easy
« Reply #28 on: January 22, 2019, 07:01:37 am »

From the last two traces we can see that your cables are unterminated, and the driver cannot supply the necessary current to drive the cable. The 6ns and 12ns numbers aren't helpful and should be ignored.
The first 2 measurements probed with the setting on 10x. The BNC cables lack such a mode so would’nt those pull more current?
Well yes but it depends on how the BNC cables are terminated as the SAG1021 is a 50 ohm source and expects to see a 50 ohm load for correct signal fidelity.
Quote
Some scopes have a 50 ohm input capability would that be in this artificial situation better? Is it possible to add a normal 50 ohm resistor, or does it need to be combined with a capacitor/inductor, or is a good old fashioned bnc terminator on a T connector a possibility?
Yes again but the source is 50 ohm and scope inputs, probe or BNC cabling all add capacitance to the load.
Have a look at this thread for example of correct scope connections for accurate reproduction of signal fidelity.
https://www.eevblog.com/forum/testgear/show-us-your-square-wave/

Most often we just want to check the type of signal in a circuit is correct and that should be enough to know if the circuit is working as it should but with the proviso that our probing technique could impact on signal fidelity.
If it does but we're aware it does we still have the result we looked for and can move on to examine further blocks in the circuit.
Just remember every measurement connection effects the measurement itself in some way.  ;)

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Offline HendriXML

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Re: Breadboard probing made easy
« Reply #29 on: January 22, 2019, 07:38:40 am »
Okay I used a T splitter and a 50 Ohm Terminator I had lying around from the days of our coax LAN.

Now the signal looks better. But using that as a baseline of comparison is not OK I guess, it is a complete different situation than measuring with the breadboard probing thingy.
« Last Edit: January 22, 2019, 07:45:13 am by HendriXML »
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Offline tautech

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Re: Breadboard probing made easy
« Reply #30 on: January 22, 2019, 07:42:02 am »
Okay I used a T splitter and a 50 Ohm Terminator I had lying around from the days of our coax LAN.

Now the signal looks better.
Yes but...........input attenuation setting is wrong.  ;)
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Offline HendriXML

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Re: Breadboard probing made easy
« Reply #31 on: January 22, 2019, 07:50:19 am »
Okay I used a T splitter and a 50 Ohm Terminator I had lying around from the days of our coax LAN.

Now the signal looks better.
Yes but...........input attenuation setting is wrong.  ;)
Depends on how you look at it, the signal generator (other scope screen) says it is outputting only half :). Oh I see, I still had 10x active. :palm:
« Last Edit: January 22, 2019, 07:52:25 am by HendriXML »
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Offline HendriXML

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Re: Breadboard probing made easy
« Reply #32 on: January 22, 2019, 08:07:50 am »
https://www.eevblog.com/forum/testgear/show-us-your-square-wave/
I don't think those example are good to watch still being content with the SAG1021. It's like some judge says: your edges are kind of dull, it won't cut, surrender your weapon.
I'll wait a decade, so I can say. Yeah my old beasty has some good 1 Mhz square wave, very good in those days. And then everybody politely nods.  :-DD
« Last Edit: January 22, 2019, 11:14:04 am by HendriXML »
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Offline HendriXML

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Re: Breadboard probing made easy
« Reply #33 on: January 22, 2019, 07:28:25 pm »
Here're the "squares" I manage to get with a Siglent SAG1021 and a Siglent SDS 1104E-E with a 50 Ohm T connection.
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Online tggzzz

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Re: Breadboard probing made easy
« Reply #34 on: January 22, 2019, 08:20:05 pm »
Here're the "squares" I manage to get with a Siglent SAG1021 and a Siglent SDS 1104E-E with a 50 Ohm T connection.

They show well-controlled outputs operating within their specification.

That's a different world to looking at general logic signals.
There are lies, damned lies, statistics - and ADC/DAC specs.
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Offline HendriXML

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Re: Breadboard probing made easy
« Reply #35 on: January 24, 2019, 12:32:37 am »

From the last two traces we can see that your cables are unterminated, and the driver cannot supply the necessary current to drive the cable. The 6ns and 12ns numbers aren't helpful and should be ignored.
The first 2 measurements probed with the setting on 10x. The BNC cables lack such a mode so would’nt those pull more current?

You need to look up the theory of digital signals and transmission lines.

Your "high impedance *10" scope probe's input impedance is lower than a "low impedance Z0 resistive divider" scope probe. Work out the impedance of a 15pF capacitor at 100MHz.

Quote
Some scopes have a 50 ohm input capability would that be in this artificial situation better? Is it possible to add a normal 50 ohm resistor, or does it need to be combined with a capacitor/inductor, or is a good old fashioned bnc terminator on a T connector a possibility?

A proper 50ohm input is ideal, but they are only found in high end scopes (and Tek 485s). In other cases the 50ohms is simply a resistor added in parallel with the scope's high impedance input (i.e. 1Mohm//20pF).

Any L/C would only be 50ohms at one frequency; at all other frequencies there would be a mismatch.

The BNC terminator plus T connector will be 50ohms//20pF, where 20pF is the scope's input capacitance. It isn't too bad at these risetimes, but becomes more problematic at higher speeds.

Quote
What would be the verdict on the probing method which started this thread? Any good?

Look at your second graph.
Okay my judgment would than be, if the transitions are faster than 3V / 50 nS ringing becomes more and more apparent on high impediance loads. Otherwise it will do just fine. I came to these values because at the rise an fall times of my sig. gen. it was stil (only) a bit noticable. In the analog domain thats is very usefull I guess.
In faster cases, normal probing with the clip lead is not advisable as well.
« Last Edit: January 24, 2019, 12:37:01 am by HendriXML »
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Offline HendriXML

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Re: Breadboard probing made easy
« Reply #36 on: January 24, 2019, 12:42:00 am »
Here're the "squares" I manage to get with a Siglent SAG1021 and a Siglent SDS 1104E-E with a 50 Ohm T connection.

They show well-controlled outputs operating within their specification.

That's a different world to looking at general logic signals.
And it keeps the ringing away,  so a nice combination with the breadbord probes.  :-DMM
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Online tggzzz

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Re: Breadboard probing made easy
« Reply #37 on: January 24, 2019, 12:58:21 am »

From the last two traces we can see that your cables are unterminated, and the driver cannot supply the necessary current to drive the cable. The 6ns and 12ns numbers aren't helpful and should be ignored.
The first 2 measurements probed with the setting on 10x. The BNC cables lack such a mode so would’nt those pull more current?

You need to look up the theory of digital signals and transmission lines.

Your "high impedance *10" scope probe's input impedance is lower than a "low impedance Z0 resistive divider" scope probe. Work out the impedance of a 15pF capacitor at 100MHz.

Quote
Some scopes have a 50 ohm input capability would that be in this artificial situation better? Is it possible to add a normal 50 ohm resistor, or does it need to be combined with a capacitor/inductor, or is a good old fashioned bnc terminator on a T connector a possibility?

A proper 50ohm input is ideal, but they are only found in high end scopes (and Tek 485s). In other cases the 50ohms is simply a resistor added in parallel with the scope's high impedance input (i.e. 1Mohm//20pF).

Any L/C would only be 50ohms at one frequency; at all other frequencies there would be a mismatch.

The BNC terminator plus T connector will be 50ohms//20pF, where 20pF is the scope's input capacitance. It isn't too bad at these risetimes, but becomes more problematic at higher speeds.

Quote
What would be the verdict on the probing method which started this thread? Any good?

Look at your second graph.
Okay my judgment would than be, if the transitions are faster than 3V / 50 nS ringing becomes more and more apparent on high impediance loads.

The problem is you don't have a high impedance load: the circuit "sees" a 15pF load - the probe tip capacitance.

If we use the rule-of-thumb BW=0.35/tr, then with tr=10ns BW=35MHz. I chose 10ns because that is typical of fast logic in the 1970s :) Today's jellybean logic is <1ns :)

Now consider the impedance of a capacitor, Z=1/2*pi*f*C, so 15pF -> 300ohms, and it gets lower at higher frequencies (e.g. 80ohms at 100MHz).

The "ringing" frequency will be f=1/2*pi*sqrt(LC), where C is the tip capacitance and L is the inductance of the ground connection, typically 90MHz for a 150mm ground lead.

Quote
Otherwise it will do just fine. I came to these values because at the rise an fall times of my sig. gen. it was stil (only) a bit noticable. In the analog domain thats is very usefull I guess.
In faster cases, normal probing with the clip lead is not advisable as well.

Whether or not it will "do fine" depends on the signal and output that you are probing. What might be fine for an AWG might not be "fine" for a logic signal.

There are lies, damned lies, statistics - and ADC/DAC specs.
Gliding aphorism: "there is no substitute for span". Retort: "There is a substitute: skill+imagination. But you can buy span".
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