Show us your square wave

[joeqsmith]

Rise time is measured at just over 5ns, which is pretty good. (Bandwidth can be approximated as 0.35 / (rise time), so we can't expect any faster than a 5ns rise on a 70MHz scope.)

I would have guessed that rule of thumb was flushed down the toilet long ago.

Is this true?

What do the specs show for DSOs made in the last 10 years?   

From Tektronix

Question:
How is bandwidth related to rise time for oscilloscopes?
Answer:

Historically, oscilloscope frequency response tended to approximately follow the rule: Bandwidth x risetime = 0.35. This corresponds to a 1- or 2-pole filter roll-off in the frequency domain. Today, at the high end, most real-time digital oscilloscopes more closely follow this rule:

Bandwidth x rise time = 0.45.

This corresponds to a much steeper frequency roll-off above the specified bandwidth. The steeper roll-off is more desirable in digital oscilloscopes that oversample by 4x, 3x, or even less because it prevents aliasing by eliminating any signal above the Nyquist frequency (1/2 the sample rate – the minimum sample rate required for accurate signal representation).

From Teledyne

Risetime-Bandwidth Product (RT*BW)
Typically RT*BW = 0.40 to 0.45 for modern high bandwidth scopes
Historically analog scopes RT*BW = 0.35

[TunerSandwich]

Saturation is a characteristic of ferromagnetic and ferroelectric materials.  Resistance wires are almost never ferromagnetic.

Tim

Nope....yer dead wrong on that one....most resistance wire has enough iron in it to do exactly what I described.  I work with the stuff everyday, and have done these experiments more times than I care to recall.  We'll build up those cables and see what happens....

[LabSpokane]

That wire might be magnetic, but if it's acting like a diode it's because of corrosion, not magnetic saturation. Colloquial terms include "rusty bolt effect" and "rusty fence effect". In radio it's called passive intermodulation.

http://en.m.wikipedia.org/wiki/Rusty_bolt_effect

[TunerSandwich]

That wire might be magnetic, but if it's acting like a diode it's because of corrosion, not magnetic saturation. Colloquial terms include "rusty bolt effect" and "rusty fence effect". In radio it's called passive intermodulation.

http://en.m.wikipedia.org/wiki/Rusty_bolt_effect

Now that is something I am willing to believe. Most of these wires oxidize almost immediately and we spend a lot of time cleaning them before lacquering. I have a spool of some quite well corroded A-1 laying around. I will build up two sets of cables, one cleaned and one dirty and lets see what happens.

[Electro Fan]

I'm way behind you guys finding all sorts of more advanced and interesting things but I thought I'd try to see how much impact scope technology and bandwidth impact the end to end results.

Below is the same 20 MHz square wave on a 70 MHz Rigol digital scope and a 400 MHz Tektronix analog scope - some difference but not a ton of apparent difference.

[Electro Fan]

Hi group,

I want to extend my previous post with some measurements.

I apologize for the blurry pictures.

I want to show just how bad the cable can be:

Here is a reference waveform made with a 50 Ohm cable. The scope bandwidth is 1 GHz and the HP8112A pulse generator has 4.5ns rise and fall times.





I then replace the cable with a really bad cable made from 75 Ohm RG6 coax with binding post adapters for the connections:

Here is the waveform:



It is similar to the waveform from the LTspice model. The notch on the leading edge is twice the time it takes for the signal to travel from one end of the cable to the other. The velocity is 66% of the speed of light.



Here is a photograph of the (terrible) construction of the cable.



If I increase the rise and falltimes to 10ns (greater than or equal to the electrical length of the cable) most of the distortion caused by the impedance miss match are eliminated.



So generally the impedance matching is important if the risetime is less than the propagation time.

Regards,

Jay_Diddy_B

I was away from the thread for a while - Jay_Diddy_B your posts are very cool.

Just to clarify, when you say propagation time are you speaking of the time it takes the signal to leave the generator and reach the scope (ie, based on the length of the cable), or something else? (Hope I got that right or I am going to incur some laughter  )

[Electro Fan]

Just one other semi-related observation - I find that scopes and other test equipment (including frequency/function generators and frequency counters) seem to consistently do well with frequency counting accuracy but struggle a little more with accurate voltage measurements (or perhaps this might just be due to my limited experience).  Further, it would seem that both frequency and voltage measurements are needed to calculate rise time. 

All that said, and no doubt cabling quality and cabling length along with impedance mismatching can have some impacts, it would seem that a generator's ability to produce good rise times (and fall times) is among the key determinants of the ability to produce a nicely square square wave.  Yes, no?

[TunerSandwich]

[T3sl4co1l]

And those frequencies are due to the wire, you say?

[Howardlong]

And those frequencies are due to the wire, you say?

Some folks get paid to make antennas like that.

[TunerSandwich]

And those frequencies are due to the wire, you say?

Some folks get paid to make antennas like that.

I will sell you this antenna for less than they will

[TunerSandwich]

 

[TunerSandwich]

And those frequencies are due to the wire, you say?

? that's at 10mV?  I doubt the wire is doing anything, other than acting as an antenna.....the pulse is buried in there somewhere.....

[TunerSandwich]

now watch what this wire does, as the power increases......this is 1Mohm termination.....the resistance wire is pretty closely matched to 4ohm per conductor (you can see in the waveform).

[TunerSandwich]

[TunerSandwich]

Here is the Titanium resistance wire my company makes....it contains no iron, nickel, manganese, chromium or aluminum/copper.  These conductors are matched to 2.5 ohms each. 

Same setting on the sigGen, same 1Mohm termination

[T3sl4co1l]

Both look about like what you'd expect from a mismatched transmission line of that length.

Tim

[TunerSandwich]

So do I win for the worst square wave possible?  Someone told me that was the point of this thread, back on page 11 or 10 ( )

[LabSpokane]

You win the prize. Absolutely no doubt about that. 

[Pjotr]

A 45 years old article about probing:
http://www.ece.vt.edu/cel/docs/TekProbeCircuits.pdf

Also worth reading this Agilent app note:
http://cp.literature.agilent.com/litweb/pdf/5990-8371EN.pdf

Maybe it becomes clear why el cheapo Chinese probes supplied with the lower end DSO's are not the ones to get most out of what these scopes are capable of regarding pulse response. And sometimes "lie" a bit.

[joeqsmith]

Pulse output feeding the two  "T"s back to back to from an "H"   One  output of the H drives the scopes input.  The remaining two have a fair amount of coax forming a loop.    Normally used for benchmarking the DSO, in this case shows how power is split between the loop and the DSO's input.  Once the power from the loop shows up, the voltage is doubled.     If the loop were long enough, you would see a second pulse (with VERY low phase noise compared with the exciter pulse.   



Nice Agilent scope Hugoneus!   

[DIPLover]

10, 20 and 40MHz square waves from Altera Stratix FPGA Enhanced PLL to Tektronix 2465. CMOS 2.5V IO standard.

The old boat anchor is still beating specs (a little) 30 years after being built (and 10 years after last pro calibration sadly  )

Just received today my Rigol DS2072A, fully unlocked to 300MHz bandwidth. Here are the results of the same measurements with identical setup.

Rise time @ 40MHz is a bit worse than the venerable Tek 2465 but quite respectable. With all the fantastic DSO capabilities, this will be a worthwhile companion to the old clunker I believe.

[-jeffB]

Alright, alright... How about an 81Gb/s circuit I made a while back with a rise time of 5.2ps?  O0

Oh, come on. There HAS to be SOMEONE around here doing millimeter-wave work...

[katzohki]

To get back on topic, here is a comparison between two different function generators, both running at 50kHz. You can clearly see that, yes indeed there is a difference in the shape "quality" between them. Both are 50 ohm terminated.

First a HP 8116A:


You can see that the square wave is a very tight shape and has rise time of 6.3ns.

Next the HP 8904A:

The edges are more rounded and the rise time is ~1.7us.

You can see a good bit of difference between them. The HP 8116A is a 50MHz generator and will, in my opinion, still output a good square wave up into 25MHz or so. The HP 8904A, on the other hand, is rated at 0-600kHz for sine and 0-50kHz for square (and ramp / triangle) so it is working at its absolute upper limit. They both are very nice units and the HP 8904A is particularly versatile, but you can see that each has its application and yes indeed not all FGs are made alike.

PS: I posted a review of the HP 8904A on my blog (shameless plug) in case you might be interested and I'm planning one on the HP 8116A as well. http://katzelectronics.blogspot.com/2015/01/hp-3904a-review.html Just in case you might be interested. 

PPS: Ignore the 2kV/div on channel one. I normally have a 1000:1 scope probe on that channel and was too lazy to change it. 

[Teneyes]

I see Rigol DSO displays are being posted on this thread and comments about overshoot.
I wish to inform All the effects of the SinX/X interpolation on the sample data for Rigol DSOs
In the displays below I used My DS2000 to show a Perfect 62.5MHZ square wave Sampled at 2 GSa/s

1st display shows the sample data in Dot Mode , perfect step transistions.
2nd display shows how the Sinx/X process is implemented on the Rigol DS2000, and shows ringing!
Owners need to be aware of this at Faster scan rates, where sample data count is low for the displayed area

a bit OT
Also someone commented that pulses are mainly used for clocking.
in the 3rd display I show how a pulse train can produce a Spread Spectrum of frequencies
for bandwidth testing.  The duality of Time and Frequency Domains