Show us your square wave

[THDplusN_bad]

Good Day,

Ok, I realize that this is an old thread, but still an interesting topic

I was hooked up by the simplicity of the 74AC14-based fast rise pulse generator design. So, same as many other members I have built my own earlier this week. Nice!
I have used a simple double-sided copper board from the scrap pile and the results of this simple thing built in "dead-bug style" are pleasing.
I have just added a "proper" power connector, a diode to protect from reverse supply voltages and some stand-offs. Used a 100 nF cap. and a 10k resistor SMDs for the oscillator, as these 0805 size components fit nicely.
The BNC output connected is mounted on an old Tektronix assembly mount, which is a left over from an older oscilloscope repair. 
Voilá - the simple thing worked right from the start and creates a nice 960 Hz square wave with an amplitude of about 3.4Vpp into 50 Ohms.

I have measured rise times between 1.8 ns and 2.1 ns (10% to 90%) per the attached screenshots. These were taken with an entry-level LeCroy Type Wavejet 334 DSO, which is spec'd at 1 ns typ rise time and 350 MHz BW.
The output signal overshoots by around 29%, but that's fine and the distinct peak makes it even useful when one uses it as a poor man's TDR.
And I had much fun when I was following Alan's (W2AEW) excellent video "#88: Cheap and simple TDR using an oscilloscope and 74AC14 Schmitt Trigger Inverter"
I was amazed to find out that this method allowed me some correct cable length measurements of two cables (an Aircell-5 and a plain-vanilla RG-58 made by HP) down to centimeter accuracy. Nice work, as always, Alan... @w2aew ! 

Cheers,

THDplusN_bad

[joeqsmith]

You may find the following of interest as well where we look at several types of gates ran over various temperatures and voltages.     

https://www.eevblog.com/forum/projects/waveforms-in-a-74ls04-ring-oscillator/

[PartialDischarge]

Somewhere in a thread user ttggzz suggested the use of LVC gates in parallel, I made a board to test the idea and I'm getting less than 500ps rise and fall times with very nice shape, tested with a 1GHz scope:

[tggzzz]

Somewhere in a thread user ttggzz suggested the use of LVC gates in parallel, I made a board to test the idea and I'm getting less than 500ps rise and fall times with very nice shape, tested with a 1GHz scope:

Here perhaps: https://www.eevblog.com/forum/testgear/show-us-your-square-wave/msg1902941/#msg1902941


Good to see the results can be replicated. It looks like the scopes I was using (not mine, very fast test) used the 20%-80% times, which would be faster than the 10%-90% times.

I used 143ohms in series since the gate output is ~7ohms. I don't think that would make too much difference in practice - decoupling would make more difference.

[PartialDischarge]

I tried with a 74VHC gate but it is slower, I'm very happy with the results since 2.5V 500ps is very useful for many applications, it is a fairly cheap solution and aberrations are minimal which I consider important

[tggzzz]

I tried with a 74VHC gate but it is slower, I'm very happy with the results since 2.5V 500ps is very useful for many applications, it is a fairly cheap solution and aberrations are minimal which I consider important

Agreed.

You can also reduce the output swing by reducing the PSU voltage, but the transition time increases. Try reducing Vcc  to 1V or so, and seeing if you get a 0.5V swing with 3ns risetime (and some "pre-undershoot" for want of a better term). You can go even lower, with some entertaining[1] aberrations The aberrations are gone by 1.7V or so, as expected.

[1] I'm easily entertained

[tchiwam]

4 outputs on top of each other

100us pulse 1 PPS rising edge ~3GHz BW

[tggzzz]

4 outputs on top of each other

100us pulse 1 PPS rising edge ~3GHz BW

4 outputs of what?

Why not use averaging?

PPS? And those figures don't seem to match the figures in the photograph.

[tchiwam]

I was comparing 4 of the 8 putputs from my PPS distribution. That's s the rising edge of a 100us pulse.

The measurements on the side are out of the window so they are wonky except for the cursors.

[bozidarms]

on CSA830A, my new toy   - 20,8ps -  rise and fall time!

[HighVoltage]

@bozidarms

20.8 ps rise and fall times ?

What generator do you use for that?
And what scope and probe?

[bozidarms]

It`s one Tektronix CSA803A with SD-24 TDR/sampling head.
However, that is my 3. SD-24, which has characteristics far more better than proclamated.
The other two wasn't so god.

That is a test pulse from SD-24 itself on SMA out, which is only shorted.
Measurement is at 10% - 90%.

[Aldo22]

- kind of curious to see what is the lowest price device is that can produce a really good looking square wave at 1, 5, 10, 20, and 30 MHz?

I don't know if "really good looking" but it's very low priced. 
The signal comes from a ESP32 microcontroller  for ~ 5$.
The oscilloscope is a handheld ZEEWEII DSO2512G for ~ 80$.

Please note:
The 40MHz measurement was made in the overclocking mode of the device (Menu Aux + ⇧ [long])
with 250MHz probes and a ground spring.

[the Chris]

My pulse generator from Leo Bodnar arrived yesterday, so I would like to contribute with my slow-but-I-am-loving-it EDUX1052A. Measurements were taken with an RS Pro 50Ohm inline terminator that I ordered from Leo alongside the pulser.

I got 5.8767ns over 112k measurements. Considering the specified 50MHz and therefore assuming a classic -3dB drop, the BW = 0.35 / r_t should be valid, resulting in 60MHz bandwidth.

This is in line with what I estimated using a 10MHz to 60Mhz sinus sweep of my UTG962E, with pretty much no drop under 10MHz (not shown) and about ~1.2V peak in amplitude at 60MHz using a N2142A 75MHz probe in 10x mode.

Thanks Leo for the fast shipping and now including the customs and taxes loads for EU members!

EDIT: 5ns/div measurement added

[David Hess]

My pulse generator from Leo Bodnar arrived yesterday, so I would like to contribute with my slow-but-I-am-loving-it EDUX1052A. Measurements were taken with an RS Pro 50Ohm inline terminator that I ordered from Leo alongside the pulser.

I got 5.8767ns over 112k measurements. Considering the specified 50MHz and therefore assuming a classic -3dB drop, the BW = 0.35 / r_t should be valid, resulting in 60MHz bandwidth.

I wonder where that aberration came from; it is much greater than I would expect considering the much faster settling time of the pulse generator compared to a 60 MHz oscilloscope.

My guess is that the oscilloscope does *not* have a Gaussian or single pole response.

[Sighound36]

Below are four examples of square wave forms I looked at earlier, one using the Siglent SDG2042A 10Mhz 4V pk<>pk / 10Mhz ultra low phase noise -124dbm@1hz to -175dbm@10Mhz/ Leo Bodnar 40pS rise time generator/ 44.1Khz word clock.

Using Wavepro 4Ghz HD scope 50 Ohm T-flex 405 18Ghz cabling, full scope BW of 4Ghz NO ERS (software enhancement) up to 20GS/s rate. Scope up to temprature and calibrated.

[Sighound36]

As we can only post 10 attachments second post last of the 44.1Khz images plus I thought if would be nice to show the eye pattern for each of these square wave sources as well

[joeqsmith]

I followed  Leo Bodnar's threads about that pulsar but never bought one.   All of my scopes are too slow to verify it. 

Many years ago, I stumbled onto an employee of LeCroy who had purchased some of their junk at an internal auction.  They had the test fixtures for the 7200A.  Another gem was they had some of the 7262 plugins.  These are the only ones I have ever seen.   This plugin includes an ECL output for TDR measurements and has an analog BW of 4GHz.   I posted some data for it recently where it measures 118ps.   

https://www.eevblog.com/forum/testgear/12-ghz-active-probe-project/msg5007958/#msg5007958

I posted about my conversations with LeCroy about the risetime and bandwidth:
https://www.eevblog.com/forum/testgear/show-us-your-square-wave/msg593071/#msg593071

Typically RT*BW = 0.40 to 0.45 for modern high bandwidth scopes

4GHz * 118ps = 0.472.   Much higher than 0.35.   

If I attach another old relic, Tektronix S-52 which has a risetime of less than 25ps, the scope measures 111ps.   Or 0.444. 

This scope was my first DSO, outside of my HP scope/logic analyzer.   Originally owned by the USAF.   Released in 1989.   She's old but fun to play with.

[Grandchuck]

Here is a result from a Tektronix Scope Evaluation board and a Siglent DS2202.  The board has a 4-bit counter with a risetime of around 2 nsec with some overshoot.

[tggzzz]

Here is a result from a Tektronix Scope Evaluation board and a Siglent DS2202.  The board has a 4-bit counter with a risetime of around 2 nsec with some overshoot.

How was the probe's shield connected to the board's ground? A 6inch ground lead is a classic cause of overshoot.

[Grandchuck]

Your guess is close ... it is a 5 inch lead.  They put the ground pad as far from TP1 as possible???
I included a shot from the manual that came with the board.

[joeqsmith]

That's funny to even see one of these boards.   I picked up what appears to be the same one directly from Tektronix when I bought my first scope (Hitachi) back in the 80s.   I think they were giving these away back then as a promotion.   
 
Connecting from pin1 to pin4 using a LeCroy PP061 resistive probe with a pretty much dead battery,  scope measure 1.65ns without the big overshoot.  The BW is much higher than what you have available and there is no ringing.  I also suspect your connection from the board to the scope. 

[tggzzz]

Your guess is close ... it is a 5 inch lead.  They put the ground pad as far from TP1 as possible???
I included a shot from the manual that came with the board.

Have a look at https://entertaininghacks.wordpress.com/2015/04/23/scope-probe-accessory-improves-signal-fidelity/

As a quick test I would wrap a length of wire around the probe body, with a short straight spike to the gnd hole on the left. In other words, a variant of the "spiked spring" supplied with the probe.

[joeqsmith]

Recently someone was asking about how to use a scope and signal generator to measure the self resonance frequency of an inductor.    I provided several examples, the later using a square wave to cause the inductor to ring at at the SRF.   

https://www.eevblog.com/forum/beginners/measuring-the-self-resonant-frequency-of-an-inductor/

As long as I have the pulser out... shown is a Mini-circuits 6.3 - 15GHz high pass.  Data sheet may be found at:
https://www.minicircuits.com/pdfs/VHF-6010+.pdf

I attached this to the old Tektronix pulser and then into my LeCroy 8500A.  This scope has a BW of 5GHz.  They claim a typical rise time of 90ps.   Obviously the filter is outside the limit of what the scope can measure.  Ignore that.  The signal was stable and I used RIS to get a clearer picture what is going on.   What I thought was interesting is it appears to have a dominant SRF of about 6.2GHz.

In the end, who smacks a filter like this anyway.... 

[Grandchuck]

tggzzz and Joe are great motivators.  Look and see what happens.
Now, why would Tektronix do that?  One would think that their pictured result would exemplify best practices???