Function generator square wave rise time

[hp3310a]

Hi everyone,

  I'm comparing specs of my UTG962E (that one I have) with the Agilent 33220a (this one I'd like) and on paper the square wave specs appear about the same. One thing puzzles me though:

At 20MHz, the period is 50ns. The rise times (0-100% or is it 10-90%, not specified) is given at <16ns for UTG962E and <13ns for 33220a. Assuming rise and fall times roughly the same, the total rise and fall times are 4*15ns=60ns for the Unitrend and 4*12ns=48ns for the Agilent respectively*. So there is no space left for a flat top on the wave.

On paper as well as on my oscilloscope, the UTG962 can't generate a square wave at 20MHz. On my scope (SDS824X-HD) it's a sine wave. The rise time (10-90%) is actually much faster than spec'd at 12ns.

What I find surprising is that the Agilent is (on paper) not much better. Is the reality different? Maybe the 13ns of the Agilent is an absolute worst case on a bad day?

* see below

[hp3310a]

I just realized that on the scope the 12ns was for the entire rise, not half of it. So on paper numbers would be 50% for rise and fall and 50% for the flat portion of the square. Which doesn't exist on my scope.

So the better question is: Will the Agilent behave more gracefully and produce something resembling a square wave at 20MHz?

[Aldo22]

It looks like “reputable” function generators don't always have particularly fast rise times (square wave).
Your two options are nothing special (according to the specs).
An FY6900 only needs <7ns and the one built into my cheap Hantek scope <5ns (screenshot, 20MHz).
No idea why this is the case.

[hp3310a]

An FY6900 only needs <7ns and the one built into my cheap Hantek scope <5ns (screenshot, 20MHz).
No idea why this is the case.

Hmm, that makes you think. What kind of FY6900 do you have, I take it the 100MHz version?

[Aldo22]

An FY6900 only needs <7ns and the one built into my cheap Hantek scope <5ns (screenshot, 20MHz).
No idea why this is the case.

Hmm, that makes you think. What kind of FY6900 do you have, I take it the 100MHz version?

The screenshot is from the Hantek DSO2000 AWG.
I don't have the FY6900, the 7ns is from the specifications.

I just wanted to give two examples, that rise times do not seem to be directly related to price or overall quality.

[hp3310a]

This is really weird, the data sheet for the Keysight 33250a 80MHz function generator (https://www.keysight.com/us/en/assets/7018-06693/data-sheets-archived/5968-8807.pdf) claims square wave up to 80MHz, yet the rise time is <8ns. The period for 80MHz is 12.5ns. WTF. How can those two things be true at the same time?

[2N3055]

I just realized that on the scope the 12ns was for the entire rise, not half of it. So on paper numbers would be 50% for rise and fall and 50% for the flat portion of the square. Which doesn't exist on my scope.

So the better question is: Will the Agilent behave more gracefully and produce something resembling a square wave at 20MHz?

Agilent will produce signal exactly as specified. 20 MHz is 50ns for full period that consists of rise for 12,5ns , than one high flat, then falling edge of 12.5ns, and then one low flat. So each flat is also 12,5 ns for total of 50ns.
It is trapezoidal signal, which every square wave is, just to different extent.

So no it won't be better, than cheap UTG962E.
UTG962E is very cute little device that is surprisingly useful and very good for it's price.
But it is a hobby level device.

From profesional type of devices, very inexpensive Siglent SDG1000X series has much better rise/fall times at 3-4 ns .
It has 2 output channels and generally better capabilities than long discontinued  33220A.

Since you say you have Siglent 800xHD scope, that Siglent AWG can also be used with scope for Bode plots...

If you want something better than UTG962E (which is really fine device for what it is) I would go with Siglent AWG.

Make note that if you need best square wave, SDG1000X series is better in that regard than new SDG1000X+ or even SDG2000X.
These 2 other AWG are better in other regards, but 1000X series had very good squarewave specs.

If you explain more your use case, maybe more help can be provided.

[2N3055]

This is really weird, the data sheet for the Keysight 33250a 80MHz function generator (https://www.keysight.com/us/en/assets/7018-06693/data-sheets-archived/5968-8807.pdf) claims square wave up to 80MHz, yet the rise time is <8ns. The period for 80MHz is 12.5ns. WTF. How can those two things be true at the same time?

They can't. In datasheet it says risetime can be down to 3.5 ns at higher frequencies. But that is also at the level of SDG1000X that has that specification guaranteed for whole freq range..

These Keysights are old discontinued products. They are good, and if you already have one it's fine. They work well. They have one good characteristic and that is output BNC ground is isolated (low voltage only) from instrument ground.
Everything is else is really nothing special today.

[DaneLaw]

Rigol DG821PRO seems to be one of the cheaper options for a general modern AWG on fast risetimes, atleast in the ballpark to a few hundred bucks.

On paper its 3ns rise/fall.. if I recall correctly.
but with the UI bug (non hacked).. it seems to do 5ns full period (200MHz) on most of the waveforms, sine, square, ramp & arb (incl. 148 built in), and 4ns on a pulse period (250MHz)
Alongside modulation on those values/waveforms.
I don't have a scope that can tolerate those fast values, so in lack of better I've been pumping it up its own cloaca-Hz-counter, that seems to play along (at least to an extent) up to 1.4GHz
- but you're obviously way out of bounds from its official tolerances.

DG821PRO Hz-counter 4.9ns period.

[hp3310a]

Agilent will produce signal exactly as specified. 20 MHz is 50ns for full period that consists of rise for 12,5ns , than one high flat, then falling edge of 12.5ns, and then one low flat. So each flat is also 12,5 ns for total of 50ns.
It is trapezoidal signal, which every square wave is, just to different extent.

So no it won't be better, than cheap UTG962E.

If the waveform on the Agilent looks like that (trapezoidal) then it's clearly much better than the UTG. Even at 10MHz there's not much left of a square. I'm not complaining, it's great still.

Make note that if you need best square wave, SDG1000X series is better in that regard than new SDG1000X+ or even SDG2000X.
These 2 other AWG are better in other regards, but 1000X series had very good squarewave specs.

I looked at all the data sheets of the Siglent series, and even the SDG5000X has inferior specs wrt. square wave. Only the SDG6000X is way better at 2ns.

If you explain more your use case, maybe more help can be provided.

There's no use case driving this, more a question if it makes sense to replace what I have with what my monkey brain says it wants.

Rigol DG821PRO seems to be one of the cheaper options for a general modern AWG on fast risetimes, atleats in the ballpark to a few hundred bucks.

Indeed, the DG800PRO (up to 40MHz) have a rise time of <=3ns. That's quite unbeatable at that price point (same as SDG1000X). The square-specs are the same for the DG900PRO series too (up to 60MHz). So a square wave should look like one on those.

[DaneLaw]

There is also Siglent's new SDG3000X with a decent big touchscreen that also in the 2.5 to 3ns ballpark (PRBS 2.5ns)

[tautech]

There is also Siglent's new SDG3000X with a decent big touchscreen that also in the 2.5 to 3ns ballpark (PRBS 2.5ns)

Could be some months before western release.

[edpalmer42]

As a general rule of thumb, don't expect a "function" generator to be a good square wave generator.  They tend to be a 'jack of all trades' box.  Yes, there are probably exceptions.

Remember that to get a good square wave you combine the fundamental frequency plus many harmonics.  So, to get a good 10 MHz square wave, you might need harmonics up to 100 MHz.  It's unlikely that the output stage of a 10 MHz function generator is designed for that.  Some high end models might have a dedicated square wave output to get around this.

Also, the same rule applies to your scope.  If the scope can't pass the harmonics, even a good square wave will look like crap.

So, what you finally see is a picture of a signal that's been distorted by the generator and the scope - and maybe the probe if you're not careful.

[hfleming]

There is also Siglent's new SDG3000X with a decent big touchscreen that also in the 2.5 to 3ns ballpark (PRBS 2.5ns)

Could be some months before western release.

Speaking of which… already have an SDG2000X but lately I have my I on the SDG6000X-series (mainly for the I-Q waveforms)… is there a new model in the works, and should I hold off for a bit longer in the hope of a new model coming up in the next year or so?

[bdunham7]

So the better question is: Will the Agilent behave more gracefully and produce something resembling a square wave at 20MHz?

Probably not significantly better and you certainly won't have a flat top.  The -3dB BW that a 13ns rise time implies is about 25MHz, so you'll get the fundamental plus almost no harmonics.  For various reasons, AWG and DDS sig gens often have rise times and BW in the pulse and square wave modes that are significantly less than the BW in other modes.  For example, my 120MHz SDG2122X has a minimum selectable rise time of 8.4ns (this is 20-80% AFAIK) which corresponds to about 40MHz.  So for a 20MHz "square wave", I get a "hump wave" even though the sig gen clearly can generate a sine wave with much faster skew.

Some sig gens may have a special square wave or pulse mode that has faster rise times, but it is going to be difficult to get a clean, crisp 20MHz square wave especially if you want a universal device that can give you arbitrary frequencies and periods. 

[tautech]

There is also Siglent's new SDG3000X with a decent big touchscreen that also in the 2.5 to 3ns ballpark (PRBS 2.5ns)

Could be some months before western release.

Speaking of which… already have an SDG2000X but lately I have my I on the SDG6000X-series (mainly for the I-Q waveforms)… is there a new model in the works, and should I hold off for a bit longer in the hope of a new model coming up in the next year or so?

SDG3000X will apparently support I-Q like SDG6000X but you need compare the specs.
You can get the translated datasheet here:
https://www.eevblog.com/forum/testgear/new-siglent-awg-sdg3000x/

[tggzzz]

As a general rule of thumb, don't expect a "function" generator to be a good square wave generator.

True.

So, to get a good 10 MHz square wave, you might need harmonics up to 100 MHz.

False.

The period/frequency is irrelevant; the risetime is the only important parameter. Even a 1Hz 74LS TTL signal "needs" 100MHz bandwidth, and a 74LVC1G signal "needs" GHz.

For a little theory and a practical demonstration, see https://entertaininghacks.wordpress.com/2018/05/08/digital-signal-integrity-and-bandwidth-signals-risetime-is-important-period-is-irrelevant/

[edpalmer42]

So, to get a good 10 MHz square wave, you might need harmonics up to 100 MHz.

False.

The period/frequency is irrelevant; the risetime is the only important parameter. Even a 1Hz 74LS TTL signal "needs" 100MHz bandwidth, and a 74LVC1G signal "needs" GHz.

For a little theory and a practical demonstration, see https://entertaininghacks.wordpress.com/2018/05/08/digital-signal-integrity-and-bandwidth-signals-risetime-is-important-period-is-irrelevant/

I agree that the period of the original signal is irrelevant.  I never stated otherwise.  However, risetime and bandwidth are two sides of the same coin.  To have a fast risetime, you need wide bandwidth.  The wide bandwidth will allow the signal's harmonics to pass which will add up to create a fast risetime.  Odd harmonics are necessary to create a square wave.  So, for my example of a 10 MHz fundamental, 30, 50, 70, 90, etc. MHz harmonics must pass through the system.  Depending on how fast a risetime you want, that might not be enough.  Theoretically, you need an infinite number of harmonics.  This is standard FFT math.  It can be easily demonstrated by looking at a square wave with a spectrum analyzer.

The post you linked to is correct as far as it goes.  But only if the system bandwidth is much greater than the fundamental frequency so that the harmonics aren't attenuated.  A point-to-point connection for a digital signal usually meets this requirement.

But this discussion is only vaguely connected to the original question i.e. performance of a square wave generator.  To generate a good square wave, the generator's output stage must have a bandwidth that's many times greater than the fundamental frequency.  Logic ICs like the 74LS and 74LVC1G that you mentioned have appropriate output stages.  Function generators typically don't.

[David Hess]

Square waves are tough for a function generator to produce because the output passes through a linear amplifier which has gain-bandwidth and slew rate limitations.  So typically the fastest square waves look more like sin waves.  Pulse generators can be much much faster.

A clean response typically requires the final linear amplifier to have a single pole frequency roll-off, which limits bandwidth, and class-AB amplifiers are tough to design past about 50 MHz, yielding a 7 nanosecond rise and fall time.  A modern function generator can do much better with an integrated amplifier, like a current feedback operational amplifier, however this will limit output voltage into 50 ohms, which may not be acceptable.

I imagine a modern discrete design using a pile of 2N4401 and 2N4403 transistors in parallel to get enough power dissipation to handle 15 volts peak-to-peak into 50 ohms.  My old fast function generators use fast metal can transistors which were discontinued decades ago.  Maybe use a pile of LT1252 or LT1253 "low cost" current feedback operational amplifiers in parallel.

[bdunham7]

Square waves are tough for a function generator to produce because the output passes through a linear amplifier which has gain-bandwidth and slew rate limitations. 

True, but for a DDS-type AWG with fixed clock, there's another limiting factor that you can see in my post above.  For square waves or pulses of arbitrary period and frequency, the desired transition from low to high or vice versa will not typically occur right at the same time as a sample.  If you just use an abrupt transition from a high sample before to a low sample after, then the high/low transitions will have a lot of jitter, up to one full sample clock period.  The fix is to always shape the transition using the three nearest sample points and the only way to acheive a uniform result is to shape it to a much slower rise time than you might otherwise get.  In my example, the output amplifier could achieve a 3ns rise time, a bit faster than the scope itself.  But the algorithm deliberately yields a much slower 8.4ns edge using a 300MHz sample clock plus a 4X oversampling 120MHz low-pass output filter.

[tggzzz]

Square waves are tough for a function generator to produce because the output passes through a linear amplifier which has gain-bandwidth and slew rate limitations. 

True, but for a DDS-type AWG with fixed clock, there's another limiting factor that you can see in my post above.  For square waves or pulses of arbitrary period and frequency, the desired transition from low to high or vice versa will not typically occur right at the same time as a sample.  If you just use an abrupt transition from a high sample before to a low sample after, then the high/low transitions will have a lot of jitter, up to one full sample clock period.  The fix is to always shape the transition using the three nearest sample points and the only way to acheive a uniform result is to shape it to a much slower rise time than you might otherwise get.  In my example, the output amplifier could achieve a 3ns rise time, a bit faster than the scope itself.  But the algorithm deliberately yields a much slower 8.4ns edge using a 300MHz sample clock plus a 4X oversampling 120MHz low-pass output filter.

Some generators put the DDS output through a low pass filter to produce a sine wave, then square that up with a comparator. The net result is that the square wave edges are not constrained by the DDS clock edges, and consequently do not have the jitter you mention.

See, for example, the application section of in https://www.analog.com/media/en/technical-documentation/data-sheets/AD9851.pdf Such applications are cheaply available on fleabay.

[bdunham7]

False.

The period/frequency is irrelevant; the risetime is the only important parameter.

I think that whether or not one would describe a particular waveform as "square" does indeed depend on the relationship between risetime and period.  If your 1-Hz signal from your high speed logic was displayed on a scope with a 1ms rise time, it would still look pretty square.  The differences in risetime would be relevant if you were interested in accurately measuring or portraying, well, risetime.

[tggzzz]

So, to get a good 10 MHz square wave, you might need harmonics up to 100 MHz.

False.

The period/frequency is irrelevant; the risetime is the only important parameter. Even a 1Hz 74LS TTL signal "needs" 100MHz bandwidth, and a 74LVC1G signal "needs" GHz.

For a little theory and a practical demonstration, see https://entertaininghacks.wordpress.com/2018/05/08/digital-signal-integrity-and-bandwidth-signals-risetime-is-important-period-is-irrelevant/

I agree that the period of the original signal is irrelevant.  I never stated otherwise.  However, risetime and bandwidth are two sides of the same coin.  To have a fast risetime, you need wide bandwidth.  The wide bandwidth will allow the signal's harmonics to pass which will add up to create a fast risetime.  Odd harmonics are necessary to create a square wave.  So, for my example of a 10 MHz fundamental, 30, 50, 70, 90, etc. MHz harmonics must pass through the system.  Depending on how fast a risetime you want, that might not be enough.  Theoretically, you need an infinite number of harmonics.  This is standard FFT math.  It can be easily demonstrated by looking at a square wave with a spectrum analyzer.

Your post mentioned only "square wave" and "frequency" (i.e. period). It did not mention the key parameter: risetime.

It is standard Fourier Transform theory, but too many people don't realise that because they only consider the special case of square waves with a 50% duty cycle.

The post you linked to is correct as far as it goes.  But only if the system bandwidth is much greater than the fundamental frequency so that the harmonics aren't attenuated.  A point-to-point connection for a digital signal usually meets this requirement.

I have no idea what any of that means, especially in this context.

But this discussion is only vaguely connected to the original question i.e. performance of a square wave generator.  To generate a good square wave, the generator's output stage must have a bandwidth that's many times greater than the fundamental frequency.  Logic ICs like the 74LS and 74LVC1G that you mentioned have appropriate output stages.  Function generators typically don't.

We don't know how the OP will use their function generator. If it is to drive modern logic (i.e. mid 80s onwards) the risetime limitations may be a problem for them.

General purpose function generators typically generate rectangular waves, with square waves being a special case. Clock generators may be different; see my previous post about DDS generators.

[MK14]

This is really weird, the data sheet for the Keysight 33250a 80MHz function generator (https://www.keysight.com/us/en/assets/7018-06693/data-sheets-archived/5968-8807.pdf) claims square wave up to 80MHz, yet the rise time is <8ns. The period for 80MHz is 12.5ns. WTF. How can those two things be true at the same time?

Although at a quick glance, it would seem to say a rise/fall time of <8ns in that spec sheet.

Rise/fall time < 8 ns4

On more careful examination, you may notice that there is a tiny '4' by it, which much lower on the data sheet, goes into more detail.  Basically, explaining that at much higher frequency settings, it really is around 3.5ns, possibly explaining your query.

From that spec sheet:

Edge time decreased at higher frequency, 3.5 nS (typical)

[tggzzz]

False.

The period/frequency is irrelevant; the risetime is the only important parameter.

I think that whether or not one would describe a particular waveform as "square" does indeed depend on the relationship between risetime and period.  If your 1-Hz signal from your high speed logic was displayed on a scope with a 1ms rise time, it would still look pretty square.  The differences in risetime would be relevant if you were interested in accurately measuring or portraying, well, risetime.

Mere appearance is irrelevant; this isn't a beauty contest

Typical function generators can create outputs where the duty cycle is far from 50%.

As for "waveforms", consider the characteristics of

• the 1pps output from a GPDSO. How that looks on a scope with a 100ms/div sweep speed is irrelevant
• a reset signal. That may have a frequency far lower than 1Hz, (i.e. period far longer than 1s), but still needs fast transitions

[edpalmer42]

Some generators put the DDS output through a low pass filter to produce a sine wave, then square that up with a comparator. The net result is that the square wave edges are not constrained by the DDS clock edges, and consequently do not have the jitter you mention.

See, for example, the application section of in https://www.analog.com/media/en/technical-documentation/data-sheets/AD9851.pdf Such applications are cheaply available on fleabay.

The Stanford Research DS345 does something very similar.  They create a synthesized sine wave with sub-Hz resolution which is great.  But then they use a comparator to square it up which adds a different source of jitter.  Noise on the sine wave - there's always noise everywhere - causes jitter in the transition points.  This jitter negates the value of the sub-Hz resolution.  I was sort of able to access the high precision signal by using the sync output and the arb waveform capability, but even that had issues.

[tggzzz]

Some generators put the DDS output through a low pass filter to produce a sine wave, then square that up with a comparator. The net result is that the square wave edges are not constrained by the DDS clock edges, and consequently do not have the jitter you mention.

See, for example, the application section of in https://www.analog.com/media/en/technical-documentation/data-sheets/AD9851.pdf Such applications are cheaply available on fleabay.

The Stanford Research DS345 does something very similar.  They create a synthesized sine wave with sub-Hz resolution which is great.  But then they use a comparator to square it up which adds a different source of jitter.  Noise on the sine wave - there's always noise everywhere - causes jitter in the transition points.  This jitter negates the value of the sub-Hz resolution.  I was sort of able to access the high precision signal by using the sync output and the arb waveform capability, but even that had issues.

Too true; TANSTAAFL

I saw an inverse variant of that with the AD9851 evaluation board from fleabay. I could see a small glitch on the sinewave output, which was caused by the comparator switching. Solution: disconnect the comparator input driven by the sinewave output.

[edpalmer42]

So, to get a good 10 MHz square wave, you might need harmonics up to 100 MHz.

False.

The period/frequency is irrelevant; the risetime is the only important parameter. Even a 1Hz 74LS TTL signal "needs" 100MHz bandwidth, and a 74LVC1G signal "needs" GHz.

For a little theory and a practical demonstration, see https://entertaininghacks.wordpress.com/2018/05/08/digital-signal-integrity-and-bandwidth-signals-risetime-is-important-period-is-irrelevant/

I agree that the period of the original signal is irrelevant.  I never stated otherwise.  However, risetime and bandwidth are two sides of the same coin.  To have a fast risetime, you need wide bandwidth.  The wide bandwidth will allow the signal's harmonics to pass which will add up to create a fast risetime.  Odd harmonics are necessary to create a square wave.  So, for my example of a 10 MHz fundamental, 30, 50, 70, 90, etc. MHz harmonics must pass through the system.  Depending on how fast a risetime you want, that might not be enough.  Theoretically, you need an infinite number of harmonics.  This is standard FFT math.  It can be easily demonstrated by looking at a square wave with a spectrum analyzer.

Your post mentioned only "square wave" and "frequency" (i.e. period). It did not mention the key parameter: risetime.

It is standard Fourier Transform theory, but too many people don't realise that because they only consider the special case of square waves with a 50% duty cycle.

Risetime is not a key parameter.  Risetime is determined by the bandwidth of the system.  If you take one of Leo Bodnar's fast pulse generators (10 MHz, 30 ps risetime, 50% duty cycle) and feed it through a 100 MHz low pass filter you won't have a 30 ps risetime any more because the system doesn't have the bandwidth to support those harmonics.  A similar result will occur if you send the pulse through a buffer that only has a 100 MHz bandwidth.  So whether you're talking about signal generation or measurement, you need a frankly ridiculous amount of bandwidth just to see a good square wave.  Your application will determine how good your square wave needs to be.

But this discussion is only vaguely connected to the original question i.e. performance of a square wave generator.  To generate a good square wave, the generator's output stage must have a bandwidth that's many times greater than the fundamental frequency.  Logic ICs like the 74LS and 74LVC1G that you mentioned have appropriate output stages.  Function generators typically don't.

We don't know how the OP will use their function generator. If it is to drive modern logic (i.e. mid 80s onwards) the risetime limitations may be a problem for them.

At this point, the OP is trying to understand how the specs relate to what he's seeing on the scope.  Typically, the answer is easier to understand if it's given in the frequency domain rather than the time domain.  Application issues are TBD.

I looked around to see if I could find an online live demo of how risetime and bandwidth interact but I didn't find anything I liked.  Does anyone know of something good?

Found one!  https://controlsystemsacademy.com/0018/0018.html  You can zoom in on the time domain plot, run the Fourier Coefficients (aka Harmonics) back and forth and then grab a coefficient and drag it up and down.  Watch what happens to the risetime as you add or remove harmonics.  Tggzzz, I realize that this isn't new to you - it's for the younger people.

[tggzzz]

So, to get a good 10 MHz square wave, you might need harmonics up to 100 MHz.

False.

The period/frequency is irrelevant; the risetime is the only important parameter. Even a 1Hz 74LS TTL signal "needs" 100MHz bandwidth, and a 74LVC1G signal "needs" GHz.

For a little theory and a practical demonstration, see https://entertaininghacks.wordpress.com/2018/05/08/digital-signal-integrity-and-bandwidth-signals-risetime-is-important-period-is-irrelevant/

I agree that the period of the original signal is irrelevant.  I never stated otherwise.  However, risetime and bandwidth are two sides of the same coin.  To have a fast risetime, you need wide bandwidth.  The wide bandwidth will allow the signal's harmonics to pass which will add up to create a fast risetime.  Odd harmonics are necessary to create a square wave.  So, for my example of a 10 MHz fundamental, 30, 50, 70, 90, etc. MHz harmonics must pass through the system.  Depending on how fast a risetime you want, that might not be enough.  Theoretically, you need an infinite number of harmonics.  This is standard FFT math.  It can be easily demonstrated by looking at a square wave with a spectrum analyzer.

Your post mentioned only "square wave" and "frequency" (i.e. period). It did not mention the key parameter: risetime.

It is standard Fourier Transform theory, but too many people don't realise that because they only consider the special case of square waves with a 50% duty cycle.

Risetime is not a key parameter.  Risetime is determined by the bandwidth of the system.  If you take one of Leo Bodnar's fast pulse generators (10 MHz, 30 ps risetime, 50% duty cycle) and feed it through a 100 MHz low pass filter you won't have a 30 ps risetime any more because the system doesn't have the bandwidth to support those harmonics.  A similar result will occur if you send the pulse through a buffer that only has a 100 MHz bandwidth.  So whether you're talking about signal generation or measurement, you need a frankly ridiculous amount of bandwidth just to see a good square wave.  Your application will determine how good your square wave needs to be.

Risetime and bandwidth are indeed interchangeable. That's precisely why it is misleading to start from a square/rectangular wave's period or fundamental frequency.

What bandwidth is needed for a "good" 1kHz square/rectangular waveform? Numbers, not adjectives, please. Is it any different for a 1µHz waveform?

But this discussion is only vaguely connected to the original question i.e. performance of a square wave generator.  To generate a good square wave, the generator's output stage must have a bandwidth that's many times greater than the fundamental frequency.  Logic ICs like the 74LS and 74LVC1G that you mentioned have appropriate output stages.  Function generators typically don't.

We don't know how the OP will use their function generator. If it is to drive modern logic (i.e. mid 80s onwards) the risetime limitations may be a problem for them.

At this point, the OP is trying to understand how the specs relate to what he's seeing on the scope.  Typically, the answer is easier to understand if it's given in the frequency domain rather than the time domain.

Not if you are using a time domain instrument such as an oscilloscope.

[Aldo22]

@hp3310a: If you simply want to see higher frequency square waves, you can use an Si5351.
You can't set the amplitude, offset or duty cycle directly, but it might be enough for simple tests.
https://learn.adafruit.com/adafruit-si5351-clock-generator-breakout

It just depends on what you need.
In the screenshot you see approx. 40MHz. That's where I reach the limitations of my cheap scope.

[gf]

Square waves are tough for a function generator to produce because the output passes through a linear amplifier which has gain-bandwidth and slew rate limitations. 

True, but for a DDS-type AWG with fixed clock, there's another limiting factor that you can see in my post above.  For square waves or pulses of arbitrary period and frequency, the desired transition from low to high or vice versa will not typically occur right at the same time as a sample.  If you just use an abrupt transition from a high sample before to a low sample after, then the high/low transitions will have a lot of jitter, up to one full sample clock period. The fix is to always shape the transition using the three nearest sample points and the only way to acheive a uniform result is to shape it to a much slower rise time than you might otherwise get.

Exactly.

Since AWGs rely on Shannon-Nyquist reconstruction of a continuous-time signal from discrete-time samples, they can only generate bandwidth-limited signals.

The Shannon-Nyquist sampling theorem does, of course, also apply when the DDS resamples the prototype waveform stored in its wavetable. If the prototype waveform isn't properly bandwidth-limited for the DDS resampling, it will suffer from aliasing, resulting in these edge jitter artifacts. Low-jitter reproduction at higher frequencies is only possible if the wavetable contains a properly bandwidth-limited square wave, which implies that it cannot have sharp edges.

The fix is to always shape the transition using the three nearest sample points and the only way to acheive a uniform result is to shape it to a much slower rise time than you might otherwise get. In my example, the output amplifier could achieve a 3ns rise time, a bit faster than the scope itself.  But the algorithm deliberately yields a much slower 8.4ns edge using a 300MHz sample clock plus a 4X oversampling 120MHz low-pass output filter.

At 300MSa/s, Nyquist is at 150MHz. If you pass an ideal step through a (non-ideal, realizable) brickwall filter with a (say) 120...150MHz transition band to get a properly bandwidth-limited step, you can indeed obtain an edge with about 3.4ns rise time, but there will be significant overshoot/ringing. If you want to have less overshoot and still not exceed the bandwidth, you will deliberately have to make the edges softer.

[bdunham7]

Risetime and bandwidth are indeed interchangeable. That's precisely why it is misleading to start from a square/rectangular wave's period or fundamental frequency.

What bandwidth is needed for a "good" 1kHz square/rectangular waveform? Numbers, not adjectives, please. Is it any different for a 1µHz waveform?

They're only interchangeable in a first-order approximation sort of way and that approximation is dependent on the characteristics of the system.  The 0.35 factor just happens to work reasonably well for a fair number of simple front-ends on common oscilloscopes from DC to a few hundred MHz.  I don't think it is misleading at all to use BW expressed in multiples of the fundamental because this both explains the Fourier representation of the square wave and comes up with a  reasonable answer that is easy to understand.

You need 10kHz BW to show a reasonably but not perfectly square 1kHz wave.  100kHz will make a nice clean, sharp picture.  It's a lot different for 1µHz.  Here is a nice clean, sharp, very square 500µHz wave with a rise time of 10 seconds.  1µHz would still look nice and square with a rise time of several minutes.

[bdunham7]

The Stanford Research DS345 does something very similar.  They create a synthesized sine wave with sub-Hz resolution which is great.  But then they use a comparator to square it up which adds a different source of jitter.  Noise on the sine wave - there's always noise everywhere - causes jitter in the transition points.  This jitter negates the value of the sub-Hz resolution.  I was sort of able to access the high precision signal by using the sync output and the arb waveform capability, but even that had issues.

That seems like an amateur mistake to make by Stanford Research.  I'd think the obvious way to make precision sub-Hz square waves would be to synthesize a sine wave at a high multiple of the desired frequency and then divide down the comparator output in logic. 

However, the real challenge of building in a sharp square wave into an AWG is the nature of the AWG and how it is used.  Users expect to be able to set at least frequency, amplitude, offset and duty cycle and then perhaps rise time and fall time separately.  This is all handled by the DAC in normal operation, but if you use a comparator IDK how you regain all that.  Generating fast pulses and sharp-edged square waves is probably best left to separate specialized equipment.

[tggzzz]

Risetime and bandwidth are indeed interchangeable. That's precisely why it is misleading to start from a square/rectangular wave's period or fundamental frequency.

What bandwidth is needed for a "good" 1kHz square/rectangular waveform? Numbers, not adjectives, please. Is it any different for a 1µHz waveform?

They're only interchangeable in a first-order approximation sort of way and that approximation is dependent on the characteristics of the system.  The 0.35 factor just happens to work reasonably well for a fair number of simple front-ends on common oscilloscopes from DC to a few hundred MHz.  I don't think it is misleading at all to use BW expressed in multiples of the fundamental because this both explains the Fourier representation of the square wave and comes up with a  reasonable answer that is easy to understand.

The 0.35 rule of thumb is based on how some scopes' front ends interact with signals; that's all.

The problem with the simplistic rules of thumb based on period are that, while strictly true, they actively and repeatedly mislead beginners with respect to important practical signals found in many systems.

You need 10kHz BW to show a reasonably but not perfectly square 1kHz wave.  100kHz will make a nice clean, sharp picture.  It's a lot different for 1µHz.  Here is a nice clean, sharp, very square 500µHz wave with a rise time of 10 seconds.  1µHz would still look nice and square with a rise time of several minutes.

I'm perfectly aware of what things look like in a picture - and don't care very much.

I care much more about how signals behave in systems.

If you are principally interested in audio signals, then square waves may be of some use. But in digital systems, square waves are merely an unrepresentative special case: most signals are rectangular with very widely varying duty cycles.

[bdunham7]

I'm perfectly aware of what things look like in a picture - and don't care very much.

Then why hijack the thread when the OP was specifically asking about the lack of "squareness" in his 20MHz example?

I care much more about how signals behave in systems.

Other than a sig-gen and scope, the OP hasn't specified a system so how could we know what is or isn't important?  But, since he at least has a scope, think about why scope calibrator outputs are square waves.  If I were using a typical 100MHz scope with a typical calibrator output with a 5µs rise time and sharpened that up to 50ns, would it matter?

[David Hess]

The 0.35 rule of thumb is based on how some scopes' front ends interact with signals; that's all.

The 0.35 rule of thumb is calculated from the bandwidth of a single-pole or cascaded single-pole response, which is not really Gaussian but close enough, resulting in a 10% to 90% transition time.  It is not only based on the front end of some oscilloscopes.

The problem with the simplistic rules of thumb based on period are that, while strictly true, they actively and repeatedly mislead beginners with respect to important practical signals found in many systems.

The math is not misleading, but manufacturers are.  If you are not marketing or sales, then you are overhead.

The 0.35 rule is still useful to estimate bandwidth from transition time for typical linear systems.  It does not apply to non-linear systems, like when slew rate limits are reached in a Rigol DS1000Z series oscilloscopes, or any sampling oscilloscope.  This distinction could be important here but we can test it.  Does the transition time of the square waves change with amplitude?  If so, then the 0.35 rule is not going to apply, and bandwidth measurements are going to change with amplitude.

Other than a sig-gen and scope, the OP hasn't specified a system so how could we know what is or isn't important?  But, since he at least has a scope, think about why scope calibrator outputs are square waves.  If I were using a typical 100MHz scope with a typical calibrator output with a 5µs rise time and sharpened that up to 50ns, would it matter?

It should not; the calibrator output is intended for low frequency compensating attenuating probes, and whether the transition time is 5 microseconds or 50 nanoseconds will not matter.  Some calibrators produce fast edges and some produce slow edges, but in both cases they have low aberrations.  Calibrating the high frequency response requires a test signal which is considerably faster.

[tggzzz]

..., think about why scope calibrator outputs are square waves.  If I were using a typical 100MHz scope with a typical calibrator output with a 5µs rise time and sharpened that up to 50ns, would it matter?

The scope cal out signal is for adjusting the low frequency response of bog-standard "high impedance" *10 probes. "Low" means time constants of ms not ns. The principal parameter is thus absence of overshoot and (time-domain) flatness of the peaks and troughs.

Many decent standard *10 probes have several internal tweakable resistors and capacitors for tuning the high frequency performance. The cal out signal is far too "leisurely" to be of use for those purposes.

[Conrad Hoffman]

I pondered this square wave limitation for a while before buying my Siglent arb gen. It seemed like a real step down from my boat anchor generators. Then I thought about my applications, mostly audio. I just don't need crazy fast rise and fall; in fact, too fast can actually be a problem because you're essentially inputting an RF signal that can cause many amps and circuits to misbehave, where they never would do so in real life. OTOH, if you were trying to do a poor man's TDR test, you might need better. Fortunately, fast square wave and pulse circuits are pretty easy to build.

[edpalmer42]

The Stanford Research DS345 does something very similar.  They create a synthesized sine wave with sub-Hz resolution which is great.  But then they use a comparator to square it up which adds a different source of jitter.  Noise on the sine wave - there's always noise everywhere - causes jitter in the transition points.  This jitter negates the value of the sub-Hz resolution.  I was sort of able to access the high precision signal by using the sync output and the arb waveform capability, but even that had issues.

That seems like an amateur mistake to make by Stanford Research.  I'd think the obvious way to make precision sub-Hz square waves would be to synthesize a sine wave at a high multiple of the desired frequency and then divide down the comparator output in logic.

I was surprised and annoyed when I realized what they were doing.  The DS345 isn't a cheap instrument - current price starts at US$2600.  It would have been easy and cheap to add a 'high quality' square wave output with CMOS levels that bypassed all the sine wave and comparator nonsense.  The existing 'function generator' capabilities would be unaffected.

Oh, well.  It is what it is.   

[edpalmer42]

Risetime is not a key parameter.  Risetime is determined by the bandwidth of the system.  If you take one of Leo Bodnar's fast pulse generators (10 MHz, 30 ps risetime, 50% duty cycle) and feed it through a 100 MHz low pass filter you won't have a 30 ps risetime any more because the system doesn't have the bandwidth to support those harmonics.  A similar result will occur if you send the pulse through a buffer that only has a 100 MHz bandwidth.  So whether you're talking about signal generation or measurement, you need a frankly ridiculous amount of bandwidth just to see a good square wave.  Your application will determine how good your square wave needs to be.

Risetime and bandwidth are indeed interchangeable. That's precisely why it is misleading to start from a square/rectangular wave's period or fundamental frequency.

What bandwidth is needed for a "good" 1kHz square/rectangular waveform? Numbers, not adjectives, please. Is it any different for a 1µHz waveform?

What?    You used an adjective, to describe a waveform and then said that I couldn't use an adjective to answer the question??  No!  To reproduce a "good" square/rectangular waveform, you need "enough" bandwidth.  The definitions of "good" and "enough" are application dependent.  I'm an amateur time-nut so my definitions will be substantially different from someone working with a 555 oscillator.

If you use the Fourier Series Demo that I previously linked to, my earlier example of a 10 MHz square wave with harmonics up to 100 MHz looks a bit sad.  It might be adequate for some applications and totally inadequate for others.

[tggzzz]

I pondered this square wave limitation for a while before buying my Siglent arb gen. It seemed like a real step down from my boat anchor generators. Then I thought about my applications, mostly audio. I just don't need crazy fast rise and fall; in fact, too fast can actually be a problem because you're essentially inputting an RF signal that can cause many amps and circuits to misbehave, where they never would do so in real life. OTOH, if you were trying to do a poor man's TDR test, you might need better. Fortunately, fast square wave and pulse circuits are pretty easy to build.

The only audio amp I've tested was a Quad 405 current dumping amp. It delivered full power at 20kHz into a 7ohm resistor. The output risetime was 10us, IIRC, a nice linear slope. I would have used a 4MHz sig gen's square wave, which was more than  fast enough for that job.

[tggzzz]

Risetime is not a key parameter.  Risetime is determined by the bandwidth of the system.  If you take one of Leo Bodnar's fast pulse generators (10 MHz, 30 ps risetime, 50% duty cycle) and feed it through a 100 MHz low pass filter you won't have a 30 ps risetime any more because the system doesn't have the bandwidth to support those harmonics.  A similar result will occur if you send the pulse through a buffer that only has a 100 MHz bandwidth.  So whether you're talking about signal generation or measurement, you need a frankly ridiculous amount of bandwidth just to see a good square wave.  Your application will determine how good your square wave needs to be.

Risetime and bandwidth are indeed interchangeable. That's precisely why it is misleading to start from a square/rectangular wave's period or fundamental frequency.

What bandwidth is needed for a "good" 1kHz square/rectangular waveform? Numbers, not adjectives, please. Is it any different for a 1µHz waveform?

What?    You used an adjective, to describe a waveform and then said that I couldn't use an adjective to answer the question??  No!  To reproduce a "good" square/rectangular waveform, you need "enough" bandwidth.  The definitions of "good" and "enough" are application dependent.  I'm an amateur time-nut so my definitions will be substantially different from someone working with a 555 oscillator.

If you use the Fourier Series Demo that I previously linked to, my earlier example of a 10 MHz square wave with harmonics up to 100 MHz looks a bit sad.  It might be adequate for some applications and totally inadequate for others.

Precisely!

Now, since you are a timenut, what bandwidth is good enough for the 1pps/1Hz output from a GPSDO?

5Hz? 10Hz? 100Hz? Surely the 99th harmonic will look good enough. Won't it?

[David Hess]

The Stanford Research DS345 does something very similar.  They create a synthesized sine wave with sub-Hz resolution which is great.  But then they use a comparator to square it up which adds a different source of jitter.  Noise on the sine wave - there's always noise everywhere - causes jitter in the transition points.  This jitter negates the value of the sub-Hz resolution.  I was sort of able to access the high precision signal by using the sync output and the arb waveform capability, but even that had issues.

That seems like an amateur mistake to make by Stanford Research.  I'd think the obvious way to make precision sub-Hz square waves would be to synthesize a sine wave at a high multiple of the desired frequency and then divide down the comparator output in logic.

I was surprised and annoyed when I realized what they were doing.  The DS345 isn't a cheap instrument - current price starts at US$2600.  It would have been easy and cheap to add a 'high quality' square wave output with CMOS levels that bypassed all the sine wave and comparator nonsense.  The existing 'function generator' capabilities would be unaffected.

I do not understand the problem.  Transition midpoint timing converters make high resolution time measurements using the same hardware in reverse, so 10 picosecond or better timing resolution should be easy with typical AWG sample rates and resolutions.  A 100 MHz ADC has 10 nanosecond timing resolution, but when used to synthesize a bandwidth limited edge, the timing resolution is increased by the available amplitude resolution and is much better than 10 nanoseconds.

The alternative of including a synthesized square wave generator is unlikely to achieve that kind of timing resolution.

[David Hess]

The only audio amp I've tested was a Quad 405 current dumping amp. It delivered full power at 20kHz into a 7ohm resistor. The output risetime was 10us, IIRC, a nice linear slope. I would have used a 4MHz sig gen's square wave, which was more than  fast enough for that job.

When I did my own audio power amplifier design, my fastest function generators were only 2 MHz, but that was plenty fast.  The trick I had to learn was that unless the design makes the mistake of limiting bandwidth with feedback, which causes all kinds of other problems, the raw bandwidth will be much higher than 20 kHz.  My designs using 2 MHz common emitter output transistors had a raw bandwidth of about 600 kHz, which was still within range of my 2 MHz function generators.

Today my fastest function generator is 11 MHz, with 16 nanosecond rise and 20 nanosecond fall times on its square wave and pulse outputs.

[bdunham7]

Now, since you are a timenut, what bandwidth is good enough for the 1pps/1Hz output from a GPSDO?

As edpalmer42 has said:

It might be adequate for some applications and totally inadequate for others.

So, good enough for what purpose? 

Here's an example of a 50ms 1PPS pulse run through a 4kHz filter and then displayed directly (a nice, clean and sharp looking picture) and then zoomed in on my scope to demonstrate that the jitter and error between the 1PPS and the scope clock+trigger is under 200ns.

[edpalmer42]

The Stanford Research DS345 does something very similar.  They create a synthesized sine wave with sub-Hz resolution which is great.  But then they use a comparator to square it up which adds a different source of jitter.  Noise on the sine wave - there's always noise everywhere - causes jitter in the transition points.  This jitter negates the value of the sub-Hz resolution.  I was sort of able to access the high precision signal by using the sync output and the arb waveform capability, but even that had issues.

That seems like an amateur mistake to make by Stanford Research.  I'd think the obvious way to make precision sub-Hz square waves would be to synthesize a sine wave at a high multiple of the desired frequency and then divide down the comparator output in logic.

I was surprised and annoyed when I realized what they were doing.  The DS345 isn't a cheap instrument - current price starts at US$2600.  It would have been easy and cheap to add a 'high quality' square wave output with CMOS levels that bypassed all the sine wave and comparator nonsense.  The existing 'function generator' capabilities would be unaffected.

I do not understand the problem.  Transition midpoint timing converters make high resolution time measurements using the same hardware in reverse, so 10 picosecond or better timing resolution should be easy with typical AWG sample rates and resolutions.  A 100 MHz ADC has 10 nanosecond timing resolution, but when used to synthesize a bandwidth limited edge, the timing resolution is increased by the available amplitude resolution and is much better than 10 nanoseconds.

The alternative of including a synthesized square wave generator is unlikely to achieve that kind of timing resolution.

The problem is that they created this really precise DDS sine wave in the digital domain and then moved it to the analog domain and used a high-speed comparator to convert it to a square wave.  By doing that they made it vulnerable to any noise that happened to be on the sine wave.  This noise converts to timing jitter in the resulting square wave.

Don't get me wrong - it works fine.  But I was hoping to get a DDS-derived square wave and I got an analog-derived square wave.  The difference is huge.  In normal operating mode, the sync output is derived from the 'analog' square wave.  I measured the Std. Dev. of the period of a 1 Hz square wave as ~100us.  In Arb mode, the sync output is derived from the DDS circuitry.  The Std. Dev. of a 1 Hz Arb-generated square wave was ~37 ps.

[tggzzz]

Now, since you are a timenut, what bandwidth is good enough for the 1pps/1Hz output from a GPSDO?

As edpalmer42 has said:

It might be adequate for some applications and totally inadequate for others.

So, good enough for what purpose? 

Here's an example of a 50ms 1PPS pulse run through a 4kHz filter and then displayed directly (a nice, clean and sharp looking picture) and then zoomed in on my scope to demonstrate that the jitter and error between the 1PPS and the scope clock+trigger is under 200ns.

That's a weird reordering of the previous conversation, presumably to (try to) make your point.

I'd prefer a timenut to comment, but that brigade seems to start at 10^-12, and my relatively crude Agilent 53310a MDA/TIA had displays with 14 decimal digits. Bloody difficult to read, since they don't have thousands separators.

Hence it looks like your concept falls short by many orders of magnitude.

[bdunham7]

Hence it looks like your concept falls short by many orders of magnitude.

You think that GPSDO individual 1PPS pulses rising edges are accurate to one part in 10¹² ? 

[tggzzz]

Hence it looks like your concept falls short by many orders of magnitude.

You think that GPSDO individual 1PPS pulses rising edges are accurate to one part in 10¹² ?

Depends on the oscillator (not GPS receiver). Have a look at some Allen Deviation charts.

[David Hess]

I do not understand the problem.  Transition midpoint timing converters make high resolution time measurements using the same hardware in reverse, so 10 picosecond or better timing resolution should be easy with typical AWG sample rates and resolutions.  A 100 MHz ADC has 10 nanosecond timing resolution, but when used to synthesize a bandwidth limited edge, the timing resolution is increased by the available amplitude resolution and is much better than 10 nanoseconds.

The alternative of including a synthesized square wave generator is unlikely to achieve that kind of timing resolution.

The problem is that they created this really precise DDS sine wave in the digital domain and then moved it to the analog domain and used a high-speed comparator to convert it to a square wave.  By doing that they made it vulnerable to any noise that happened to be on the sine wave.  This noise converts to timing jitter in the resulting square wave.

Don't get me wrong - it works fine.  But I was hoping to get a DDS-derived square wave and I got an analog-derived square wave.  The difference is huge.  In normal operating mode, the sync output is derived from the 'analog' square wave.  I measured the Std. Dev. of the period of a 1 Hz square wave as ~100us.  In Arb mode, the sync output is derived from the DDS circuitry.  The Std. Dev. of a 1 Hz Arb-generated square wave was ~37 ps.

Ah, I see the difference then.  They needed to create a bandwidth limited edge with multiple points rather than the slow edge of a sine wave.  I wonder why they did not.

[MarkusAJ]

An FY6900 only needs <7ns and the one built into my cheap Hantek scope <5ns (screenshot, 20MHz).
No idea why this is the case.

Hmm, that makes you think. What kind of FY6900 do you have, I take it the 100MHz version?

The screenshot is from the Hantek DSO2000 AWG.
I don't have the FY6900, the 7ns is from the specifications.

I just wanted to give two examples, that rise times do not seem to be directly related to price or overall quality.

@hp3310a
I have the FY6900 60 MHz, I bought mine in 2020 for $120 from Amazon, it is branded "DOMINTY".
I made for you few measurements using the Siglent 2504X+ scope and attached screenshots of 100Hz, 1kHz, 10kHz, 100kHz, 1MHz, 10MHz, 20MHz, and 25MHz  2.5V(p-2-p) square waves.

The FY6900 was connected to the scope with 1ft. RG58 cable and input was 50Ohm terminated.
Scope setup: sin(x)/x interpolation OFF, acquisition: normal, bandwidth limit: OFF.

I attached also screenshot with FFT of 1kHz sinusoidal wave, it will give some information about harmonic distortions.
FFT: window: frequency span: 500Hz - 30kHz, flattop, length: 2Mpts, ERES 3 bits enchantment.

Edited: I just noticed that mistakenly I connected the FFT (F2) directly to C1 and not to the ERES3  (F1), sorry.

[edpalmer42]

Now, since you are a timenut, what bandwidth is good enough for the 1pps/1Hz output from a GPSDO?

5Hz? 10Hz? 100Hz? Surely the 99th harmonic will look good enough. Won't it?

No, for a Time-Nut, there is no limit.  I want all the bandwidth!  But, for this application, it makes more sense to look at the time domain, i.e. risetime rather than the frequency domain, i.e. bandwidth.  To get the best measurements, you need to minimize jitter which means the fastest possible risetime.  Is that what you wanted me to say? 

Your discussion with bdunham7 brought up a few questions that caused me to make some measurements.  The results are interesting.

1.  Start with a Trimble Thunderbolt GPSDO.  Risetime on the 1 PPS signal is ~2 ns as measured on a 1 GHz LeCroy 9384L scope.
2.  Use a Fluke PM6681 Timer/Counter/Analyzer to make 100 measurements of the period of the Thunderbolt 1 PPS signal and then display the Std. Dev. of the measurements.  The PM6681 has a resolution of 50 ps.
3.  Make the measurements with and without the PM6681's 100 KHz low pass input filter.  With the filter:  ~0.5 ns.  Without the filter:  ~0.1 ns.

So even a 100 KHz filter results in unacceptable degradation of the measurements.

To summarize, for some situations, LIKE THE OP's INITIAL QUESTION, it's easier to understand what's happening by using the frequency domain i.e. bandwidth.  For other situations, like the measurements above, the time domain offers better clarity.

[BillyO]

The FY6900 was connected to the scope with 1ft. RG58 cable and input was 50Ohm terminated.

<5ns is pretty impressive.  I was deciding between the FY6900 and the PSG90XX and chose the PSG9080.  It's risetime measures around 9ns, so not a killer WRT square waves, but sufficient for what I need this unit for considering I have better AWGs.  I mainly chose it for the UI.  The keypad was the deciding factor.

But, yeah, <5ns is very nice for the price you paid.

[bdunham7]

I have the FY6900 60 MHz, I bought mine in 2020 for $120 from Amazon, it is branded "DOMINTY".
I made for you few measurements using the Siglent 2504X+ scope and attached screenshots of 100Hz, 1kHz, 10kHz, 100kHz, 1MHz, 10MHz, 20MHz, and 25MHz  2.5V(p-2-p) square waves.

One of the issues with the FY6900 and its ilk is that if the frequency you select is not an even submultiple of the sample clock, then it will use a modified edge to get the middle of the transistion to be at the correct time.  It does this in an ad hoc manner, not consistently like better AWG designs.  This results in mostly faster but inconsistent rise times between different periods.  You can see this in your numbers, even though the pictures look good--at least the low frequency ones. 

Here is a rise and fall time screenshot of a 10.0001kHz square wave selected on an FY6900.  Note that the displayed frequency is off a bit, the FY6900 clock isn't all that great.

[bdunham7]

So even a 100 KHz filter results in unacceptable degradation of the measurements.

That's what I was driving at--a slowed rise time measured in microseconds gives you an error measured in picoseconds.  But we've gone far enough off topic, IMO.

[hp3310a]

@hp3310a
I have the FY6900 60 MHz, I bought mine in 2020 for $120 from Amazon, it is branded "DOMINTY".
I made for you few measurements using the Siglent 2504X+ scope and attached screenshots of 100Hz, 1kHz, 10kHz, 100kHz, 1MHz, 10MHz, 20MHz, and 25MHz  2.5V(p-2-p) square waves.

The FY6900 was connected to the scope with 1ft. RG58 cable and input was 50Ohm terminated.
Scope setup: sin(x)/x interpolation OFF, acquisition: normal, bandwidth limit: OFF.

I attached also screenshot with FFT of 1kHz sinusoidal wave, it will give some information about harmonic distortions.
FFT: window: frequency span: 500Hz - 30kHz, flattop, length: 2Mpts, ERES 3 bits enchantment.

Edited: I just noticed that mistakenly I connected the FFT (F2) directly to C1 and not to the ERES3  (F1), sorry.

Thanks a lot for taking the time to put this up.

One of the issues with the FY6900 and its ilk is that if the frequency you select is not an even submultiple of the sample clock, then it will use a modified edge to get the middle of the transistion to be at the correct time.  It does this in an ad hoc manner, not consistently like better AWG designs.  This results in mostly faster but inconsistent rise times between different periods.  You can see this in your numbers, even though the pictures look good--at least the low frequency ones. 

Here is a rise and fall time screenshot of a 10.0001kHz square wave selected on an FY6900.  Note that the displayed frequency is off a bit, the FY6900 clock isn't all that great.

Also thanks for that! Never in my wildest dreams would I think of things like that, so great to know.

This discussion is very interesting to follow. Turns out there are a lot more "moving parts" to consider in a frequency generator than I thought.

[tautech]

Turns out there are a lot more "moving parts" to consider in a frequency generator than I thought.

And uses !

Back when I fixed CRO's for a hobby having a pulse generator to use instead of a Mark space generator would have been very useful and when I got into this game my 1st but only half decent AWG was the quite ordinary SDG1010 with only a single higher amplitude output. 

Since then several have stuck for a while and now settled on an unhacked SDG6022X and a RF gen for any HF work.

However customer needs always interest me and the most recent unusual was the need to burst 3 short pulses at a specified rate with SDG1032X.
Had them bluffed but it was quite easy with the pulsing channel burst for 3 cycles and triggered at the required repetition rate by the 2nd channel where we left it self triggering whereas we could also manually or externally trigger it.
After the little usage lesson they were on their way wagging their tails like happy pooches. 

[Aldo22]

However customer needs always interest me and the most recent unusual was the need to burst 3 short pulses at a specified rate with SDG1032X.
Had them bluffed but it was quite easy with the pulsing channel burst for 3 cycles and triggered at the required repetition rate by the 2nd channel where we left it self triggering whereas we could also manually or externally trigger it.
After the little usage lesson they were on their way wagging their tails like happy pooches. 

Many people do not know the capabilities of their  (even very limited)  tools.
What you describe can also be done with a poor FY3200S, you just have to find out how to do it. 

[tggzzz]

Now, since you are a timenut, what bandwidth is good enough for the 1pps/1Hz output from a GPSDO?

5Hz? 10Hz? 100Hz? Surely the 99th harmonic will look good enough. Won't it?

No, for a Time-Nut, there is no limit.  I want all the bandwidth!  But, for this application, it makes more sense to look at the time domain, i.e. risetime rather than the frequency domain, i.e. bandwidth.  To get the best measurements, you need to minimize jitter which means the fastest possible risetime.  Is that what you wanted me to say? 

Your discussion with bdunham7 brought up a few questions that caused me to make some measurements.  The results are interesting.

1.  Start with a Trimble Thunderbolt GPSDO.  Risetime on the 1 PPS signal is ~2 ns as measured on a 1 GHz LeCroy 9384L scope.
2.  Use a Fluke PM6681 Timer/Counter/Analyzer to make 100 measurements of the period of the Thunderbolt 1 PPS signal and then display the Std. Dev. of the measurements.  The PM6681 has a resolution of 50 ps.
3.  Make the measurements with and without the PM6681's 100 KHz low pass input filter.  With the filter:  ~0.5 ns.  Without the filter:  ~0.1 ns.

So even a 100 KHz filter results in unacceptable degradation of the measurements.

To summarize, for some situations, LIKE THE OP's INITIAL QUESTION, it's easier to understand what's happening by using the frequency domain i.e. bandwidth.  For other situations, like the measurements above, the time domain offers better clarity.

Bingo! We've got there, finally.

Note that when my response[1] corrected your statement, there was no definition of "this application". I've forgotten whether the OP has subsequently added a specific application.

[1] reply #16 https://www.eevblog.com/forum/testgear/function-generator-square-wave-rise-time/msg5849241/#msg5849241

[DaneLaw]

@hp3310a: If you simply want to see higher frequency square waves, you can use an Si5351.
You can't set the amplitude, offset or duty cycle directly, but it might be enough for simple tests.
https://learn.adafruit.com/adafruit-si5351-clock-generator-breakout

Got a few of those..Si5153B with PLL, VXCO.  https://www.eevblog.com/forum/testgear/adjustable-8ch-clock-generator-2-5k-200mhz-0-5ppm-(si5351bpll)
Doesn't cost a lot (20 to 40$ incl. VAT) these TypeC units fitted with a small screen where you individually can adjust the variables on the 8 channels (2.5k to 200MHz) and 5-way phase difference ability, and 7 channel division reference-Hz-sett & clock  [some time since I looked at it]

[BillyO]

If the OP  needs REALY nice square waves then I'd suggest this: https://www.leobodnar.com/shop/index.php?main_page=product_info&cPath=124&products_id=406&zenid=7e46e2ec849efabc0612f5d4abf56be6

[joeqsmith]

...
Some sig gens may have a special square wave or pulse mode that has faster rise times, but it is going to be difficult to get a clean, crisp 20MHz square wave especially if you want a universal device that can give you arbitrary frequencies and periods.

... +  offset, amplitude, markers, slew rate ....

If OP just needs a fixed logic level, maybe just buffer the output.

[ozkarah]

Siglent SDG2000X series can do 8ns rise/fall time for square wave signal.

But, using ARB Mode (DDS - Duty 50) can go as low as 4ns. However, the signal is not a perfect square wave in DDS mode.

[bdunham7]

But, using ARB Mode (DDS - Duty 50) can go as low as 4ns. However, the signal is not a perfect square wave in DDS mode.

You'll also have the same DDS issue that the FY6900 has--unless you pick certain specific frequencies, you'll have a lot of jitter.  And even if you pick a magic frequency, you might still get strange occasional jitter due to how the clock correction works.

... +  offset, amplitude, markers, slew rate ....

If OP just needs a fixed logic level, maybe just buffer the output.

Yes, all those things too.  The OP seemed mostly interested in getting better performance out of an AWG, but clean fast rise times are tough to do when you expect to just be able to dial in anything from 2mV_p-p to 20V_p-p.  This shows 5GV/s or 5kV/µs slew rate. 

[tautech]

But, using ARB Mode (DDS - Duty 50) can go as low as 4ns. However, the signal is not a perfect square wave in DDS mode.

You'll also have the same DDS issue that the FY6900 has--unless you pick certain specific frequencies, you'll have a lot of jitter.  And even if you pick a magic frequency, you might still get strange occasional jitter due to how the clock correction works.

Is that FY6900 jitter ? 
Is anything 10 MHz referenced or just free running ?

[bdunham7]

Is that FY6900 jitter ? 
Is anything 10 MHz referenced or just free running ?

Sorry, I didn't make that clear!  That is the SDG2122X using the stored ARB Duty50 waveform at 9.9MHz.  Essentially the same setup as ozkarah posted except I changed the frequency from 10MHz to 9.9 and increased the amplitude to show the slew rate capability.  And I used a 10x probe so my scope/probe rise time is 1.5ns at best.

[gf]

Sorry, I didn't make that clear!  That is the SDG2122X using the stored ARB Duty50 waveform at 9.9MHz.

Using which TrueArb interpolation mode?
(0-order, linear, sinc, sinc27, sinc13)
Zero-order hold is, of course, expected to jitter and linear interpolation is not expected to provide the best image/alias rejection either. But does it also happen with the sincXX interpolation modes?

Edit: Looking at the datasheet, it seems that the SDG2000X does not support the sincXX modes, but only the 6000 and 7000 and the new 3000 models do?

Edit: Or do you talk about traditional DDS mode (not TrueArb)?

[bdunham7]

Using which TrueArb interpolation mode?

None of the above, this was DDS.  TrueArb only does 75MSa/s and the Duty50 stored arb waveform has 16384 points and that translates to ~4.5kHz and about a 13ns rise time.

[Mahagam]

And I used a 10x probe so my scope/probe rise time is 1.5ns at best.

Use BNC cable and 50 Ohm scope internal termination.

[gf]

Using which TrueArb interpolation mode?

None of the above, this was DDS.  TrueArb only does 75MSa/s

Oh, only 1/4 of the 300MSa/s DDS/DAC sample rate
[ Edit: To avoid confusion: I just mean the input sample rate of the DAC, not its oversampling rate. ]

I compared the SDG2000X/6000X/7000A datasheets, and interestingly it seems that only the SDG7000A supports TrueArb sample rates up to the full DAC sample rate.

Also, neither the SDG6000X datasheet nor the User Manual mentions sincXX interpolation, although there was apparently a firmware upgrade that introduced it. Since the docs seem to be outdated, I wonder if this SG6000X firmware upgrade also coincidentally introduced TrueArb sample rates up to 1.2Gsample/s? Does anybody know?

[ I speculate that the sincXX interpolation modes and the support of TrueArb sample rates up to the full DAC sample may be related, because as long as only linear interpolation is supported, I see a good reason for limiting the TrueArb sample rates. ]

Edit: According to
https://www.eevblog.com/forum/testgear/new-siglent-sdg3000x-arb-waveform-generator/?action=dlattach;attach=2515697
the forthcoming SDG3000X also appears to support both, sincXX interpolation modes and up to full 600MSa/s TrueArb sample rate.

[gf]

But, using ARB Mode (DDS - Duty 50) can go as low as 4ns. However, the signal is not a perfect square wave in DDS mode.

You'll also have the same DDS issue that the FY6900 has--unless you pick certain specific frequencies, you'll have a lot of jitter.  And even if you pick a magic frequency, you might still get strange occasional jitter due to how the clock correction works.

Try the attached custom DDS waveforms instead [ I hope I got the file format right ].
At frequencies <= 10MHz, the 2nd one should not jitter, and the first one should not jitter that much.