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