What do you guys like on function generators?

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J

Jidis

Well-known member
Joined
Jul 21, 2008
Messages
143
Figured it better not to clutter up the forum with another similar thread. I'll tack the new question at the end of this thread.

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I did an opamp-based one last night and it seems to do pretty well in the 200Hz range, but above and below that, the wave tops seem to ramp in one direction or the other. I guess that's kind of pointless if you're trying to use it to see how something else distorts the squares.

Do the 555 based circuits (or others) produce something more accurate? Didn't see a sweepable one, but I can go with a few switched steps if needed.

Thanks!

 
A tilting or sloping square wave suggest you are AC coupled somewhere.

Opamps are not optimal for making square waves, unless you bandpass limit the square wave rise time to below the opamps slew rate limit.

The tilt in square waves is an old school way to see the LF bandwidth of a path.

JR
 
Thanks John,

This attached thing is all it was. Came from some PDF full of opamp circuits. Interestingly, a few different opamps had different ranges where the wave was flat, and a couple also had some weird overshoot thing happening at the start of the wave.

Opamps are not optimal for making square waves
Click to expand...

Yeah, that's what i was figuring, I just didn't have a 555 or anything handy. There was another opamp circuit out there which claimed to be able to create squares from a sine input but I didn't try it. I probably need to break down and grab a function generator. I've been using your TS-1 the past couple decades, but in addition to the squares, I've also got a used distortion meter here whose calibration routine calls for a pair of low distortion generators, so I may need to find some dual output job if there is such a thing.

Take Care
 

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  • Square Wave.jpg
    Square Wave.jpg
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Thanks again John, that definitely helped!

It now flattens out around 1k and stays that way from there upward, but the lower range still slopes a bit. May not matter though as I'm not sure yet what ranges I'll be using it for.

I'll probably still be looking for a used generator, so if there's a common old one that people like here, I'd be interested. It looks like the cal routines for that distortion meter may only use a pair of sines at two different frequencies, so it may even be something I can do with my DAW, but it would still be nice to have a real one.

Take Care 
 
I'm still thinking about getting something that can do nice low distortion sines and squares. I'm mainly looking to use it right now for testing modded circuits for any "improvements". My current scope is only a 20MHz. Is there something common, popular and inexpensive to look for? I'm also figuring many of you are aware of that new breed of import microcontroller-based generators that are all over eBay. Has anyone used one or heard anything about how they compare? Rod Elliot also has a promising looking DIY circuit here, but he sounds like the squares aren't really its strong point.

-Thanks!

George
 
Jidis said:
I'm still thinking about getting something that can do nice low distortion sines and squares.
A low distortion sine wave has merit... Just one frequency. A low distortion square wave is hard to measure, and not that meaningful..
I'm mainly looking to use it right now for testing modded circuits for any "improvements".

Testing audio paths, with non audio waveforms (like square waves can lead to confusing results. If you want to use square waves proabaly a good idea to rise-time or LPF the HF content, and AC couple or HPF the DC content. 
My current scope is only a 20MHz. Is there something common, popular and inexpensive to look for? I'm also figuring many of you are aware of that new breed of import microcontroller-based generators that are all over eBay. Has anyone used one or heard anything about how they compare? Rod Elliot also has a promising looking DIY circuit here, but he sounds like the squares aren't really its strong point.

-Thanks!

George
Click to expand...
You can do much of what you meed using computer files and a sound card or music player of some sort.

I used to make low cost test gear, I wouldn't try now when you can do it almost free with a smart phone or similar gadget.

JR
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Thanks John,

The other day, I coincidentally was wondering what Nuendo's version of a square wave looked like (from their test generator plug). On the scope, it looked like it was clearly made up of higher frequency sine waves with rippled tops and bottoms. I didn't look into other software options, but I'd like to have something which could do clean squares.

Still wondering on those new eBay gadgets though. The price is definitely right, as they're about half of what an old simple function generator costs on there, but if they're noisy or distorted, I don't guess there would be any point.

George
 
Google "Gibbs phenomenon" Square waves are made up if multiple different sine waves. When you perfectly filter out the highest frequencies (way above audio), what left is the square wave minus the ripples.. this is normal .

A "square" square wave has higher frequency content than normal audio, and higher than you need. 

JR
 
Happy 7777 John  :)

It's just that I was reading on ways to test for distortions and oscillation, and was seeing people talking about pushing squares through circuits way outside the audible range. I figured that might be something worth learning about.

George
 
This little guy should do the trick.  It's based on a now discontinued EXAR chip.  I guess Jameco kept some in stock.

http://www.jameco.com/webapp/wcs/stores/servlet/Product_10001_10001_20685_-1

Bri

 
FWIW, I've been a big fan of a function generator for literally decades.  Approx. 30 years ago, I built one of these kits, like the unit shown here:

http://www.ebay.com/itm/HEATHKIT-SG-1271-IG-FUNCTION-Sine-Square-Triangle-Generator-Calibrated-RESTORED/231128896814?_trksid=p2045573.m2042&_trkparms=aid%3D111000%26algo%3DREC.CURRENT%26ao%3D1%26asc%3D27%26meid%3D3897088707558631108%26pid%3D100033%26prg%3D1011%26rk%3D1%26rkt%3D3%26sd%3D231128896814%26

(Egads...hope that long URL works....)

To this day, it still is one of my mainstays.  The circuit is really odd all discrete transistors, versus opamps, but it works like a champ.  It is my "go to" generator when aligning analog tape recorders because all output levels (in particular for tape machines, the sine wave output) have essentially "zero second" settling times.  So, as I sweep frequencies, I don't have to wait for the sine wave output level to quit bouncing around/settling.  In addition, the sine wave output levels are ULTRA FLAT throughout the audio spectrum and beyond.

One can argue that the distortion of the sine wave signal of a function generator (maybe 0.5% THD on a good day) sucks for audio applications, but actually is just fine for basic frequency response sweeps.  FWIW, all MRL alignment tapes sold the past few decades are created with the sine wave output created by a computer-controlled function generator.

If I want to do ultra-low THD/IMD (and admittedly ultra low noise floor) measurements with the gear that I own, then I fire up my ancient Amber 3500 test set, which cost me (new) in excess of $2000 back in the 1980's.  But, the frequency response flatness and settling time of the Amber's low distortion sine wave signal isn't as good as the lowly function generator.

As for square waves, they are an excellent testing tool.  However, the signals you can generate with an average sound card or typical "studio" D/A really suck for analog testing applications.  The Gibbs Effect "stuff" in the signal source makes it not really useful when looking at an analog Gizmo on a 'scope, unless your D/A is running at ...I dunno....help me here JohnR !...maybe a 394K sample rate??

Bri


 
Thanks for all the info Brian!

Re: That settling time thing, I've often wished my Loftech had preset switches for a few frequencies for tape alignment and such. Bouncing back and forth quickly is sort of a pain with a dial, but I guess that's what our computers are for now.  :)

I looked at that Jameco thing you linked. Looks like a predecessor to those import things I was talking about. Since you've been looking at eBay units, I'm guessing you've seen some.

They range from basic generator boards with none of the panel parts for under $10 USD:

http://tinyurl.com/lvr8rcm

to full units with LCDs:

http://tinyurl.com/l9tpq24

and ones with enclosures:

http://tinyurl.com/m4x4gt9
http://tinyurl.com/ks7xs8p  <---- I like the style of this one

Some don't give distortion specs, but the ones that do look to claim less than 1% or .8% for a 1k sine. There's actually too many to pick from and it looks like whoever is pumping them out is constantly adding functions as they evolve. One of the raw boards with the LCD even does two simultaneous outputs.

Take Care

George
 
Jidis said:
Thanks for all the info Brian!

Re: That settling time thing, I've often wished my Loftech had preset switches for a few frequencies for tape alignment and such. Bouncing back and forth quickly is sort of a pain with a dial, but I guess that's what our computers are for now.  :)
Click to expand...
Since I designed the TS-1 I can comment on that... The TS-1 was designed to be simple to use while delivering good, but not full test bench instrument performance. The sine wave generator was actually pretty difficult, Covering a more than 1000:1 range on a single sweep pot was not trivial, and hanging a frequency counter on the output encourages customers to attempt to dial in exact frequencies.

The settling time was a pure trade-off between low distortion and fast settling. I eventually compromised on roughly 0.15% THD, to provide a decent settling time. I could hear the .15% THD in side by side listening tests, but that was low enough to provide accurate general purpose level measurements, and listening for gross distortion like speaker rubs, etc. 

If you are willing to live with higher distortion you could speed up the AGC and enjoy faster settling.
I looked at that Jameco thing you linked. Looks like a predecessor to those import things I was talking about. Since you've been looking at eBay units, I'm guessing you've seen some.

They range from basic generator boards with none of the panel parts for under $10 USD:

http://tinyurl.com/lvr8rcm

to full units with LCDs:

http://tinyurl.com/l9tpq24

and ones with enclosures:

http://tinyurl.com/m4x4gt9
http://tinyurl.com/ks7xs8p  <---- I like the style of this one

Some don't give distortion specs, but the ones that do look to claim less than 1% or .8% for a 1k sine. There's actually too many to pick from and it looks like whoever is pumping them out is constantly adding functions as they evolve. One of the raw boards with the LCD even does two simultaneous outputs.

Take Care

George
Click to expand...

I guess we will agree to disagree over the merit of wide band square wave vs a proper rise time and LPF limited square wave.

It is useful to reject/filter out above band content in audio paths.

JR

PS: I've designed cheap line up oscillators for use inside old school console master sections. These generally used the function generator ICs (like old 2206 or whatever) that used higher distortion diode-break or other approximations to make the crude sine waves from triangle waves.
 
John,

I noticed last night they do give rise time specs on a few of those, but I didn't know what was considered "good speed" there.

JohnRoberts said:
I guess we will agree to disagree over the merit of wide band square wave vs a proper rise time and LPF limited square wave.
Click to expand...

No, I don't mean to sound like I'm arguing 'for' those Chinese things. Just sort of intrigued by them and couldn't help but wonder how the quality of their output compared to some of the lower-end used units. I also liked the "portability" factor.

You've got me wondering now what that DAW generated wave might have looked like with an LPF attached. I'll try that later to see how clean it can get and what range it can stay clean in.

Take Care
 
In theory a perfect square wave has an infinite rise time or edge rate. In audio terms that means the square wave has energy at silly high frequency, way above audio importance.

Opamps are designed to reproduce audio, and infinite rise time square wave will make a trapezoidal shaped waveform. With added distortion at the start and stop of the slew limiting. By low pass filtering the infinitely fast square wave we get a rise time limited square wave. This is a good waveform that audio amps can reproduce as long as the steep part of the exponential rise time is within the slew rate speed. 

Using "too fast" signals is an old school way to look for circuit instability, there are ways to design audio paths where the first stage harmlessly removes this above band content.

JR
 
JohnRoberts said:
Using "too fast" signals is an old school way to look for circuit instability, there are ways to design audio paths where the first stage harmlessly removes this above band content.
Click to expand...

Yeah, that's what I'm hoping to learn how to do (among other things).

By the way, in that Rod Elliott generator circuit above he mentions the same issue you had with the Loftech in trying to cover such a wide range. He ended up splitting the range into overlapping halves with a switch on that one.

George

PS- I bet you've got one of those cool-ass "enhanced" 1U rack versions of the Loftech don't you? Always wanted that.  8)
 
Jidis said:
JohnRoberts said:
Using "too fast" signals is an old school way to look for circuit instability, there are ways to design audio paths where the first stage harmlessly removes this above band content.
Click to expand...

Yeah, that's what I'm hoping to learn how to do (among other things).

By the way, in that Rod Elliott generator circuit above he mentions the same issue you had with the Loftech in trying to cover such a wide range. He ended up splitting the range into overlapping halves with a switch on that one.

George

PS- I bet you've got one of those cool-ass "enhanced" 1U rack versions of the Loftech don't you? Always wanted that.  8)
Click to expand...
Nah, I'd have to build an ark to keep one of everything I ever designed or helped design laying around. I don't even keep schematics or much of anything to remember them by. IIRC the rack mount TS may have also been done after my time associated with the company.

I do have an old stock TS-1 on my bench that I still use from time to time as I find it a handy general purpose tool.

JR
 
That's interesting to hear John. I had always wondered how that sort of stuff worked for you guys, like if you had a whole rack of those Peavey AMR "PME" EQ's, or if you had one (or none?). If you're at the design level of stuff like that, and you say you want a few when they're done, do most companies just give them to you, or do you just get some special discount?

Sorry for the goofy questions.  ;D
 
Jidis said:
That's interesting to hear John. I had always wondered how that sort of stuff worked for you guys, like if you had a whole rack of those Peavey AMR "PME" EQ's, or if you had one (or none?). If you're at the design level of stuff like that, and you say you want a few when they're done, do most companies just give them to you, or do you just get some special discount?
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I knew a marketing guy who collected one of everything he worked on, and he had a pile of gear.. (He did some work for one of my early companies so he had several pieces of my design).  I don't roll that way, but I have my own pack-rat/Hoarding issues. As it is I have to constantly force myself to throw stuff away. Some day I will die and don't want to make the task of sorting through my pile of trash even harder.

No I did not get free gear, but I was generally able to buy stuff at a fair price. I recall while doing some outside design work for Bozak, Rudy sold me some of his low end bookshelf speakers at a good price, when I gave them away as Christmas presents one year. Another time I bought a very nice Acoustic guitar (made in Finland IIRC) to give away to niece as a wedding present. I got it at a reasonable price because these were slow moving inventory that Peavey was trying to blow out. For some reason people don't think "Peavey" for quality Acoustic guitars (my niece loved it). About the only Peavey gear I bought for my personal use was a pair of AMR 308, studio monitors that do not suck, and are still my living room hifi set up. The 308 replaced a nice 6" 2-way (Fourier) designed by a different friend of mine (I traded for a MC phono preamp kit of mine.). These days I do 99.9% of my listening over a decent relatively premium computer speaker system that was a gift, in exchange for some free consulting to that company (4 small satellite speaks with a sub).     
Sorry for the goofy questions.  ;D
Click to expand...
I don't gain much pleasure from ownership of gear.. If I need something I can generally build my own, while I stopped rolling my own speakers back in the '80s.

JR
 

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