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Comment by aDfbrtVt | Hacker News Reader

It's not really a function generator without DC offset and proper amplitude control. I don't want to be overly negative, it's a cool project. I think there's definitely something to be said about having polished interface to a low end function generator. However, there needs to be more information on specification such as ENOB, sample rate, max Vpp, max offset and bandwidth.

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Hi! Author here. You are absolutely right, and I called out a lot of those limitations in my original design doc.

http://cushychicken.github.io/bfunc-design-doc/

I'm just starting to get to work on the second version of this board, and I'm planning on addressing all of these in that design.

Wouldn't that be pretty easy to do as well? The most crude way to have a DC offset is to just pipe the output through a voltage divider with a potentiometer no? As for amplitude control, an op amp can be used again with a potentiometer to control the resistors needed for a specific gain right?

Sure, the voltage divider works ... if the load impedance is high. Not a good idea for a 50 Ω load.

Op amp needs a dual-rail power supply, so it needs some auxiliary power, then power electronics to get both rails. Stuff starts to get complicated, and now that $1500 Keysight box starts to looks worth its price...

This. A function generator is not a trivial piece of digital electronics. It's a complex, highly analog beast, where the digital component of the circuit is actually rather small. Nowadays a dedicated IC (such as the XR2206) is the starting point for a home-made FG.

Here's one kit:

http://electronics-diy.com/electronic_schematic.php?id=128

I'd even argue that the XR2206 is pretty obsolete when it comes to function generation methodologies.

For one thing, it's very limited in frequency - 1MHz max, and dependent on analog components to generate it.

For another thing, it's really limited in waveform types. Practically, you can only generate sine, triangle, and square waves. Modulation is possible, but requires extra hardware to pull off.

The advantage I see of the DDS is that, once the signal generation portion of the product is software based, you can push upgrades to functionality via software updates. Not something any XR2206 can do!

In theory yes. The issue is the different load conditions will mess with the voltage divider bias. Check out page 272 and onward of the HP33120A manual https://coupp-docdb.fnal.gov/cgi-bin/RetrieveFile?docid=206&...

Why compare a commercially available top of the line function generator with a hobby project?

At this level you can add a couple of simple components and get 95% of the functionality and accuracy of the commercial one for a couple of bucks. Go higher than that and you might as well shell out the money for an army surplus one, but that does not get you what he wanted: an opportunity to tinker and learn.

Why compare a commercially available top of the line function generator with a hobby project?

Because as soon as you do, you start to realize that you can get quite a ways with commodity hardware. Hence, why I started this project. The research I've done on the frontend of this on what's available in the market suggests to me that there's a niche for something that's a little cheaper, and a little easier to use. I also have a hunch that portability is a nice feature for most folks as well.

(Hi Jacques!)

There is a whole branch of Chinese manufactured pc peripherals aimed squarely at this space, digital scopes, spectrum analyzers and so on, don't they make a function generator?

They do, but the usability leaves something to be desired.

Please don't take the comments I wrote above as overt criticism, I was trying to point out some of the potential pitfalls in DIY function generators. As I said your project seems really interesting, especially if the software interface is polished.

Not at all! They're all things I'm learning myself. Some were lessons I knew I was going to have to learn, but there have been a few I didn't really expect to learn about.

That HP manual was a great link. I've been studying it over the last few days. Thanks for sending it. :)

The only one I'm really kicking myself over is not including an antialiasing filter at the DAC output. Seems like a bushleague mistake considering I paid all that money to get this EE degree.

I can definitely sympathize, but anytime you take on a new project like this for the first time you're bound to at least make some stupid mistakes. I'll be following the project closely, hopefully you manage to overcome all the difficulties!

I'm not comparing it to a commercial product, I was linking a datasheet that describes how the output stage of a function generator is built. It takes a lot more than "a couple simple components" to get 95% of the functionality. I think building your own function generator is a great hobbyist project, I just wanted to point out some not so obvious gotchas.

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