Function Generator Opinion?

[Strube09]

Hello everyone,

I am looking for a function generator and came across this one:

http://www.mastechpowersupply.com/function-generator/5mhz-sweep-function-generator-/-signal-generator-/-frequency-counter/prod_53.html

I haven’t heard of the brand, but it seems to have a lot of function and a low price... maybe that should be a sign enough to stay away from this.

Just looking for your opinions.

Thanks

[DJPhil]

I've run into that brand name before while looking for soldering irons . . .

Hakko 936 (known and respected)
Quakko 936 (scroll down, sparklies! wtf?)

I just kind of assumed they were low grade knockoffs, though I could be wrong. I remember the name because it was hilarious.
I can't seem to find a company website with a little work. If it were me, I'd keep shopping.

[logictom]

Damn, my soldering iron never came with any sparkles
Maybe it helps with keeping the magic smoke in??

Have you considered Dave's 10MHz DDS over on altzone? I'm in the process of acquiring the parts to play with

[eswets]

I bought a cheaper function generator about the same price and it will work for what I need, but Im guessing it will be junk in a few years.  They work ok, but they are built like crap.  The first day the knob fell off and the pot the knob is attached to, is not attached to anything so I can feel the whole thing moving.  I will not buy a off brand cheapo again.

[mlaargh]

Damn, my soldering iron never came with any sparkles
Maybe it helps with keeping the magic smoke in??

Have you considered Dave's 10MHz DDS over on altzone? I'm in the process of acquiring the parts to play with

I've been considering that one as well.  A shame about the MAX038 being discontinued... I'm having a bit of trouble convincing myself to pay $35(US) for one on eBay - Which seems to be really the only source for it these days. If anyone knows of any other sources for that part, I'm all ears!  I've also been sniffing around some manufacturer websites (LT, Analog Devices, etc.) to find something similar, but so far, no dice.
I'd be open to suggestions regarding that as well. My only function generator at the moment is an HP 200CD... Not very feature rich, to say the least - haha.

[alm]

The modern way would be to use DDS. AD has a few DDS chips that can do sine/square/triangle (eg. AD9834). They do need a micro controller to configure them, and a proper filter if you want sine waves at anything close to the Nyquist frequency. Plus an amplifier to remove DC offset and change the amplitude. Definitely more complex than a MAX038, but much better performance. For a sample schematic, see this ELV manual. The text is in German, but it does include full schematics.

[DJPhil]

I've been considering that one as well.  A shame about the MAX038 being discontinued... I'm having a bit of trouble convincing myself to pay $35(US) for one on eBay - Which seems to be really the only source for it these days. If anyone knows of any other sources for that part, I'm all ears!  I've also been sniffing around some manufacturer websites (LT, Analog Devices, etc.) to find something similar, but so far, no dice.
I'd be open to suggestions regarding that as well. My only function generator at the moment is an HP 200CD... Not very feature rich, to say the least - haha.

I know what you mean about the MAX038, I was baffled myself when I learned of it. Alm was right about DDS, but if you're looking for an analog alternative for tinkering with there's Exar's XR2206. I've seen them for sale at several larger supply houses for about $7US, more or less depending on if you want DIP or SOIC. It doesn't seem quite as awesome as the MAX, but I could live with myself if I cooked one tinkering with it. I haven't tried Exar's sample system, but you might be able to get a couple free, especially if you can get a good open schematic for their chip out in the world for people to build. I'm sure they'd love that.

Hope that helps.

[alm]

If I recall correctly, the XR2206 is quite inferior to the MAX038, max. 100kHz or so, and pretty lousy sine waves. Might be OK for audio frequencies, as long as you don't need low distortion. It's quite an old chip (from the eighties?), so I'm sure there are plenty of designs floating around. The MAX038 was much newer and better, but was manufactured on a discontinued process.

[saturation]

There are so many brands coming out of China its hard to know.  You'd have to test each to know it delivers as specified.

If you are looking for low cost function generators and know what higher end frequency you want, I would look at B&K Precision's equipment line.

Its no frills when it comes to the case and external parts, but they are electronically sound, and if you are in the USA, its easy to return it if you're not satisfied.  As a company, they've been around for over 30 years, so they must be doing something right and they are still the same company, not bought out.

The key feature in function generator performance is distortion free wave forms and frequency stability. If you select sine wave at 1MHz you want to get 1MHz and you do not want it generated with distortion, thats a basic test, and you don't want the frequency drifting up and down either.  If you read the spec sheet, the device should truly deliver those specifications to you.  B&K is a popular FG maker because most low end designs work well with their product line.

Switch the FG to square wave, and retest.  If it delivers a good square wave, then you're good to go.  Any distortion in a square wave reflects poor frequency response.

The next item is about shielding and safety.  If the unit is t shielded well, or your probes bad, the high frequency waveforms will be distorted, and it won't be the fault of the FG.  You also need a good scope to read those waveforms without distortion.

http://www.bkprecision.com/generator-guide

For most at home design work something like this is all you need:

http://www.bkprecision.com/products/model/4001A/4-mhz-sweep-function-generator-with-dial.html



Or the next model up, the 4003A.

Once you start working on RF frequency designs, you start to get a bunch of problems with shielding, drift, and board layout.  So you need to prototype it on design software really well before building it.  If you make it that far, you should jump up to a professional grade FG by Agilent or others.

If you can find a DDS based FG that >= features/cost as the low end 4001 B&K [~$US 200], such as the those Chinese branded FGs [$190 pictured below], its a better deal, but I don't know how well they'll live up to their spec or their warranty.



Its a matter of trust, unless someone reviews their models like our hero, Dave!

[DJPhil]

If I recall correctly, the XR2206 is quite inferior to the MAX038, max. 100kHz or so, and pretty lousy sine waves. Might be OK for audio frequencies, as long as you don't need low distortion. It's quite an old chip (from the eighties?), so I'm sure there are plenty of designs floating around. The MAX038 was much newer and better, but was manufactured on a discontinued process.

Datasheets: Max038, XR2206, and just for fun Intersil ICL8038

It's a little bit better, with a listed max frequency of 500kHz (min) and 1MHz (typ). Nothing like the Max's 20/40MHz to be sure, but Intersil's (also discontinued) version was 100/300kHz.
The sine is listed with 2.5% distortion to the Max's 2%, but can be reduced to 0.5% with external adjustment. I'm not sure how much of a pain that would be, but I agree that there's better ways of getting a low distortion sine wave. It has a couple of perks over the Max: the sweep ratio is larger, temperature stability better, wider duty cycle adjustment, and a handful of modulation options. That's pretty much where it ends though, and I think the better specs are largely due to it's lower bandwidth. The Max also has a pulse output, but that's easy to generate from the other waveforms and you'll have more control over your pulse characteristics if you do it manually.
I can only find rumors, but it appears that Exar discontinued it for a while and brought it back. Their most recent datasheet is from '08, so it may have seen improvements since back when due to modern manufacturing.

I apologize, I really shouldn't have implied that there weren't a heap of designs out there already. It's the sort of thing I should have googled before speaking. There's a lot of information available that goes over the quirks, limitations, and applications out there in the googlesphere.