cheap function generator advice

[frank26080115]

I know you guys are not keen on cheap equipment

My current problem is that I have a chip that needs a 6 MHz crystal, but for some reason, the oscillation for the crystal does not start (I see the impulse on my oscilloscope, but it doesn't start the sine wave as expected). The hardware obviously has problems and I already have a revision of the PCB planned, so I'd rather continue on with firmware work rather than battling with the crystal.

I know I can just hook up a 6 MHz full swing oscillator to it for about $3, but it got me thinking, I might want to have a function generator someday.

I'm not rich, the most expensive electronics tool I own is a DS1052E and that was free.

So I've found three interestingly cheap options on eBay

http://www.ebay.ca/itm/New-AD9850-module-modest-capacity-AD9851-DDS-Function-Generator-up-to-40MHZ-ahg-/190857915186?pt=LH_DefaultDomain_0&hash=item2c7004ab32&_uhb=1

There are a lot of these on eBay, I think from a usability point of view, they kind of suck, I'd probably need to breadboard it up every time I want to use it. But you can't beat that price.

http://www.ebay.ca/itm/New-DDS-Function-Signal-Generator-Module-Sine-Square-Sawtooth-Triangle-Wave-/161090477412?pt=LH_DefaultDomain_0&hash=item2581bd9d64&_uhb=1

Plenty of these, a lot of ATmega + LCD + some synth chip for under $20

But I'm kind of leaning towards this:

http://www.ebay.ca/itm/AD9854-DDS-Function-Signal-Generator-Module-1HZ-100MHz-PC-Control-Software-/161090927292?pt=LH_DefaultDomain_0&hash=item2581c47abc&_uhb=1

Seems useful and a great deal for the price, the software looks like some guy dragged-n-dropped a bunch of buttons in Visual Studio (but I'm a Windows user anyways, and I'm always near my laptop so having no knobs is no problem). What do you think?

I don't think I want to spend $300 on a proper lab bench generator if I just need to generate some clocks once in a while, I just need enough accuracy and stability to meet USB spec, and 2.4 GHz spec (but before the PLLs, I don't actually need to generate 2.4 GHz).

Since I am going for such cheap equipment, I should also keep a frequency counter handy, correct? what do you think of this cheap one? http://www.ebay.ca/itm/VC3165-Radio-Frequency-Counter-RF-Meter-0-01Hz-2-4GHz-Victor-Professional-Tester-/330995504851?pt=Motors_Automotive_Tools&hash=item4d10de96d3&vxp=mtr&_uhb=1

[Rerouter]

i can vouch for the AD9850 modules, if you want something more permanent, and arduino is your tool of choice, get a uno, a breadboard shield and a lcd shield, download the bog standard code, run out an encoder or keypad off the breadboard shield and boom variable frequency source, then add some op amps to level shift and amplify and you have sine and square covered,

i went a little further with one of these writing various patterns more relevant to my job,

[Mr Smiley]

What's driving the crystal, is it a rtc, a micro, a feedback loop 

Is the crystal a self contained oscillator can, or one that needs load caps 

 

[Mechatrommer]

I know you guys are not keen on cheap equipment

says who? different school different thread. ad9850 is cheap, but not a complete unit, you need to do more work to make it standalone. mcu or arduino programming + more robust output circuit. my suggestion is always DDS3x25 if you can tolerate PC dependency. @ ~$150 @ 100MHz sine, around 10MHz arbitrary, you will need sometime before the next upgrade. though you still need to work on "more robust output circuit" side (which is lacking in all cheap kit type FG) if ±3V output is not your game. though there are other standalone, complete suggestion, cheap enough unit compared to 3x25 but they only go to few MHz, an upgrade is inevitable if you need more in later time. cant recall the brand you may search in eevblog, siglent atten bk precision or what, cant recall, please consult saturation if he's around.

[kripton2035]

http://www.ebay.ca/itm/New-DDS-Function-Signal-Generator-Module-Sine-Square-Sawtooth-Triangle-Wave-/161090477412?pt=LH_DefaultDomain_0&hash=item2581bd9d64&_uhb=1

Plenty of these, a lot of ATmega + LCD + some synth chip for under $20

I would go on this one
the first one you need an arduino or so to use it - will suck
the last you need a pc to use it - will suck too ! ( and if the software is bad you cannot change it for sure)

[frank26080115]

DDS 3X25 looks like a better option, thanks! I think the brand is "Hantek".

PC dependance isn't too much of a problem. My laptop is always near by. Both the Hantek and the AD9854 board have an interface for adding stuff. Plus I sometimes use my smartphone to RDP into my laptop anyways.

What do you suggest I do for the output buffer? Pick out a >100 MHz bandwidth opamp and add a few pots to manually calibrate away any offset and gain errors? Is there any off-the-shelf options? Should I just perfboard one? If a perfboard will degrade the signal too much then maybe a custom PCB?

[Mechatrommer]

What do you suggest I do for the output buffer? Pick out a >100 MHz bandwidth opamp...

yes. i used $10 a pop opamp from TI. THS3091 iirc. but then you need external ±15V power supply rail to power it.

and add a few pots to manually calibrate away any offset and gain errors?

no need, offset is handled in software. just an opamp with gain to boost the ±3V output voltage.

Should I just perfboard one?

not an option! used either pcb or dead bug dangling in the air style.

i'm not sure what you are capable of and i hate to fell guilty if someone figure out that my suggestion not suite them. so the $20 http://www.ebay.ca/itm/New-DDS-Function-Signal-Generator-Module-Sine-Square-Sawtooth-Triangle-Wave-/161090477412?pt=LH_DefaultDomain_0&hash=item2581bd9d64&_uhb=1 might be another better option? from the look of it, its a standalone :shrug:

[frank26080115]

and add a few pots to manually calibrate away any offset and gain errors?

no need, offset is handled in software. just an opamp with gain to boost the ±3V output voltage.

I don't really want to put in a x-volt offset just to get 0 offset, unless the software has both "before buffer" offset and "after buffer" offset. I'd much rather just build a 10x gain buffer and make it transparent to the software. (OK so 10x is 30 volts, maybe not 10, maybe just 2 so I can use a wall-wart)

i'm not sure what you are capable of and i hate to fell guilty if someone figure out that my suggestion not suite them. so the $20 http://www.ebay.ca/itm/New-DDS-Function-Signal-Generator-Module-Sine-Square-Sawtooth-Triangle-Wave-/161090477412?pt=LH_DefaultDomain_0&hash=item2581bd9d64&_uhb=1 might be another better option? from the look of it, its a standalone :shrug:

Don't worry about me, I can do some crazy stuff. The Hantek seems like a great choice right now. I also think I need to do stuff like generate repeated short impulses and stuff like that so having arbitrary waveforms is definitely a plus.

[Mechatrommer]

Don't worry about me, I can do some crazy stuff.

then read this 35 pages https://www.eevblog.com/forum/chat/review-hantek-dds-3x25-anyone-own-one as i've built a standalone ad9850 module + atTiny + pots. i found out that i reach it more frequently, quick to turn on no need PC, since i seldomly use arb these days. but in case i need it, 3x25 is always there. here's my built ad9850 FWIW... https://www.eevblog.com/forum/projects/ad9850-module-and-dds3x25-dds-based-fg-compared/ my 3x25 is modded to provide buffer pretty much as you have stated, just a gain amplifier on its output.

[jonmarx]

http://www.ebay.ca/itm/New-DDS-Function-Signal-Generator-Module-Sine-Square-Sawtooth-Triangle-Wave-/161090477412?pt=LH_DefaultDomain_0&hash=item2581bd9d64&_uhb=1

Plenty of these, a lot of ATmega + LCD + some synth chip for under $20

I just bought one of those.  It appears to be a rip-off of the design at http://www.scienceprog.com/avr-dds-signal-generator-v20

The one I ordered had a few teething problems (dodgy on/off switch, a couple of joints that needed re-soldering), but as a cheap-and-cheerful signal generator for £15 (UK) it does the job.

By the way there is no synth chip; the ATmega generates signals directly via a crude DAC (an 8-bit resistor ladder), which probably explains why the maximum output frequency is only 65kHz. 

[frank26080115]

Ah so looking around the internet, Mechatrommer good sir seems to be the king expert on the Hantek. I've read about the sync issues, and found one Youtube video of somebody mentioning your fix for it. But looking through that 35 page thread is a pain in the ass... Is there an official webpage for that fix?

That cheap 65KHz unit isn't gonna cut it

[Mechatrommer]

By the way there is no synth chip; the ATmega generates signals directly via a crude DAC (an 8-bit resistor ladder), which probably explains why the maximum output frequency is only 65kHz. 

i take my word back. 65KHz is only as good for audio, but then audio FG is another arena requiring nasty spectrum purity. buying and modding the ad9850 is a worthier deed, but thats just me.

But looking through that 35 page thread is a pain in the ass... Is there an official webpage for that fix?

you said you can do crazy stuff? reading the whole 35 pages is a crazy stuff . i prefer to call it a hack or "extension", there is no real fix for that, as in hardware or firmware fix. and there is no official page for that, except the provided 35 pages thread and my download and some simple explanation at http://www.soasystem.com/eng/goltek but then i cant guarantee its "permanent existence" since its in a "free web package". hope it helps.