Single-chip Frequency Synthesis - Page 1
Forum section: electronics / beginners

author: sub
author_rank: Regular Contributor
timestamp: November 12, 2010, 10:31:21 pm
content: Has anyone had any experience with ICs such as the AD9834 as a replacement for a bench signal generator? With only ten bits of output resolution, it seems it might have somewhat more distortion than a good analogue oscillator, but at $14 from Farnell, it almost seems livable.Failing this, I suppose either a Wein Bridge or waveshaping circuit is the way to go.

author: DJPhil
author_rank: Frequent Contributor
timestamp: November 13, 2010, 04:04:34 am
content: Dave's function generator project used an AD9835, probably a good place to start. I think the difference is that the '35 has an onboard DAC, but I didn't look into it too deeply.For most things you probably won't need low (<1%) distortion numbers, especially out of the audio range. Square waves are the easiest thing to generate by far, so most analog generators will use some sort of wave shaping on a square wave for other waveforms. It gets tough to do a good all around job when you want multiple wave shapes, a wide frequency range (more than a few MHz), and fancy stuff like sweeps, ramps, and pulses. If you want to build one monolithic device to do it all it's a big project, and you may be left wishing you'd just ebay'ed an '80s HP generator.Wein bridge oscillators can be temperamental when you make them tunable, but it's not too hard to build one with inaudible levels of distortion. The main difference between them is the method used to stabilize the feedback loop. If you decide to use a thermistor or lightbulb don't expect a lot of frequency agility and forget sweeping altogether! Search for 'amplitude bounce' and you'll see what I mean.If all you need is audio I would consider using a computer soundcard. It's a full on 16bit DDS with the right software though you'll have problems with harmonics higher than the soundcard can render. This will make your square waves look horrible over about 1kHz.When I go to tackle this problem I'll probably make or buy a few different devices that play to different strengths. It's going to be fun whichever way I finally go.Hope that helps some.Edit: Forgot to include URL for Dave's project

author: sub
author_rank: Regular Contributor
timestamp: November 14, 2010, 11:26:24 am
content: Thanks for the advice. It looks then as though the DDS IC will be the way to start, then. You're likely right about the utility of separate devices, and I suppose that if digitally programmable they could be muxed together in any case.Beyond that, there seems no shortage of programmable clock generators to use as the basis for a waveshaping oscillator.

author: Zad
author_rank: Super Contributor
timestamp: November 14, 2010, 05:06:19 pm
content: If you need something with more resolution, use the AD 9951, it has 14 bit output and can run up to 400MSPS. They are surprisingly cheap at $20 or so.

author: NiHaoMike
author_rank: Super Contributor
timestamp: November 14, 2010, 10:12:26 pm
content: There's also the option of using a counter to cycle through a RAM or ROM used as a lookup table in order to drive a DAC. It can even be done entirely in software using a microcontroller. For low frequencies, about the easiest way is to generate a square wave then use a switched capacitor lowpass filter.

author: sub
author_rank: Regular Contributor
timestamp: November 17, 2010, 11:26:51 am
content: I have since discoveredthisbreakout board on Sparkfun. It ought to save soldering the TSSOP, though there is no way to use an external reference oscillator without modifying the board.