Pseudo VCO using DDS

[Mikebits]

Greetings all My 30 yr old son asked me awhile back to help him build a modular synth project. Some sorta music gizmo and the more I read on the subject the more I see it can get fairly complicated.

Things I been working on are analog modulators/multipliers (RF guys call em mixers) audio guys call combiners mixers, so that confused me as well.
My son also said he needs to have a oscillator to make sinewaves, squarewaves, ramps, and triangular waveforms. I thought I had the perfect idea and ordered a DDS AD9851 breakout board, and a simpler version the AD9833 which is only a 10 pin part.

Well as I was talking with my son recently he asked what kinda control voltages will the oscillator take? Huh, Control voltages ?
Ah crap sounded like he wanted a audio VCO, and the DDS did not have a pin for external voltage. I really want to stay in the digital domain for much of this project. I think this project will get quite involved as I learn more about how synthesizers are done these days, so I will probably be asking many questions in the future. So now for my immediate question on a idea I am throwing around in my head.
So let's say I use the DDS chip, but want to control it via a voltage--now I am using a Cypress micro-controller that has ADC's (SAR and Delta Sigma). What if I have a control voltage enter the board and drive a ADC, then use the digital word to setup the frequency of the DDS chip. I know this would work, but would the system be able to keep up with say a 1 KHz ramp, in other words, would I be able to read in the voltage perform the conversion, and then send the appropriate data to the DDS, and can the DDS sweep at various speeds. Think voltage to frequency converter.
So what do ya guys think, could this work, or is this just really stupid? I am not very good at articulating myself so I am attaching a drawing of what I mean. One other thought I had was, what if I vary the reference clock to change the frequency? I can find off the shelf VCO's that run around 25 MHz?

 

[dr pepper]

I have a sweep gen that uses a ad8950, it works well, however the '8950 only makes sine and square, the latter only to a couple of mhz, so you'd have to make a saw or whatever from the sine.

 

[Mikebits]

Well, thanks but I have the waveform outputs taken care of with the AD9833 part. This is from the data sheet of that part.

The AD9833 is a low power, programmable waveform generator capable of producing sine, triangular, and square wave outputs.

I assumed the AD9850 would do the same. Hmm

 

[alec_t]

would the system be able to keep up with say a 1 KHz ramp

Don't see why not. From the datasheet it looks as though you can input a 40-bit control signal in <1uS.

 

[dr pepper]

Nope the '9850 is sine/square.
If the dds technology is the same then your task ought to be doable.
Thats an interesting thing I just found, the AD9833 having extar waveforms, I have a '9850 module in a crystal oven as a signal source, if the frequency control word is the same I could add the extra waveforms to it by swapping to the '9833, sawtooth has been one I've wanted a couple of times.

 

[tomizett]

I've no experience with DDS, but it might be worth having a look/listen to what the waveform does during frequency updating before investing too much into a DDS based design. If you end up with the output being audible during the periods when the frequency is being updated, you'll want to be sure it won't generate pops and crackles while the update occurs (I imagine this would happen if the DDS reset its internal "phase" counter during the update). Just wondering.

On the positive side, you'd certainly get better stability then with a VCO - analogue synths (as you've probably read) are notorious for drifting out of tune with time and temperature.

 

[large_ghostman]

How many bits is the ADC and what speed is the conversation? I recently used a Cortex M3 with ADC of ju7st over 100,000 mps at 10 bit and 250k sps at 12 bit. The synth chips are pretty neat but it does look a nightmare

 

[dr pepper]

Your right tommi, the consideration to be made is whether glitches are acceptable, I've used my sweep gen to look at aerial impedance and to test if transformers on old radio sets, for that it has beem fine, if you were using it to operate a receiver as a scanner then it may be unacceptable.

 

[Mikebits]

How many bits is the ADC and what speed is the conversation? I recently used a Cortex M3 with ADC of ju7st over 100,000 mps at 10 bit and 250k sps at 12 bit. The synth chips are pretty neat but it does look a nightmare

2 SAR ADC 12 bit @ 1Msps
1 Delta Sigma ADC 8 - 20 bit
8 bit @ 384 Ksps, 12 bit @ 192K, 16 bit 48K, 20 bit 184 sps.

 

[large_ghostman]

2 SAR ADC 12 bit @ 1Msps
1 Delta Sigma ADC 8 - 20 bit
8 bit @ 384 Ksps, 12 bit @ 192K, 16 bit 48K, 20 bit 184 sps.

Green with envy nice specs 12 bit@1Msps! I hope thats real and not the 1Msps microchip keeps dreaming of lol , maybe one day they either crack it or not put it in the data sheets only to have to correct it in the errata! How many years they been using that trick anyway?

20bit ADC!! decent speed on 16 bit and enough resolution for most apps.

 

[dr pepper]

The thing is LG the faster they make micro's, the more demanding the application we will find for them.

 

[large_ghostman]

The thing is LG the faster they make micro's, the more demanding the application we will find for them.

Yes people must miss the days of the Ox drawn cart, just off down the road for a paper dear.see you next week , now you got 20 mins or the handbag is out lol

 

[gophert]

The thing is LG the faster they make micro's, the more demanding the application we will find for them.

I think it is the reverse. The faster and cheaper they make micros, the less demanding applications they end up in. Micros are approaching the price of simple logic chips. The mentality is becoming, why use two logic chips when you can put a micro there and save 0.25 cm2 of board space.

 

[mikep56]

Hi Mikebits,
Check out the website http://www.musicfromouterspace.com/. It has everything that you could ever want to know about electronic music.
Regards,
mike