[sdiy] Hi frequency VC clock

[rburnett at richieburnett.co.uk]

DDS instead?

1. Available (Analog devices AD9834, etc.)
2. Frequency range of DC to tens of MHz is trivial
3. Frequency agile (no PLL locking issues)
4. Excellent linearity
5. Milli-Hertz freq resolution possible
6. Absolute frequency accuracy measured in ppm if master clock derived 
from a decent crystal oscilaltor
7. Frequency drift measured in ppm/'C if master clock derived from a 
decent crystal oscillator

Not as cheap as a free-running VCO though :-(  Particularly if you have 
to budget for a micro to talk to it, if your design doesn't already 
include a suitable microcontroller.

-Richie,


On 2017-06-21 12:25, Tom Wiltshire wrote:
> The reasons are pretty straightforward:
> 
> 1) Cheap
> 2) Available
> 3) VCO to 10+MHz.
> 
> Find me another chip that fulfils the above criteria and I'll use it! 
> ;)
> 
> But yes, I agree with you. We're using the chip for stuff it wasn't
> intended for. As we both noted, the absolute frequency isn't that big
> an issue for a PLL, but it matters a lot for a musical application.
> 
> Tom
> 
> ==================
>        Electric Druid
> Synth & Stompbox DIY
> ==================
> 
> 
> 
> 
> On 21 Jun 2017, at 11:20, chris <chris at chrismusic.de> wrote:
> 
>> To be honest, I never really understood why people resorted to use a
>> 4046 as a stand-alone oscillator in the first place.
>> That's not what it has been designed to do. Ok, it *can* be used like
>> that, but in the intended PLL usage, the absolute frequency isn't
>> determined by the osc intself but some input frequency, usually 
>> divided.
>> The osc only needs to "go somewhat faster with a higher CV", 
>> everything
>> else is fixed by the feedback loop and comparator.
>> 
>> In a way it's similar to a DCO wave shaper circuit - it doesn't have 
>> to
>> be precise, but just needs to create a saw from a digital square with 
>> a
>> precisely given frquency. If you chose to make a stand-alone osc from
>> such a sloppy wave shaper by adding some feedback, you wouldn't get 
>> far
>> either.
>> 
>> Chris
>> 
>> 
>> 
>> On Wed, 21 Jun 2017 11:03:02 +0100 Tom Wiltshire 
>> <tom at electricdruid.net>
>> wrote:
>> 
>>> They're all "good" in the sense that they do what they say on the 
>>> data sheets. It's just that the data sheets vary between 
>>> manufacturers, and also leave out many significant pieces of 
>>> information. Notice, for example, that the datasheet doesn't actually 
>>> provide a equation relating the component values and the VCO 
>>> frequency. They focus much more on the linearity, but not the actual 
>>> frequency. Makes sense for a PLL, perhaps, but not so good for us.
>>> I did find that Texas Instruments chips were more linear than NXP 
>>> ones. This was discovered by testing a couple of each and writing 
>>> down a series of data points and then getting the computer to 
>>> generate a line of best fit.
>>> 
>>> I was looking for something that could provide a couple of octaves of 
>>> exponential frequency modulation at 8MHz for top octave generation. 
>>> There's an old thread on here "4046 VCO overclocking a PIC" if you 
>>> want to find it in the archives. Some manufacturers versions of the 
>>> 74HC4046 claim to be able to go to 10MHz, so those were the ones I 
>>> chose. I was using a simplified variant of the Thomas Henry 
>>> exponential control. What I found was that the actual frequency range 
>>> is extremely variable between individual chips, and is even worse 
>>> between manufacturers. For something that needed to be tuned to a 
>>> musical pitch, this was hopeless.
>>> 
>>> I still think that what I was trying to do should really be possible, 
>>> but I wasted quite a bit of time on it and got nowhere, so eventually 
>>> I got discouraged and gave up. I might go back to it at some point.
>>> 
>>> Tom
>>> 
>>> On 21 Jun 2017, at 00:03, mark verbos <markverbos at gmail.com> wrote:
>>> 
>>>> I’m using a Texas Instruments one. It’s SO16. I hope I can make 
>>>> these work, because I bought a full reel of them!
>>>> Which brand are the “good ones”?
>>>> 
>>>> Mark
>>> 
>>> 
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