[time-nuts] Re: shoestring budget & jitter AVR328p

[Bob Camp]

Hi

A lot depends on how you are dividing the clock signal. The most likely case here is a simple divider that just does integer division ( = the typical timer output on an MCU). You start from a 16 MHz master clock and divide by 4 to get 4 MHz or by 8 to get 2 MHz. 

The jitter on the output will be from multiple different sources:

1) The clock that is driving the MCU. If you are running on the “internal clock” that some MCU’s have, this will be a big deal. 

2) The VCO in any PLL multiplier in your MCU. You might have a 4 MHz external clock that is multiplied to 16 MHz. With a good external clock, this can easily be the dominant source of jitter. 

3) The output circuits on the MCU. Typically this is not a big contributor. 

Yes, this list could go on quite a bit further. 

Simple answer is that with a proper crystal oscillator into the MCU, your jitter should be << 1 ns. 

Jitter is a bit strange. You have the same time jitter regardless of how you divide. Phase noise scales as you multiply or divide. That’s just how the math works out. 

Phase noise / phase modulation is a more normal way to look at signal “pollution” in a case like this than jitter. It gets you to numbers that are a bit easier to understand. Saying that a noise contribution is 100 db down puts it on a scale you can understand. ( something like : if your system noise floor is 80 db down …. this doesn’t matter). This is often the better way to dig into a question like this. 

Bottom line: No, it’s not a big deal compared to the noise floor’s on the chips you are looking at. That’s true looking at jitter or at phase noise. 

Bob



> On Oct 16, 2023, at 7:11 AM, folkert via time-nuts <time-nuts at lists.febo.com> wrote:
> 
> Hello,
> 
> As an electronic music-enthousiast I also tinker with sound-chips of
> the 8- and 16-bit age. Circuit bending for example. And about that I
> have a question.
> On my website I published a page describing in a nutshell how tweaking
> the clock-frequency of a Philips SAA-1099p soundchip gives interesting
> sounds ( https://vanheusden.com/electronics/SAA1099-clock/ ). Here I use
> a timer of the Arduino Nano (AVR 328p) as a clock for the SAA1099p
> soundchip.
> A friend of mine read this and asked me if I have any ideas about the
> jitter introduced. Like: if I set the clock to 4MHz, how much jitter
> would this give. Now I read somewhere that delays of less than 5ms are
> usually not audible but does that also apply to jitter?
> First step in the investigation of that is to quantify how much the
> jitter introduced is (I guess). I know that when you have a PPS
> signal, that you can easily feed that to code that calculates the allan
> deviation, but how about clocks in the MHz range? If I divide the clock,
> wouldn't that average out any jitter?
> My budget is limited and/but I (do) have a hantek DSO-6022BL
> oscilloscope, some PicDivs and a 10MHz TCXO.
> 
> 
> Regards,
> 
> Folkert.
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