[sdiy] XR2206 temeperature stability within +/- 5%

[Tom Wiltshire]

That was my first thought too, Richie!

My experiments have lead me to prefer Pulse Density Modulation over Pulse Width Modulation.

With PWM, the ripple is worst for the 50%/50% case - your midpoint value. With PDM, the ripple is worst at the extremes when the pulses are least dense, and at that point, it degenerates to a similar signal to PWM. For other cases, what you’re essentially doing is taking the ‘high time’ and the ‘low time’ of your PWM signal and breaking it up into little pieces and rearranging them, instead of having all the high time together and all the low time together, as you would with PWM. That increases the apparent output frequency enormously and helps a great deal when it comes to the filtering. A simple RC lowpass would do.

Best of all, the NCO peripheral on the PIC can generate these waveforms directly in its “fixed width” mode. You can easily get 12-bit or better resolution out of it, a decent step up from the 10-bit PWM.

> On 2 Jul 2020, at 22:29, Richie Burnett <rburnett at richieburnett.co.uk> wrote:
> 
> Have you thought about doing this digitally with a low-end micro? You didn't say how pure you need the sine wave to be, but you could generate it by PWM. (You can filter the hell out of something that only needs to go up to 60Hz maximum and doesn't contain any harmonics.)
> 
> The very low maximum frequency spec is amenable to software DDS with PWM output, provided the purity spec isn't ridiculous. 
> 
> -Richie,
> 
> Sent from my Xperia SP on O2
> 
> ---- Jean-Pierre Desrochers wrote ----
> 
>> I need a pure sine wave generator adjustable from 10Hz to 60Hz  at around 5v
>> peak.
>> The adjusted frequency should stay stable within +/- 5% temperature wise.
>> The project inside metal enclosure could have temp of up to around +40deg C.
>> I have a bunch of XR2206 VCO IC on hand to use.
>> I wonder if the best capacitor to be used here would be
>> A polystyrene type for temperature stability.. (??)
>> I'll use 1% metal film resistors for the rest of circuit..
>> 
>> 
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