[sdiy] Square to Sine

[Magnus Danielson]

Andy,

On 04/05/2017 09:05 AM, Andrew Simper wrote:
> On 5 April 2017 at 13:46, Magnus Danielson <magnus at rubidium.dyndns.org> wrote:
>>
>> The backside of that would be that the 2 pole gives only 12 dB/oct compared to the 3 pole giving 18 dB/oct, thus giving less suppression of the square overtones. Their roll off would be 18 dB/Oct vs 24 dB/Oct from the filters. You would have to add a passive RC-link to achieve the same performance.
>>
>> The simplicity of the RC link setup is good starting point from a learning perspective.
>>
>> Cheers,
>> Magnus
>>
>
> The original passive 3 pole filter was drooping a bit much for Elaine,

With the buffer that I was proposing, there would not be much of a drop 
except of what the filter cuts of the fundamental. I was proposing a 
simple amplifier setup as a variant (add two resistors) to compensate 
with some minor gain.

> I was just proposing a very simple way of swapping in a BJT sallen key
> section cascaded with a passive low pass to keep the signal level a
> bit higher while still delivering the same 3 poles of filtering.

You where talking about a 2 pole filter, which is different from the 
original 3-pole filter, or at least that is what I saw from your 
contribution.

> I'm not following what you're talking about with the 12 dB/oct vs 18
> dB/oct vs 24 dB/oct, can you please let me know where the fewer / more
> poles are coming from?

2 pole => 12 dB/oct
3 pole => 18 dB/oct

Square has a 6 dB/oct roll off in itself, thus combined:

square + 2 pole => 18 dB/oct for overtones
square + 3 pole => 24 db/oct for overtones

Now, a smarter approach is to use a resonant lowpass filter setup, as 
the resonance located on the fundamental will provide an additional 
fundamental gain offset for all overtones, and then the roll-off takes 
care of the rest. This is fairly feasable to do for a fixed frequency 
thing like the organ/tone-wheel thing.

Cheers,
Magnus

> Cheers,
>
> Andy
>
>
>>
>>
>> On 04/05/2017 04:41 AM, Andrew Simper wrote:
>>>
>>> One way to avoid some of the volume drop is to use a 2 pole sallen key
>>> BJT filter with a cascaded passive stage. This is a common circuit
>>> used around BBD chips, eg:
>>>
>>> http://www.hobby-hour.com/electronics/s/schematics/boss-ce2-chorus-schematic.gif
>>>
>>> Andy
>>>
>>> On 4 April 2017 at 19:50, Elaine Klopke <functionofform at gmail.com> wrote:
>>>>
>>>> Thanks for the info Tom!
>>>>
>>>> Magnus, the sources are fixed square waves coming from a set of PIC chips. So the fixed filtering doesn't bother me a bit. One filter for one frequency. What worries me is the volume drop.
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