[gta02-core] ECN0019 (was Re: ECN0010: LDO2/LDO4 swap implemented)

[Rene Harder]

Werner Almesberger wrote:
> Rene Harder wrote:
>   
>> Rene Harder wrote:
>>     
>>>> Joerg Reisenweber wrote:
>>>>         
>>>>>   we should replace (A)-R4305-(B) by (A)-R4305-(X), (X)-C4306_NC-(GND),
>>>>> (X)-R4399_0R-(B)
>>>>>   move C4303, C4304, R4305 next to wm8753, behind B3005
>>>>>     
>>>>>           
>>> I think that's a really good idea. In this configuration you completely
>>> prevent RF from getting into MICBIAS through JK4401.
>>>   
>>>       
>> I rewrote ECN0019 to reflect those changes and reopened it for further
>> discussions.
>>     
>
> Hmm, I think this is quite different from what Joerg had in mind.
> Here's my attempt at making sense of it:
> http://svn.openmoko.org/trunk/gta02-core/docs/ecn/ecn0019/swap.ps
>   

First of all, Sorry there was a typo in it. C4306 will be moved not
C4307.  fixed!

Apart from that, it's exactly what I had in mind, except for relocating
the choke. I would prefer to handle this in a different ECN, combined
with moving C4301/2 and C4305 to the codec side of the choke.


> Note that this drawing doesn't include ECN0042 or ECN0043.
>
> Also, the description of how the the buzz fix works isn't quite
> correct - it's not blocking RF noise coming from the microphone,
> but it's filtering the (audio frequency) buzz already riding on
> MICBIAS, originating from the jack.
>   

Okay I'm still not completely sure were the buzz gets into the system
but if audio frequency is already on MICBIAS (buzz coming out of the
codec) this change does not make much sense to me or maybe I don't get
the point.
I think, the idea was to build a RC filter (R4305 & C4306) with a really
low cutoff frequency to get rid of any noise introduced by the MICBIAS
line.
However, in this case R4310=0 Ohm fcut->inf (MICBIAS internal resistance
not take into account). So to get the same effect as in the current
design we would need to increase R4310 which is not possible because
this would also increase the voltage drop across R4310 and R4305. 
Currently, we are already operating the microphone near the minimal
required voltage so an additional decrease will probably result in
malfunctioning of the mic.