[sdiy] Spice vs real life noise problem

[Magnus Danielson]

Check your voltage supplies.

Cheers,
Magnus

On 06/06/2014 03:51 PM, Rutger Vlek wrote:
> Hi guys,
>
> I recently wrote about making some progress with a variation on the "discrete OTA" idea, well...now it seems I'm stuck. I've done a few weeks of debugging and simulating, and I just can't figure out
> what's going on. I have a VCA running, and that works pretty well (given known limitations of OTAs and discrete components) but has a problem in the output stage. There's a constant level of white noise on the output, that is quite a bit higher than I was expecting. It is totally uncorrelated with the VCA's control voltage, so it's even there (at the exact same level) when the VCA is maximally closed. I have several of these VCAs on a board, and as far as I can measure, each circuit behaves exactly the same. Because of this, and because of the "white" nature of the noise, I excluded layout-problems and interference as a possible source (am I right to do so?). Other possible sources I suspected were: opamp-noise (there's one 1/4 of an TL074 opamp in the output stage) or resistor noise. I changed the opamp for a very high quality one and no difference, which was also supported by some Spice modeling I've done (first in B2 Spice, later in TINA-TI with an opamp noise-model by Art
>    Kay: ftp://ftp.ti.com/pub/linear_apps/noise_article_series/article11-noise4-jan2007.pdf). Conclusion: it's not the opamp. And according to Spice: it's no resistor noise either. Strangely, the Spice model doesn't show the amount of noise I'm experiencing in the real world. This has made me really confused..... what noise source can be white, and doesn't show up in Spice? My strongest suspects for the source of this noise are at the moment the transistors in the current mirror of the OTA, but I believe that thermal and shot-noise are well defined in the Spice model of a BJT, right? I've entered some extra parameters in the BJT model for the flicker-noise components, but this type of noise is clearly not as strong and as "white" as I have seen in the real world. Is anyone experienced with such issues and interested in helping out? I would prefer talking privately over mail with a few people about this in depth, rather than having large public discussions on the list. Although
>   if we do any shocking discoveries or gain insights, I will of course share.
>
> Best,
>
> Rutger
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