Hi all! I'd need some help, as usual. I'm trying to help Dag Lem, author of reSID and Simon White, author of Sidplay2 to implement realistic SID filters to their emulators. In short, the SID is a hybrid digital/analog design manufactured in pure NMOS design (8580 is HMOS). In the filter part, biased NMOS inverters are used as op-amps and simple NMOS FETs as voltage controlled resistors :-O. Because of this, the filter doesn't seem to match any known filter responses ;-). It's indeed very nonlinear; its response highly depends on the amplitude of the signal that is fed through the filter. The current implementation models an 'ideal' two-integrator-loop biquadratic filter with a measured cutoff-to-register_value mapping. I'm aiming to model the filter part in a simulator, but I'd definitely need some help for increasing my effectivity ;-). I'm not much familiar with FETs; I used to know their overall features, but have certainly never digged deep into FET based analog tech. At the current standing, my questions are aimed to the following: - Does anybody know the overall parameters of the FETs used in Commodore NMOS chips? They would be needed, since the filter is an analog circuit. It seems like the FETs get overdriven almost everytime during normal operation; the filter doesn't act as expected except the lowest signal levels (not to mention, that the OSCs have DC offsets, helping the FETs going out of their operating point even easier). Resistance, the typical gate opening voltage and so on. Typical gate-capacitance (seems like it's significant) and its behaviour in function of the gate voltage. - How does a Commodore MOS inverter look like _exactly? AFAIK, similar NMOS FET is used as pullup, but is it a similar one as the 'active', controlled pulldown FET or something else, what is its typical resistance in both states and so on... I'd be interested in all nuances, since any of them could have a noticeable effect inside the SID filter... - Is it possible to create real (linear) resistors in NMOS manufacturing? AFAIK it's quite hard, so it used to be cheated with using NMOS FETs, but I'm not sure and I must know this, also... If no resistors are possible, how is it cheated and so on... - And finally, what major differences are between the different manufacturing technologies of Commodore? (NMOS-x and HMOS, in particular). AFAIK, the 8580 was manufactured in HMOS, and it is indeed different to all 6581s. Similarly, different 6581 revisions should have been manufactured in different NMOS tech. (They also sound different; R4s have the strongest filter resonance and said to be the 'best'). I'd be happy if I could solve this modelling. After all, if I'd make a reasonable model (with also some know Commodore NMOS manufacturing data), I could also do some measurements on the real thing - both to gain new data and to compare them to each other. Also, I'd be glad if someone could point me to a webpage with 'standard' NMOS FET tech, to become more familiar with this subject (NMOS based analog example circuits and so on...). Any help is appreciated, thanks... L. - This message was sent through the cbm-hackers mailing list. To unsubscribe: echo unsubscribe | mail firstname.lastname@example.org.
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