[sdiy] Gilbert Cells and VCAs with Predistortion

[Ryan Williams]

hi Magnus,

Magnus Danielson wrote:
> I want the OTA-experts here to comment on this, because I have seen the same
> things (including the book, which I have in my "library") and also wondered if
> not that would make it sensible. One comment that I think I've heard was that
> the diff current source was component hungry.

this might explain why I haven't seen it before. Either that, or my lack 
of experience. My next task is to figure out how to make the diff. 
current source in an easier or better way. the version shown in that 
simulation was from a circuit labeled 'Voltage-Controlled Attenuator' in 
the MAT-04 data sheet but I attenuated the input slightly to allow large 
input range. It seems ok but my goal is to have something that works 
properly at DC also.

> As I was hinting, tossing in a clean sine (low THD) and measure the overtone
> spectra would give you a really good idea of the linearity. Do that with
> different input signal strengths and different modulation levels.

thanks for the hint. Hopefully I can get these circuits ready to build 
in the next few weeks on CNC milled circuit boards.  My school has all 
the stuff i'll need for measurments (the best thing about being in 
college).  I think it would be neat to have some useful data to compare 
these and other OTA and VCA circuits to see what really works.

> Is really nobody doing the DBX VCA anymore? Comments?

I still have about 15 THAT2180 VCAs but I'm saving them for situations 
where I want exponential control.  I'll probably just use them in other 
synth modules that need an on board VCA.

-ryan williams