[sdiy] Gilbert Cells and VCAs with Predistortion

[Magnus Danielson]

From: Ryan Williams <destrukto at cox.net>
Subject: Re: [sdiy] Gilbert Cells and VCAs with Predistortion
Date: Sat, 02 Jul 2005 20:25:56 -0500
Message-ID: <42C73EA4.6040508 at cox.net>

> 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.

Build some traditional OTA/VCA designs onto the same board and make the
comparision in the same project.

A classic way to do THD is to subtract the output signal with the input signal
and balance them towards minimum. The residue signal should be the non-harmonic
(not all true, but ah well, it's an approximation) The ratio between the
signals is the THD.

On my network-analyzer/spec I have a little program that measure the 10
overtones and calculates the THD from that. Lovely to have gear that can be
taught to think for themselfs... ;O)

> > 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.

Well, the VCA is 2 PNP and 2 NPN and an inverter as "active" components, so it
was easy enought to do. The linear control input can be acheived by using a
diode to logarithmize the signal. Remember, over there some temperature
dependence might not be the worst thing in the world.

Cheers,
Magnus