[sdiy] Temperature Compensated Exponential ConverterUsingSSM2164

[David G. Dixon]

And another thing...

It is very easy to add Linear FM to this 2164 expo converter.  All you have
to do is to connect another 1M (or whatever) resistor into the input of the
second VCA in parallel with the 1M resistor from the negative rail, and feed
a signal or voltage to it through an attenuator.  The input of the VCA acts
as a current summer at virtual ground, just like an opamp summer.  Positive
voltages into the Linear FM input increase the frequency, and vice versa.


> I've just simulated my own version of the "Hoshuyama Expo + Saw Core VCO",
> and it is very nice indeed!  A 1nF timing cap gives 100Hz/uA, which sets
> the
> 0V CV frequency at 150Hz for a 1M reference current resistor.  This puts
> the
> audible range of CV between about -6V and +4V.  A CV of -15V gives a
> period
> of about 20s, which is a pretty decent bottom end for LFO use.  Also, it
> would appear that this VCO will not require high-frequency compensation.
> 
> The simulation confirms that the tempco voltage must be within a fraction
> of
> a mV for good tuning.  Here's how I organized the hypothetical tempco
> voltage divider feeding the buffer (all 1% resistors, of course):
> 
> +15V --> 15k --> 150R --> 100R trimmer <-- 250R <-- GND
>                                 |
> 
> This should allow for relatively easy adjustment, and puts the
> hypothetical
> voltage of 290 mV right in the middle of the trimmer.  Of course, in real
> life the small resistors may have to be altered, but I doubt it.
> 
> For the CV summer feedback resistors, I selected a 53.6k 1% resistor and a
> 2k multiturn trimmer.  Again, the sweet spot is right in the middle.
> 
> Anyone interested in the complete schematic can email me offline.  I'm
> going
> to breadboard this hopefully within a few days to confirm the design (I'm
> supposed to be doing "real" work today -- oh well!).
> 
> 
> > Sorry, I blew it!  The temperature coefficient of both metal film
> > resistors
> > and high-quality trimmers is 100ppm max (so long as the resistor value
> is
> > less than about 100k).
> >
> > > Incidentally, I have looked at the temperature sensitivity specs of
> > metal
> > > film resistors (~1.5 ppm) and 18-turn trimmers (~100 ppm max), and as
> > long
> > > as the bulk of the voltage drop is taken across the metal film
> resistor,
> > > the effect should be minimal.