[sdiy] Temperature Compensated Exponential Converter Using SSM2164

[Magnus Danielson]

Ian Fritz wrote:
> At 11:32 AM 8/30/2009, David G. Dixon wrote:
>> Missing the next octave by 1 Hz is no big deal
>> if your target is 1760 Hz, but it is a very big deal if your next 
>> octave is
>> 55 Hz.  Capiche?
> 
> Ummm ... no.   :-)
> 
> What is most objectionable about mistuning is the beating it creates.  
> In your example there would be a 1 Hz beat rate (against a perfect 
> reference)  for either case.  Not bad at all.
> 
> This why I set my VCOs up so that deltaVbe at the converter pair 
> corresponds to something around 2 kHz.  (deltaVbe = 0 --->  
> exp(a*deltaVbe/kT) = 1, so there is no T dependence at this point)  
> Drift above this point isn't too important, because you rarely use 
> fundamental frequencies much above 3 kHz.  Drift below this point 
> becomes less and less critical as you go down, because a 1 Hz beat 
> represents smaller and smaller fractional error.
> 
> Just my personal philosophy, I guess.  :-)

I think I have almost never seen any good motivation for this "0 
modulation frequency" to have any particular value... other than "this 
is in our neighborhood so let's settle with that one". I agree with your 
reasoning and even a few octaves down should still work well.

I have proposed and used a three-trimmer setup in which one of them is 
the reference current and hence the modulation free frequency. Makes 
trimming a much simplified procedure.

Cheers,
Magnus