[sdiy] Chroma charge pump VCO & hi-freq tracking error

[Massimo Ischia]

I've a question about the well known high freq. tracking error in MOOG like VCO: I think that is not Rbe but the timing capacitor discharge time that degrade the VCO linearity at high frequency !
 
Looking at Chroma VCO I found that instead a classic comparator + fet / bjt for the timing capacitor discharge they used 
a charge pump IC (4151). This make the frequency linear because (balancing the charge into the capacitor):
 
 Iexp * T cycle = Ipump * Tpump

where Iexp is the current foem the expo converter, Tcycle the total charge discharge time, Ipump the current from the Charge pump and Tpump the duration of the Charge pump impulse. Thus:

 f = 1/ Tcycle = Iexp  / ( Ipump * Tpump) 

and the terms Ipump and Tpump are costants ! A very smart solution that eliminate the effect of a costant discharge time.
Right or I've omitted something ?
 
Now I want to try the LM331 that appear to be pin compatible with 4151 and make some test.
Ideas and suggestions are well accepted

Bye

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