[sdiy] quadature oscillator

[Michael Bacich]

On Apr 27, 2006, at 7:22 PM, sadiq sadiq wrote:

>
> We are trying to replace some resistors in an
> oscillator circuit with voltage controled resistors
> using the LM 3700 OTA.  What is the best way to do
> this

Shall we assume that you have already looked at and tried the voltage  
controlled resistor circuit examples on pages 10 and 11 of the  
LM13700 datasheet?

BTW, does your quadrature oscillator application depend on accurate  
tracking of several of these voltage controlled resistors?  Could be  
a tough problem, depending on how much accuracy you need over the  
entire operating range.

To measure the operation of your voltage controlled resistor, I think  
what you need to do is:  Supply a known amount of voltage through a  
known resistor to get a known current (for instance, 10 volts through  
a 10K resistor to get 1 mA current).  Supply that current to the VCR  
input and measure the current at the output.  Use Ohm's law to  
determine the overall resistance of the circuit (V / I  =  
resistance).  So, if you supply 1mA at 10 volts to the input, and you  
measure 0.5 mA at the output, then you know that the equivalent  
resistance of the OTA is 20K Ohms.  Take a series of these types of  
measurements with different amounts of bias current at the OTA's  
control pin to determine the working range of your voltage controlled  
resistor.


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