<HTML><BODY style="word-wrap: break-word; -khtml-nbsp-mode: space; -khtml-line-break: after-white-space; "><BR><DIV><DIV>On Apr 27, 2006, at 7:22 PM, sadiq sadiq wrote:</DIV><BR class="Apple-interchange-newline"><BLOCKQUOTE type="cite"><P style="margin: 0.0px 0.0px 0.0px 0.0px; font: 12.0px Helvetica; min-height: 14.0px"><BR></P> <P style="margin: 0.0px 0.0px 0.0px 0.0px"><FONT face="Helvetica" size="3" style="font: 12.0px Helvetica">We are trying to replace some resistors in an</FONT></P> <P style="margin: 0.0px 0.0px 0.0px 0.0px"><FONT face="Helvetica" size="3" style="font: 12.0px Helvetica">oscillator circuit with voltage controled resistors</FONT></P> <P style="margin: 0.0px 0.0px 0.0px 0.0px"><FONT face="Helvetica" size="3" style="font: 12.0px Helvetica">using the LM 3700 OTA.<SPAN class="Apple-converted-space"> </SPAN>What is the best way to do</FONT></P> <P style="margin: 0.0px 0.0px 0.0px 0.0px"><FONT face="Helvetica" size="3" style="font: 12.0px Helvetica">this</FONT></P> </BLOCKQUOTE></DIV><BR><DIV>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?</DIV><DIV><BR class="khtml-block-placeholder"></DIV><DIV>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.</DIV><DIV><BR class="khtml-block-placeholder"></DIV><DIV>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.</DIV><DIV><BR class="khtml-block-placeholder"></DIV><DIV><BR class="khtml-block-placeholder"></DIV></BODY></HTML>