This circuit uses the versatile MAX038 function generator. Although in this circuit some of the advanced characteristics of this IC are disabled, you can generate Sine, Triangle, Square waves (adjusting A0 and A1 pins see datasheet on www.maxim-ic.com if you want other waves, use a switch).
The signal is amplified through a TCA0372 (from ONSEMI) Power opamp with current capability up to 1A and bandwitch up to 1 MHz.
I selected this particular frequency (122 Khz) because i needed a cheapo ESR-o-meter for my electrolytic capacitors to monitor their health as they have to discharge tens of amperes in less than 2 ms. At 122 KHz capacitive reactance is very low, and inductive reactance isn't so high, so forcing a current (es 200mA, using a precision resistor) through a capacitor and reading AC voltage drop accross it gives me an estimation of ESR (Vdrop/current). Of course inductive and capacitive reactance are still present, but negligible.
Let's back to the circuit.
Operation:
The 122 khz 2V p-p sine wave is generated by the MAX038 IC, its frequency can be calculated by the formula Freq (MHz) = Iin(uA) / C6 (pf) . Iin = 2,5V / R1 (25Kohm default). So the freq is 0,122 MHz . The resistor is for small adjustments, don't go under 10000 Kohm or above 40000 Kohm because the accuracy will drop. If you want multifrequency just use the multiposition switch with 820 pF, 8,2 nF , 82nF , 820 nf for 122Khz range 12,2Khz range 1220 Hz and 122 Hz. Fine tuning can be done adjusting R2 , the frequency can vary from 1,7x (Vfadj = -2,4) to 0,3x (Vfadj = 2,4) of the main frequency (when fadj is at 0V).
The sine wave output is feed into a TCA0372 1/2 opamp to achieve a gain from 1 to 5 (2V p-p, 10 V p-p), adjust the potenziometer and into a TCA0372 2/2 opamp buffer stage also present on the same IC.
Important:
Adjusting the frequency needs a frequency counter, so this circuit should be used on conjunction with a freq couter. The max current is 1A, but i would suggesto to not go above 0,5A to remain accurate. Needs a computer power supply with 12V,5V,-5V,-12V,GND to be operated, if you don't have one just use a multivoltage mains transformer (15 watt is enough) diode bridges (low current 1-2 Amps), smoothing capacitors 10000uF 16V, and voltage regulators such as LM7905 and LM7912.
Source: Electronics Lab
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