Sine wave oscillator  (first stage of ESR meter) - Page 1
Forum section: electronics / projects

author: erikbrenn
author_rank: Regular Contributor
timestamp: August 12, 2018, 01:33:54 am
content: Hi,I want to build a simple ESR meter, preferably using a circuit that I actually understand. I liked the minimalistic design by Larry Coyle in this article in Nuts and Volts magazinehttp://www.nutsvolts.com/magazine/article/january2016_ESRMeterforTestBenchTo learn/understand the circuit I started with the oscillator and created it in LTSPICE. I changed the output resistors/caps a bit to get closer to the 100KHz target frequency.Anyhow, although the sinewave is decent looking I came across another similar oscillator circuit that made me curious. It also uses an opamp but the feedback loop involves a JFET. Here is the schematic with both oscillators.I have compared the circuits in ltspice and it seems the circuit using the jfet yields a smoother sine (blue=out1, green=out2)Plotting the FFT of both outputs also shows some more harmonics for the first circuit.However, the second oscillator outputs only around 15kHz and I need 100KHz, and I don't really understand the second circuit. I've tried to modify the caps and the resistors without success, oscillation stops etc.How can the second circuit be changed to yield a higher frequency? (I realize this might affect signal output quality, but I would like to learn)cheers.(NOTE: The +5V and -5V labels at the opamp supply voltages are incorrect. The voltages at these nets are +15V and -15V , from the spice directives V98 and V99.)

author: oPossum
author_rank: Super Contributor
timestamp: August 12, 2018, 02:21:43 am
content: The second circuit is a Wien bridge oscillator. They can produce a very clean output.You must keep C6 & C7 the same value, and also R11 and R12 the same value.

author: RoGeorge
author_rank: Super Contributor
timestamp: August 12, 2018, 02:22:57 am
content: The second one is called a Wien bridge oscillator.https://en.wikipedia.org/wiki/Wien_bridge_oscillatorThe JFET is there to adjust the gain of the amplifier in order to keep the amplitude of the output oscillations.

author: erikbrenn
author_rank: Regular Contributor
timestamp: August 12, 2018, 04:41:35 am
content: aha, ok thanks. So I've been reading about the WB oscillator for an hour now and it is stated that the frequence is governed by 1/(2*pi*RC), and thus can be easily changed.But I was not able to increase the frequencey just by lowering the cap values as things got unstable, or the sine was clipped. I also had to tune the feedback resistor divider, and that is extremely sensitive it seems. So implementing the circuit I'll probably need an accurate pot-meter there and tune things on mye scope, fair enough.Anyhow, this is what I came up with to yield a nice ~100KHz sine in ltspice.It also has lower harmonics at the 300KHz mark compared to the the original one without the FET switcher.

author: rhb
author_rank: Super Contributor
timestamp: August 12, 2018, 07:30:49 pm
content: Read "Max Wien, Mr. Hewlett and a Rainy Sunday Afternoon" by Jim Williams.You can also measure ESR using a pulse from an MCU and measuring the rise time curve and switching a 2nd resistance in and out of circuit. But read Jim's article no matter what you do.

author: ogden
author_rank: Super Contributor
timestamp: August 12, 2018, 09:33:54 pm
content: Quote from: erikbrenn on August 12, 2018, 01:33:54 amCheck this ESR meter:https://www.eevblog.com/forum/projects/5-transistor-esr-meter-design/