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Overly sensitive circuit question
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Chris Wilson:
I got some great, patient advice re this project about a year ago. It's a phase meter circuit that looks at the phase on an antenna with 1 kW on it at 137khz and the circuity drives a variometer outside to bring the antenna back into phase as the weather or whatever changes. With advice from Ian.M I got it working quite well, but what I have found is it's overly sensitive. A snowflake circuit ;) By this I mean it hunts about as the wind blows the antenna. This is annoying and wearing out the variometer and motor drive. It is stable without minor outside environment changes though. Is there a way to make it less sensitive to small phase changes please? The circuit for the phase meter has no meter installed, but a 150 Ohm resistor in its place, still retaining the series 180 Ohm resistor as well. I also, on advice, added a 0.1uF across the output socket of the (separately, metal boxed) phase circuit, and floated its ground to match that of the motor drive circuit's floating ground. I did this with a 0.1uF and 470pF in parallel in breaking the ground where the ground symbol is shown on phase.jpg. I don't think this is the issue, it seems more likely I need to make the circuit less "accurate" so it takes more of a phase change to actuate the motor. I do not know if it's easily done, nor how to do it though. Thanks.
dmills:
Adding an extra diode or two in series with each of D1,D2 will make the thing need more integrated phase error before it begins to move the motor.

Making R1 a 10k pot will let you vary the sensitivity, and increasing the value of the cap across the first opamp will slow down the integrator.

Regards, Dan.

Chris Wilson:

--- Quote from: dmills on May 16, 2018, 02:28:30 pm ---Adding an extra diode or two in series with each of D1,D2 will make the thing need more integrated phase error before it begins to move the motor.

Making R1 a 10k pot will let you vary the sensitivity, and increasing the value of the cap across the first opamp will slow down the integrator.

Regards, Dan.

--- End quote ---


Hi Dan, many thanks for the reply, but could you please explain the difference between making it need more phase error and making it less sensitive? Does this mean a bigger voltage is needed to R1 before anything happens at all if I increase R1 in value, and if so how does that differ to a bigger phase error? Also why would you think BAT42's are specced for the diodes? Finally, (sorry...) should I ground unused OPAMP inputs to the negative rail or to the virtual ground? Thanks, sorry to be obtuse!
T3sl4co1l:
Reduce the 1M to, say, 100k, or even 10k.  This reduces the error amp gain, so that it only crosses the threshold for larger phase errors.

Tim
Ian.M:
Here's the original topic, for reference: https://www.eevblog.com/forum/beginners/250mv-and-_-from-a-single-output-bench-supply/

Unless the specific OPAMP datasheet says otherwise, its generally preferable to strap the -in of a spare OPAMP in a quad package to its output to configure it as a unit gain buffer, and take the +in to a voltage within its permitted common mode range.   If the common mode range includes the negative rail, great, otherwise use your virtual ground, or any other convenient node that's at a DC level.
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