Overly sensitive circuit question - Page 1
Forum section: electronics / beginners

author: Chris Wilson
author_rank: Super Contributor
timestamp: May 16, 2018, 01:30:43 pm
content: 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 circuitBy 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.

author: dmills
author_rank: Super Contributor
timestamp: May 16, 2018, 02:28:30 pm
content: 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.

author: Chris Wilson
author_rank: Super Contributor
timestamp: May 17, 2018, 11:03:56 am
content: Quote from: dmills on May 16, 2018, 02:28:30 pmHi 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!

author: T3sl4co1l
author_rank: Super Contributor
timestamp: May 17, 2018, 10:18:24 pm
content: 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

author: Ian.M
author_rank: Super Contributor
timestamp: May 17, 2018, 11:18:21 pm
content: 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.

author: Chris Wilson
author_rank: Super Contributor
timestamp: May 18, 2018, 10:06:17 am
content: Quote from: T3sl4co1l on May 17, 2018, 10:18:24 pmHi Tim, thanks for helping. At the risk of me sounding completely stupid, are you saying replace the 1 meg pot with a 100k or 10k pot? I don't see why this is any different to reducing the resistance of the existing 1 megohm pot by simple adjustment, as it's got one side connected to the slider... Sorry, but I need to also understand what I am doing in order to learn from this and I am unclear as to whether you mean adjust or change the pot. Thanks.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: May 18, 2018, 06:35:15 pm
content: Yes, adjust it down.Alternately, increase the 1k.You might not have much resolution, say in the first 10% of range, in which case changing the range (either way) might help. It's not a precision control, as far as I can see.Tim

author: Chris Wilson
author_rank: Super Contributor
timestamp: May 18, 2018, 09:04:28 pm
content: Quote from: T3sl4co1l on May 18, 2018, 06:35:15 pmThat worked fine, thanks for the explanations, but I have spotted an annoying possible issue. I have fixed up an old, but originally quite expensive current and voltage source and reader. It's a Haven Minical MKII and it gives a positive and negative source with excellent accuracy. My motor drive circuit has a red and green LED to show power is being sent in one direction or the other to the variometer drive motor. What I am seeing is at -5.25mV the green LED will light. In theory at +5.25mV the red LED should light, but it does not, it only needs +1.06mV to light. I will check what sort of mV levels come from the bridge in a bit, I can't recall the levels, but where would I look for the inaccuracy, and where do I probe with a DVM , ground wise, to see the first opamp is unbalanced (I can't see how it cannot be, but hey, what do I know!). I am guessing any imbalance is in the circuit around the final two op amps? Diodes or 4.7k resistors? Thanks again

author: T3sl4co1l
author_rank: Super Contributor
timestamp: May 19, 2018, 02:50:14 am
content: What was the opamp? RTFDS.Tim

author: Chris Wilson
author_rank: Super Contributor
timestamp: May 19, 2018, 03:12:19 am
content: Quote from: T3sl4co1l on May 19, 2018, 02:50:14 amOpamp is a TL084 I have added pin numbers to the sketch and the link to the datasheet is herehttp://www.ti.com/lit/ds/symlink/tl082.pdf