Noise in oscillating circuit  - Page 1
Forum section: electronics / rf-microwave

author: snoozy
author_rank: Contributor
timestamp: April 11, 2021, 03:42:35 pm
content: Hey, Its me again...I'm looking for some help to improve my circuit.Actually only the OPamp U401 and the following oscillating circuit C401 and L401 as well as the resistor R402 are relevant.So, the circuit is a sensor circuit and I'm measuring the phase difference between the Signal OSZ_IN (1.4 MHz) and the singal at the sensor circuit.Unfortunatly, with a low resistance R402 the phase of the sensor circuit is not changing.I think this is because of the low impdance of the op amps nature.If I rise the resistance to a value like 1k, 10k or 100k, my sensor circuit gains sensititivy.But for sure the amplitude of the signal gets smaller and I see a lot of noise.In the screenshot you can see the signal OSZ_IN (green) and the signal at the sensor circuit.Is better way to get loose coupling to the opamp?Thank you in advance

author: T3sl4co1l
author_rank: Super Contributor
timestamp: April 11, 2021, 05:24:58 pm
content: Assuming U401 is doing what it should be doing, it seems your R402 or C401 are wrong.R401 and R403 seem to be superfluous.If R402 is as given, then it seems the "sensor" manifests as... around 0.47 - j1.63 ohms, or 0.47R + 70nF. (Phase shift seems to be about 74°.)The given values suggest a very broad bandwidth, and presumably your "sensor" varies one or both of C401 or L401 so that corresponds to poor sensitivity as you say. Sensitivity will be much higher with larger resistors, assuming the component Q is high enough to permit it. For values as shown, Zo = 345R, so (R402 || R403) >> 345R will give proportionate Q. In general, there is some equivalent parallel resistance (R403 but incidental to the components) which limits gain.You may find it's better to make an oscillator and count its frequency, than to measure the phase of a network at a fixed frequency. (Or maybe that's what you're doing, that phase detector would be fine into a VCO. I have no idea what's outside this thing..)Tim

author: E Kafeman
author_rank: Regular Contributor
timestamp: April 12, 2021, 05:14:28 am
content: Too low value for R402 can give problem with noise in voltage supply lines and heat problem if input amplitude is big enough. Assume there is decoupling.At 5V RMS must R402@47Ohm be able to handle 0.5W as almost no power will be dissipated over L401. Higher value recommended.Function of R401 and R403 is unclear also for me.U401, U401 and U403, all three had I added diode clamping to protect from overvoltage and 1kOhm serial resistors in serial with non limited input voltages.U402 and U403 may need C/R DC-decoupling.Decreasing C somewhat and it will be better resonance at 1.4MHz and better suppression of unwanted frequencies.A suggestion: R402=1-100 kOhm C401= 220 pF and 6x1n4148If everything is correct, at 1.4 MHz should measured amplitude and phase be almost same at pin3/U401 as after resistor R402, even if its value is 10 kOhm or more and R403 is removed.

author: snoozy
author_rank: Contributor
timestamp: April 13, 2021, 03:48:26 pm
content: Thank you guys a lot for your answers. That is really kind.What I missed saying is, that the change of inductance of L401 is the actual sensor. R401 is nonsense, I think I had it only for the simulations I did before.This sentence I didn't understand:If R402 is as given, then it seems the "sensor" manifests as... around 0.47 - j1.63 ohms, or 0.47R + 70nF. (Phase shift seems to be about 74°.)How did you calculate that?This is a university project. I have another aproach with a PLL similar to your suggestion. This was a tip from the forum, tooThanks to E Kafeman, too. It is true, there might be a very high current, if R402 is low.There is always a voltage drop at the resitor R402. Therefore a higher value gives a lower amplitude.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: April 13, 2021, 09:13:27 pm
content: If the voltages are as measured, then R402 into L401 || C401 (and whatever other equivalents) acts as an impedance divider, 47 ohms into Z, and the input and output voltages (including phase) are as given on the oscilloscope.A simple AC steady state analysis gives the result.I took the shortcut of ignoring the divider, as the ratio is pretty small, assuming instead R402 gives a constant current. This gives an error on par with the ratio, i.e., for the load being around 2 out of 50 ohms or 4%, the error is expected to be about as much i.e. it might be closer to 0.45 ohms.If you can adjust the frequency, you can use this to solve for the impedance as well:https://www.seventransistorlabs.com/Calc/RLC.html#frqIn short, there'sno way in hellyou got those measurements AND the values are as given-- something's wrong. It could be a short circuit, it could be just utterly wrong components, it could be the probed signals aren't as stated, I don't know.Tim

author: snoozy
author_rank: Contributor
timestamp: April 14, 2021, 06:44:21 pm
content: Hey Tim,thanks again for the explanations. The screenshot from the scope was done with a higher value for R402 to show the noise.Anyway, I tried to do the calculation, to see how low the voltage gets and to check the phase.I did it for a low value like 47 Ohm and a high value 10k.I'm not sure, if this gives me an answer about how to improve the circuit.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: April 14, 2021, 08:47:33 pm
content: Oh, so it wasn't as shown... well that would explain that, wouldn't it.So what's the problem with the noise? Like I said, sensitivity goes up with Q, and so does gain, so you can afford larger R402. I have no idea what your sensor is/does/looks like, if it's susceptible to ambient noise, or can't easily be changed to increase Q, or what.The noise on the scope, may or may not be there in circuit. Scopes typically have in the ballpark of 1mV RMS noise. That looks a bit more than that, I don't know. Noise can be present due to ambient sources (faulty SMPS, radio transmitters), or other sources in the circuit (parasitic oscillation, or anything else really).Noise can even be present in the circuit, without appearing across the nodes you think you're measuring, and still show up on the scope (or while probing) -- common mode noise (voltage between circuit ground, and scope ground or other surroundings) manifests in this way. The trick is you'll see CM noise when probing ground itself (probe tip to ground, clip open; or even with clip and tip both grounded, particularly at higher frequencies).Noise can be filtered, say by adding another R-(L||C) after this one, at modest expense to Q and gain, or with an active filter (including, or after, the buffer; note that noise can then affect the buffer, which may be undesirable).Since you have the excitation source, you can also sense it synchronously, which would consist of using a PLL to double the frequency, then a flip-flop to halve it, and a few gates to generate quadrature pulses. (A phase shift network could also be used, if the frequency range is narrow.) The quadrature clocks drive pairs of analog switches which pass or invert the signal (after the buffer), and the results are filtered (averaged). This gives the I and Q components of the analytical signal, which can be used directly as a complex number (I + jQ), or processed further (e.g. atan2(Q, I) to get the argument aka phase angle), which probably isn't too useful for analog purposes (that's a complex function), but can be done digitally. This has the advantage of extreme sensitivity (lock-in amplifiers use this or related techniques to very high sensitivity indeed), freedom from spurious modes (as the XOR phase detector may give) and freedom from edge sensitivity (as an edge-sensitive (type 2) phase detector has). The downside is the complexity, which should be very much overkill for a simple problem like this -- but again, as I have no idea what you're doing, it might pay off to see adjacent approaches.Tim

author: snoozy
author_rank: Contributor
timestamp: May 01, 2021, 06:54:24 pm
content: Hi Tim.thank you for your explanations an tips. And sorry for the late response.I got my sensor working so far. Some problems with the output signal conversion.I tried different coils and resistior combinations and found a solution for now.There are some pictures attached, so maybe it is a bit more easy to image.GreetingsSteffen