Active twin t notch filter oscillation - Page 1
Forum section: electronics / projects

author: DThodoris
author_rank: Newbie
timestamp: March 18, 2017, 05:22:38 pm
content: Hi, i have designed a tunable notch filter circuit for an application where i need to filter out a specific frequency.It is the well known active twin t, i just added a few multiturn trimmers to make it tunable in frequency and Q.The circuit is not very easy to tune, but once you tune it it stays there and achieves a good deep and sharp notch.I used 1% resistors and 1% C0G capacitors.The circuit exibits a 'weird' behaviour when i apply power to it.It oscillates for some time and then operates normaly.The duration of the oscillation is directly proportional to Q and inversely proportional to the supply voltage.For example:(@Vcc=9V)When i set the Q too high (trimpot R11 at 100%) it oscillates for quite a long time (tens of seconds).When i set it at 80% the duration becomes 1-2 seconds.(@Vcc=16V) The oscillation times reduce by a factor of 10The frequency of the oscillation is in the audio band,about 5-10% above the notch frequency.These measurements were done with the input shorted, and the output directly to the oscilloscope.Can anybody please explain this behaviour and maybe suggest a solution?Thank you in advance!

author: nctnico
author_rank: Super Contributor
timestamp: March 18, 2017, 05:43:21 pm
content: Did you do a stability analysis? I think you want the filter to be way too sharp with only a few components.

author: f5r5e5d
author_rank: Frequent Contributor
timestamp: March 18, 2017, 07:45:55 pm
content: sounds like too much positive feedback - other places I have seen only 10-20x Q multiplication with the twin Thttp://www.tronola.com/moorepage/Twin-T.htmlyou really need to think over your real requirement for how narrow/high Q the notch needs to bee and use the minimum value to avoid long settling transientsalso the TL072 are poor at these performance levels - at least step up to OPA1642 if not OPA827 op amps - "best" would be supply bootstrapped composite op amps rather than singles

author: T3sl4co1l
author_rank: Super Contributor
timestamp: March 18, 2017, 09:06:44 pm
content: Why did you reference input and output to GND when you have a perfectly serviceable "VB" there?Tim

author: MagicSmoker
author_rank: Super Contributor
timestamp: March 18, 2017, 10:31:55 pm
content: Personally, I'd be looking at a switched capacitor filter IC if I needed to adjust the Q and frequency of a higher order filter independently (or you could go the really modern route and implement a digital filter on a MCU or DSP or FPGA). Something like the venerable (albeit not so great performing) Texas Instruments MF10 or the newer Maxim MAX7490.The reason why your twin-t filter is oscillating is the same reason any active op-amp circuit oscillates: there is 360 degrees of total phase shift when loop gain is 1 or higher.

author: DThodoris
author_rank: Newbie
timestamp: March 20, 2017, 02:09:26 am
content: Hi guys thank you all for your time!As most of you suggest it seems that i am pushing the circuit beyond its inherent limitations...The reason i chose this topology in the first place is it's simplicity (easy design equations and low component count) and the high Q values it can achieve (or so i thought ).From a practical point of view though, the circuit does the business; at the lowest Q setting (R11 @ ~80%) that satisfies the application AND Vcc=16Volts it oscillates at 4kHz but only during power up (duration of oscillation is less than ~100ms) and then works fine. I was just wondering if there is a way to push this topology further and get rid of the oscillation completely.Anyway, thanks a lot guys!PS The reason i needed such a filter in the first place is a topic in itself!I ll open a new thread in the appropriate category for that and maybe we can discuss it there

author: DThodoris
author_rank: Newbie
timestamp: March 20, 2017, 02:16:30 am
content: Quote from: T3sl4co1l on March 18, 2017, 09:06:44 pmIts because the source is already referenced to GND (the same power supply feeds the previous stage and the signal is GND referenced)Does this have to do with the oscillation in some way?

author: T3sl4co1l
author_rank: Super Contributor
timestamp: March 20, 2017, 02:42:12 am
content: Can you rewire that stage, then? ...And so on?The oscillation described doesn't sound like it's necessarily the direct cause, but startup transients will definitely excite the system's resonance every time. You can at least reduce the amplitude by arranging the circuit to not have an initial voltage offset; namely, by grounding everything to VB, so the supply acts like a split supply.Tim