Bandpass filter oscillations - Page 1
Forum section: electronics / beginners

author: Tobias89
author_rank: Contributor
timestamp: April 14, 2015, 02:28:21 pm
content: Hello,I have designed a cascade of 4th order Chebyshev BP filter (using MFB section), inverting amplifier and analog comparator.The 4th order filter itself is designed to have a center frequency 129kHz, passband around 30 kHz and gain in pass band around 0 dB, and amplifier around 40 dB. GBP of used op amps is 50 MHz.Now, if i ground V_IN node (input of this cascade), according to PSpice, I should have around 2.502 V DC on the output of BP filter (V_OUT2 node). However, I get a steady sine signal about 40 mVpp and 110 kHz frequency. The same signal is (as expected) amplified on 3rd stage (amplifier).I calculated a few more BP filter configurations (different center frequency for about 3 kHz and different passband, ergo different Q-factor), and same thing would happen, only with different sine signal frequency.However, if I just disconnect last op amp in cascade (comparator), this signal would disappear.I know my BP filter is stable (has all poles in left half plane etc.), and I can't figure out what could be the problem.Has anyone seen this before? What could be the problem? Thank you for any help!

author: TimFox
author_rank: Super Contributor
timestamp: April 14, 2015, 02:32:31 pm
content: Besides the 1uF capacitor in the lower right corner, do you have a reasonable (maybe 0.1 uF) capacitor at each IC, with short leads between the positive and negative power pins of the device?Is your actual circuit built on PCB, perf board, or solderless breadboard?

author: rs20
author_rank: Super Contributor
timestamp: April 14, 2015, 02:34:44 pm
content: How can you be so sure that the design's poles are all on the left half plane when you don't know:a) the real component values (as opposed to nominal)b) parasitic valuesc) phase delay in the opampsEtc?

author: deephaven
author_rank: Frequent Contributor
timestamp: April 14, 2015, 02:45:54 pm
content: You've got a massive amount of effective gain on your comparator. Try adding a bit of positive feedback to it to introduce a touch of hysteresis.The preceding amplifier stage might benefit from turning down it's HF gain a bit with a small C across it's feedback.

author: tggzzz
author_rank: Super Contributor
timestamp: April 14, 2015, 02:59:42 pm
content: The circuit layout will be important, particularly decoupling, ground plane and near the opamp inputs. I presume you are not using one of the wretched spring clip breadboards; people spend more time debugging those than debugging their circuitOften a clue can be gained by poking various nodes and components with your finger, and seeing what changes.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: April 14, 2015, 11:59:13 pm
content: Which PSPICE analysis/es did you use?

author: Tobias89
author_rank: Contributor
timestamp: April 15, 2015, 08:02:03 am
content: Quote from: TimFox on April 14, 2015, 02:32:31 pmI have that 1uF capacitor right next to each IC (have 2 multipart chips, each with 1uF decoupling capacitor). My actual circuit is built on single layer PCB.Quote from: rs20 on April 14, 2015, 02:34:44 pmThe values on attached schematic are the actual values I put on my PCB (resistors have 1% tolerances and capacitors have 5%). Do you think that could decrease stability?Besides, isn't the definition of stability that the system gives a bounded output signal for any bounded input signal (bounded in amplitude)? I tested this circuit with function generator for sine input from 1 kHz to 1 MHz, and it gave a bounded output signal every time.Quote from: deephaven on April 14, 2015, 02:45:54 pmYes, I could add a hysteresis to a comparator and a C across amplifier's feedback (thanks for advice), but I don't think that would solve my current issue.The previous revision of this circuit consisted only of current third stage amplifier and comparator, and it worked exactly like planned. In this revision, we only wanted to become more resistant on interferences (our useful signal is between 124 kHz and 134 kHz), so we added first two stages of BP filter.Quote from: tggzzz on April 14, 2015, 02:59:42 pmLike i mentioned, circuit is built on single layer PCB, so there is a possibility a ground plane could help.I'll try poking it a little bit, thanks.Quote from: T3sl4co1l on April 14, 2015, 11:59:13 pmFirst, I checked AC sweep for a frequency characteristics. Then a transient analysis with DC signal on the input (as expected, output of comparator is at 0, and output of an amplifier is at 2.502 V), and last Monte Carlo and Worst Case analysis with my component tolerances.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: April 15, 2015, 10:59:11 am
content: Quote from: Tobias89 on April 15, 2015, 08:02:03 amHmm, single layer? No ground plane? That sounds suspect to begin with.Goodsingle layer layouts are hard to do, when you have GBW up into (and beyond) the 10s of MHz range.It could very well be that your bypass caps are resonating (forming a high Q loop against trace inductance), or other considerations (common mode at the inputs? RFI?).Show your PCB, layout, whatever -- you can never supply too much information!QuoteThat definition is also assuming a linear, time invariant (LTI) condition, and it's also not a very subtle condition -- your circuit can ring like a bell, but as long as the ringing decays, it can still be considered unconditionally stable.Real circuits can never be perfectly linear; the output is always bounded, so in that sense alone, even a proper oscillator would be stable.A slightly more practical definition might be, a system whose impulse response asymptotically goes to zero over time. But this, too, is an unrealistic definition, because real circuits also exhibit noise, so the asymptotic response can never truly be zero, either! Argh!A typical case for amplifiers is, it's stable at zero signal (or zero load or offset), but goes unstable at certain portions during an input waveform, or only up to a certain amplitude. You can think of it this way: the signal level, or offset, or oscillation amplitude itself, effectively causes the poles to shift, so that the oscillation becomes self-limiting at some point -- this is a nonlinear system, and often time-variant as well. The output remains bounded, but the circuit is said to oscillate (or if it's during portions of the input waveform, it's called squegging).So between these two cases -- marginal stability and noise -- you may well have a problem at work.But about noise -- it's noteworthy that it may be completely normal. I don't know offhand the noise amplitude expected from your choice of op-amps and filter design, but it is nonetheless true that it should have residual noise, which will be bandpassed by your filter of course, so that rather than random white noise, it looks like an erratic sine wave -- the noise is in the frequency and amplitude.This is characteristic of radio IF stages, where the gain is very high (typically millions!) and the bandwidth is narrow, so the output always looks like a sine wave, whether it's receiving a signal proper, or just receiving, well... noise!Offhand, 100s of mV sounds like way too much for that, though.(Aside about ringing: in your case, it's *supposed* to -- that's what a bandpass filter's step response looks like! Of course, it's only bad when it's doing something it's not intended to do. Or, to put a more refined point on it: it's *supposed* to ring, but just enough, and at just the right frequency -- if it's too sharp or too weak, or at the wrong frequency, your gain and bandwidth will be all kinds of wrong, which is bad.)QuoteAh, transient is noiseless so with DC input, you'll never see anything, whether it's unstable or not! You can add a small (~1mV?) noise source, or a pulse or something, to give it some "movement". And see how that goes.The transient simulation's equivalent of a real world test might be, use a noise source, then plot the output frequency spectrum. After long enough run time (it can take quite a long simulation to gather enough data, unfortunately), the spectrum should be fairly flat, except where it naturally conforms to the frequency response of the bandpass filter. Try to avoid using too small a signal level; it should be large enough to exercise a reasonable voltage swing, say Vout(p-p) about half the supply voltage. (Which, for a Gaussian noise source, should be around Vrms ~ 1/10 of supply voltage, assuming sigma around 3 or so.)Tim