Damping Crystal Filter Oscillation (SOLVED) - Page 1
Forum section: electronics / rf-microwave

author: jpanhalt
author_rank: Super Contributor
timestamp: May 05, 2017, 02:43:28 pm
content: I built a 500 kHz crystal filter using somewhat "matched" ceramic resonators (schematic attached). The design is based on a series of articles by Wes Hayward found in QST (See: QST May, 1982). When sweeped, it shows a satisfactory passband for my needs.When challenged with a burst (Rigol DG1032Z), it continues to oscillate for about 2 ms after the stimulus (attached).Legend: Yellow = filter output; Blue = input burst at 500 kHzIs such slow damping an inherent characteristic, or can I reduce it and how? Would an actual quartz crystal filter show faster damping?Regards, John

author: T3sl4co1l
author_rank: Super Contributor
timestamp: May 05, 2017, 03:02:00 pm
content: Read up on the Fourier transform.If you want a short pulse, you need a wide bandwidth.If you need a narrow bandwidth, you need a long pulse.This is true for baseband (step response, pulses, general signals: a center frequency of 0Hz), as well as RF (the same sorts of signals, around a carrier; a center frequency higher than the signal bandwidth). So, a wavelet with narrow bandwidth must ring for a long time.If you need short impulse response, consider a Bessel type bandpass. Don't be surprised if the frequency response turns out horrible, though.Likewise, we can see what your above measurement has produced: notice the tone burst starts on a sharp corner -- it's differentiated through the network's series capacitance, not "filtered" at all. (One property of the crystal ladder filter is, the asymptotic response -- the attenuation at frequencies far from Fc -- is not very good, typically 20dB per crystal used, with peaks and dips at various spurious frequencies where the crystals have secondary resonances. They are best used when combined with a fairly sharp, modest order, LC filter.) This is also why the amplitude is so small: the tone burst length needs to be on the order of 1/BW to see the output ring up to its full output. The ringing seen afterwards is the impulse response (since such a short burst is near enough an impulse, being much shorter than 1/BW), which will have the same form (decay over time) for a longer burst, as is seen here.Finally, your scope measurement will need to reflect the signal as well. I suggest triggering on the generator's GATE output signal (or AUX or whatever it's called), setting horizontal timebase on the order of 1/BW, and setting acquisition to peak detect mode. (An analog scope will read the RF envelope just fine; a newer scope may not need peak detect if it has deep memory, samples faster than the center frequency, and displays the waveform with antialiasing.)CheersTim

author: jpanhalt
author_rank: Super Contributor
timestamp: May 07, 2017, 02:17:38 pm
content: Thank you. I thought what I was seeing (misinterpreting) on the oscilloscope was too good to be true. The filter output at that time was just a mirror of the input (blue trace) and was not sensitive to the burst frequency set on the signal generator. When I extended the burst to 1000 cycles, the crystal started up, and response to the SG frequency was as expected. Attached is a screen shot of that result.John

author: T3sl4co1l
author_rank: Super Contributor
timestamp: May 07, 2017, 05:28:09 pm
content: Ah, nice simple single pole envelope.(Which means it's actually a complex pole pair, because it's oscillating all the while. But there aren't any pole pairs near that pair, otherwise the envelope would be different, it might bounce a bit, or be more rounded. The envelope is the Fourier transform of the bandpass frequency response, so it follows the step response of whatever filter prototype you have.)Tim

author: Richard Head
author_rank: Frequent Contributor
timestamp: May 08, 2017, 08:37:39 pm
content: A carefully designed and built noise blanker will make a huge difference.