Damping Crystal Filter Oscillation (SOLVED)

[jpanhalt]

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 kHz 

Is such slow damping an inherent characteristic, or can I reduce it and how?   Would an actual quartz crystal filter show faster damping?

Regards, John

[T3sl4co1l]

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.)

Cheers

Tim

[jpanhalt]

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

[T3sl4co1l]

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

[Richard Head]

A carefully designed and built noise blanker will make a huge difference.