Sonar : Problem with a resonant circuitry

Hello Simon,

Ok, I am from the medical side of ultrasound but we have the same problem, the need to see tissue that's right under the scanhead. What you can try is to arrange your TGC amp so it loads down the transducer heavily, at least for the first few usec (after the T/R switch).

However, the real issue is the backing material of the transducer. That's a science in itself and the recipes are closely guarded. You could talk with the folks that process PZT at companies such as Parallel Design or TRS. Or with experts at universities like Penn State.

Regards, Joerg

At Stanford in the 80s they used epoxy loaded with tungsten powder for a nice absorber that impedance-matched to zinc oxide.

Cheers,

Phil Hobbs

You might be looking at the natural ringing of the transducer. Your transducer may not be appropriate for your task if it isn't damped enough for your application. What sort of minimum range do you need and at what frequency? If the transformer is resonating with the xdcr, your transformer may be improperly designed (resonance too close to the operating freq). If shorting the primary is helpful in reducing the ringing, try using a H-bridge instead of a push-pull or half-bridge. In a H-bridge, you can leave either the top or bottom switches (FET's) on during the receive period which effectively shorts the transformer primary.

Hello Phil,

Yep, that's a start. There is also a lot of black magic to processing that, for example making sure there aren't the tiniest pockets.

Regards, Joerg

Hello Mark,

Shorting isn't always going to cut it. The energy from the ringdown needs some place to dissipate, ideally a resistance. It's like the shock breakers in a car which transfer vertical ringing of the suspension into heat after driving through a pothole. If you'd 'short' them all the stuff the trunk and the passengers would go flying about.

Regards, Joerg

At EMI we just used resin-bonded tungsten powder - mostly bonding it with regular epoxy resin. You'd want to de-air it (stick the mixture in a vacuum desicator (or some other chamber tht you can see into and pump down) and evacuate it until the mix starts frothing, the evacuate more slowly so that the froth doesn't overflow your container before it breaks.

--=20 Bill Sloman, Nijmegen

Another trick we used at EMI was to cast a quarter-wavellength-thick layer of resin-bonded tungsten powder and glue that on the front of the transducer as an impedance matching device. Apparently that helped too.

Hello Bill,

Yep, that's another trick. But can cause trouble in very wideband PZT-5H apps. We used to call that acoustic lens, it doesn't have to be a constant thickness layer.

Regards, Joerg

You shouldn't have, it is strictly an impedance matching device, equivalent to the anti-reflection layer on the face of an optical lens.

It does if you want to improve the coupling between the transducer and the fluid in which you are doing your imaging. An acoustic lens doies something rather different.

Simon,

the fastest way to stop the transducer ringing is to drive it with another signal that is 180 deg out of phase with the original signal...for a very short time....just until it stops ringing...

Mark

Hello Bill,

Reason is that it isn't always a flat surface but.

Yes, but there is always the trade-off between resolution, focus and matching. In the near field matching isn't too important (provided you have a good receiver circuit) but focus is. Imaging in our case is mostly through tissue with very little in fluids. Except to some extent in coronary imaging and Ob-Gyn during pregnancy.

Regards, Joerg

Interesting theory. Have you evr tried to reduce it to practice?

There's the acoustic transit time, at a minimum.

In general it's probably better to subtract the ringing out in software--but that requires really good control, i.e. keeping the transmit pulse exactly the same each time--same phase, same length, same transients.

Cheers,

Phil Hobbs

Hello Phil,

The best result I achieved was with some nice resistive damping right before the first receive amp which is then gradually eased off towards the long range echoes.

Look at it this way: You can't really control the oscillations of a car suspension after hitting a pothole via slamming an 'inverse pothole' into it. To some extent you can regulate like it was done on the Citroen DS21 (the big one) but at the end of the day the shock absorber, or a simulated one, does the lion's share. Ringdown is energy and the only way to get rid of some of it is dissipation.

Also, transducers don't like to be slammed into reverse, they have a limited bandwidth. For stuff such as PZT-5H that's usually around 30-40% at -6dB, not matter what the glossy marketing materials say. And it varies a bit from one transducer element to the next. PVDF is a lot wider but that doesn't have much efficiency to write home about.

Well, to an ultrasound guy keeping pulse pattern uniformity should come as natural as balancing a wheel comes to a mechanic. If it doesn't, it's best to get help.

Regards, Joerg