Positive Vs Negative TX Waveform

[B^C]

Could somebody explain the advantages & disadvantages of having a positive going vs a negative going TX waveform.

[Tinkerer]

Could somebody explain the advantages & disadvantages of having a positive going vs a negative going TX waveform.

Hi B^C,

for a negative going TX pulse we use N type Mosfets. There is a wide choice available and they are easy to find.

For a positive going TX it takes a P type Mosfet. They are often hard to find and the choice is much restricted. There are some of the ultimate generation now that are nearly as good as the N type.

Tinkerer

[Carl-NC]

If we're talking about the flyback on the TX coil, the NMOS causes a positive flyback. I think most people are using NMOS.

- Carl

[B^C]

Thanks guys,

Good guess Carl:
It is for flyback reasons i asked the question, i read an article some time ago where it was stated that the flyback is not required to stimulate the target, when i test, the flyback is the only way to stimulate the target at depth.
Maybe they had the polarity wrong on there setup for flyback stimulation?

I have read before also that some believe the flyback to be just an unwanted side effect of the circuit--i don't see this at all or maybe there is another reason they state this, early sampling problems maybe.

Maybe i am looking at this all wrong.

[Carl-NC]

Yes, this has been "discussed" before! It is the abrupt turn-off of the coil current that produces the fast-changing magnetic field we need to stimulate the target. The flyback is a side effect of this, yes, but I wouldn't call it unwanted. But it does get in the way of super-early sampling, if that is what you want to do.

- Carl

[B^C]

Gday Carl,

I wasn't trying to revisit what was discussed before but more trying to see how the flyback can be advantageous to us, some i talk to think it's an issue with early sampling but maybe not so.

If we look at the Inductor analogy regarding a flywheel where on turn off it wants to keep spinning like the Current in an inductor wants to keep constant, by applying a brake to the flywheel we can progressively slow it down or speed the process up by applying the brakes harder, this works the same for the Inductor.

In this case the Transient spike Voltage is the braking on the Current, so the Voltage causes a progressive reduction in Current. The higher the Transient Voltage spike the faster we can get the current & the spike back to 0.
In this senario the flyback is very important indeed, if the Duty Cycle of the signal & the Current determines the flyback or flyforward as some say & the Transient spike brakes the Current so it drops to zero quickly surely we can use the flyback to our advantage. The higher the Transient spike voltage the steeper the current will fall & the spike will return to 0 faster.

The Transient spike voltage will take on any value to keep current continuity so i believe we can use this to our advantage for very early sampling.

It would be interesting to hear some thought's.

[wyndham]

B^C
In your analogy to braking, the energy needed to stop the forward momentum of the mass is equal to the energy of the mass in motion, correct, or close to it. The braking energy also produces heat which has to be dissipated.
Does the flyback produce noise(heat) or a TX signal that we use for the target?
Wyndham