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Since the Whites TDI PI has come on the market and may not have the TX punch that the ML does, is it reasonable to make a signal amplifier that boost the TX signal after it leaves the box and before the coil so as to get additional depth, or are the problems more that the benefits of this idea? Wyndham
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Hi wyndham,
I would try the other way instead. Reducing the power on the coil and sampling as fast as possible. The dU/dt voltage change is much higher on early sampling time after switching the TX coil off. The produced target signals are much higher on early stages.
There is also no need for low Rdson MOSFET's. Low Rdson MOSFET's has a quite big chip die and has therefore more gate capacitance. It takes much time to charge (switching on) and to discharge (switching off) the gate capacitor. Using a very fast switching MOSFET with less gate capacitance will probably be better.
The second way is using of low noise amplifiers (e.g. AD797). As you have lower TX pulse power, you can compensate this with increased signal-to-noise ratio.
The third way is keeping the coil capacitance low and making fast switching possible.
Has anybody more tips?
Regards,
Aziz -
Not necessarily Aziz, take for instance the FQP4N90 fron ON semiconductors it has 3.3 Ohms RDSon, 860pF input capacitance but it switches on in 60ns and with a rise time of 70ns. Now take the IRF6636 from International Rectifiers with 3.3 mOhms RDSon, that's right 1000 times lower RDSon, it has an input capacitance of 2420pf but it switches on in 14ns and with a rise time of 19ns. So, might reconsider your above statement and take into account VDS as well.Originally posted by Aziz View Post
By the way with what sound card are you going to sample 19ns rise time?
RegardsComment
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You might want to take a look at this:Originally posted by wyndham View Post
http://www.avtechpulse.com/papers/classd/
I built it, it works and it is quite easy to put together.
RegardsComment
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Are you kidding?Originally posted by gwzd View Post
Of course, by increasing the gate-source voltage, the charge on the gate can be moved quicker to achieve higher switching time. Normally, you cannot change this control voltage too much. On some MOSFET H-Bridges, the upper stage of NMOS MOSFET's has its own switching voltage supply (charge pump).
May be, this could be reasonable, to make a pure switching voltage regulator.
It is more practical, to use fast switching MOSFET's taking some Drain-Source resistance into account instead. It should not too much, as you mentioned in the example above.
AzizComment
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About what? About the values I posted? no, dead serious, check yourself. I happen to use these devices in my work, that's how I know what I'm talking about, do you?Originally posted by Aziz View Post
It's not about voltage, it is about current ...Of course, by increasing the gate-source voltage, the charge on the gate can be moved quicker to achieve higher switching time.
As I said it's not about voltage, but even if it was why not? Most MOSFET drivers allow you to control their output. Heck, in a pinch you could use an LM317 to control the supply to the driver!Normally, you cannot change this control voltage too much.
who was talking about H-Bridges?On some MOSFET H-Bridges, the upper stage of NMOS MOSFET's has its own switching voltage supply (charge pump).
MOSFETS are happy when you treat them as switching devices, but again the OP wanted an amplifier and I suggested one.May be, this could be reasonable, to make a pure switching voltage regulator.
That's most of the time true, but not always, and as I showed to you in my previous post there are faster MOSFETs with very low RDSon.It is more practical, to use fast switching MOSFET's
I didn't quite understand your point here.taking some Drain-Source resistance into account instead. It should not too much, as you mentioned in the example above.
Aziz[/quote]Comment
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Hi Wyndham,
Enjoyed visiting your store today, and our brief chat.
Per the original question, with PI you can't simply amplify the transmit signal. It's not like audio, where you create a signal, amplify it, and send it out to a speaker. With PI, the coil is an integral part of creating the transmit signal. You pump a current through the coil, and then shut it off. This creates the transient magnetic field that induces eddy currents.
The transmit signal strength (transient magnetic field) depends on how much current is
flowing right at turn-off, and how fast you turn it off. Also the coil inductance. It is proportional to L*di/dt. Increasing L gets you into trouble fast with switching speed, so most people either try to pump more current, or switch off faster. BBSailor's work on fast coil designs will help with the latter. Also faster MOSFETs as mentioned above.
Of course, increasing TX strength is an exercise in futility, because it takes a whole lotta more signal strength to get a whole tiny more depth. A lot of work has also been posted on better (lower noise) preamp stages and other tricks to increase RX sensitivity. Also faster sampling techniques to get smaller targets.
- CarlComment
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Carl, thanks for dropping by. Enjoyed the visit also, hope you had a chance to get to Salisbury. Please let me know if it worth a little run away time.
After our visit I went over to the Reed Gold mine, it was closed on Mondays but an old farmer let me try my Gold Bug out for a few mins. It was over 100 deg so no reason to stay out and fry.
Thanks again WyndhamComment
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