PI output & rx power?

[wyndham]

Tried a search for this but must not have worded it properly.
When the PI transmits a pulse, what is the power that is going to the coil? and conversely what signal strength that comes back on the rx. I'm still trying to visualize the power going in & back. I realize the rx is weak.
I would think the signal is generated then amplified at the last stage before tx, right/wrong?
Thanks Wyndham
PS I'll have this down pat in a 100 yrs or so.

 

[Reg]

Hi Wyndham,

Sorry, but your questions do not have a pat answer for any of them. As an example, take the transmit power as an example. Now, this is a function of the coil resistance and inductance, the applied voltage, the time the pulse is on and any any other resistances in the transmit loop. This includes any series resistor and the FET resistance, just to name a couple.

As far as the receive signal strength, this is a function of the transmit power, the distance any target is from the coil, the type of coil used, plus several other things.

Now, in a nutshell, the receive signal is extremely small and need some serious amplification before it can even be "seen". As an example, even with an amplification of 500, it is almost impossible to see the "ground signal", yet it can cause some serious side effects.

Now, probably the best way to get a better handle on what is going on is to jump in and build a PI unit. I would recommend the Hammerhead project by Carl Moreland. This is a very good basic unit that has clear instructions on about every aspect. I am not sure if you have been to his website, so here is a link to his main home page.

http://www.thunting.com/cgi-bin/geotech/pages/common/index.pl?page=main&file=main.dat

This project along with some decent test equipment and you will be able to see just what I mean. The "hands on" technique is really the best way to learn.

Reg

 

[wyndham]

Thanks Reg, What I meant by the question is a range that the TX works in. Example: If the signal produced from the pre amped PI is , say, 2v and after the amp in 300v, I would understand the ratio of pre and post amplification. If its.03v up to 4v is would see the relationship even though there is a range to consider.
Since you mentioned the RX boost x 500(example) what is the signal strength coming in .01,.001.000?1 volts?
The idea I have about the computer and the dsp card is the reason to get a baseline for the signal in & out strength because I have no idea what the signal/voltage range is. I hope this make sense. Wyndham

 

[Reg]

Hi Wyndham,

Here is a previous post by Eric that might provide some of the info you are looking for.
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Re: Tech stuff
Posted by: Eric Foster <Send a PM> (IP Logged)
Date: December 26, 2003 03:26PM


Hi Reg and Chris,
With my ultra sensitive PI, 30mV at the integrator output sends the meter on the dc amp full scale. To get a signal I can see on the front end amplifier, I increased the signal from the target to give 1V at the integrator output. This gives me a deflection of 20mV at the amplifier output. Divide by the gain and you get 78uV from the coil. The signal is decaying fast during the sample period, so this is approximate. For the full scale meter deflection, where there is only 30mV at the integrator, the coil voltage works out to 2.3uV. For 10% deflection, which is a repeatable, not to be missed signal, the target voltage from the coil is 0.23uV.
Hope I have done the sums right, but you can see the order of signals we are dealing with.
Eric.;

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I am sure you can find more info by expanding your search, but the numbers in the uvolt range are reasonable for the receive signal at the coil. So, you will l have to have a gain of a 1000 just to get millivolt signals, or a gain of a million to get something even close to the volt range. Now, if you are trying for serious sensitivity, then a gain of a million might get you .23V or possibly less.

Reg

 

[wyndham]

Thanks, now I can "see" what's going on.
Unfortunately, I'm one who's brain is locked into "seeing" a concept before it sinks into the gray matter.
This also tells me that the signal input and output from a "DSP" card or sound card would be adequate for the testing job.
BTW I have seen several "Basic compilers" vs "interprets" that may compile into very tight code. I just have to see what calls are implemented for those ports or libraries that are available.
If I can write a simple pgrm to TX & RX, even very slowly, then I can put it into a faster working model
Thanks for the info and the time, Wyndham