chance pi build

[baum7154]

That is very interesting! My stated damping resistor values are measured at the coil jack with the coil disconnected so I don't see how I am at about 600 ohms. Is it because of the opamp inverting input being at VIRTUAL ground?

[green]

That is very interesting! My stated damping resistor values are measured at the coil jack with the coil disconnected so I don't see how I am at about 600 ohms. Is it because of the opamp inverting input being at VIRTUAL ground?

I think so.

[bbsailor]

What a pity the pictures have gone from that thread.

If anyone is interested in knowing a little more about this, look up on the web "lock-in amplifier". This is a piece of test equipment used to extract very weak signals in the presence of noise. By sampling and averaging by using an integration stage, the noise which is broadband and random can be minimized by always extracting the sample from the same location in a high frequency pulse train. This is sort of like what PI detectors do when they integrate many samples while over a target. This means that the RX sensitivity is based on the coil size, sweep speed and the time that a potential target is within the the TX/RX field. In Eric's 5 us PI design he can integrate about 8,000 to 10,000 samples while the target is under the coil. The short TX pulse also make the for less response from the wet salt that is typically near a 10 uS delay.

What depth he does not get from a higher TX power pulse, he makes up for by allowing the RX circuit to receive very tiny signals from very tiny gold targets. You can even see how Carl's Hammerhead design uses an integration stage to do something similar but the Hammerhead would need to be modified to use a higher PPS rate for this to be more effective.

bbsailor

[kt315]

last Eric's designs were with high PPS 8,000-10,000 and with a coil inductivity 1mH. he did see something that we are not able.

315

[bbsailor]

[The present 2 stage amp modification uses 2 OP37 amps with the first stage inverting amp set at a gain of 18 and the second stage non-inverting amp set to a gain of 28. Positive voltage is limited in the first stage to 1.4 volts by placing two diodes in series across the feedback resistor to be in forward conductance on the positive going voltage. The original negative voltage limiter diode is also used in the feedback loop of the first amp keeping the negative to 0.7 volt . These measures keep the first amp out of saturation and the low gain keeps amplification of noise down]

Some thoughts, maybe correct maybe not. With the two stage amp the bandwidth might be high enough that the input resistor acts as a damping resistor. The damping resistor and input resistor are in parallel giving you a damping resistance closer to 600 ohms. I doesn't matter in the circuit just the thinking on how it works. The circuit I've been playing with uses the input resistor only, allowing a lower value input resistor (less resistor noise).

Anyone wanting more information about opamps, check this web link out. http://waltjung.org/index.html
bbsailor

[baum7154]

[The present 2 stage amp modification uses 2 OP37 amps with the first stage inverting amp set at a gain of 18 and the second stage non-inverting amp set to a gain of 28. Positive voltage is limited in the first stage to 1.4 volts by placing two diodes in series across the feedback resistor to be in forward conductance on the positive going voltage. The original negative voltage limiter diode is also used in the feedback loop of the first amp keeping the negative to 0.7 volt . These measures keep the first amp out of saturation and the low gain keeps amplification of noise down]

Some thoughts, maybe correct maybe not. With the two stage amp the bandwidth might be high enough that the input resistor acts as a damping resistor. The damping resistor and input resistor are in parallel giving you a damping resistance closer to 600 ohms. I doesn't matter in the circuit just the thinking on how it works. The circuit I've been playing with uses the input resistor only, allowing a lower value input resistor (less resistor noise).

What wattage receive input resistor are you using? Does it get warm when operating for a long time indicating it is dissipating coil energy?

I have a working spare CHANCE PI board equipped with the 390 ohm resistor and when I ohm it it reads 390 ohms to board ground. So I cut the resistor out and ohmed it again and got 440 megohms using the ohmmeter leads in both polarities on all my measurements. I don't think I am seeing the Rcv amp input 1k resistor across the coil at all. I believe the OP37 is a FET input op amp so there is a lot of input resistance. Probably what is happening is the negative going diode of the back to back 1N4148 pair is conducting down to .7 volt during decay damping and then shutting off below that voltage. Does your scope trace show diode cutoff at 0.7 volt during damping?

Dan

[green]

What wattage receive input resistor are you using? Does it get warm when operating for a long time indicating it is dissipating coil energy?

I have a working spare CHANCE PI board equipped with the 390 ohm resistor and when I ohm it it reads 390 ohms to board ground. So I cut the resistor out and ohmed it again and got 440 megohms using the ohmmeter leads in both polarities on all my measurements. I don't think I am seeing the Rcv amp input 1k resistor across the coil at all. I believe the OP37 is a FET input op amp so there is a lot of input resistance. Probably what is happening is the negative going diode of the back to back 1N4148 pair is conducting down to .7 volt during decay damping and then shutting off below that voltage. Does your scope trace show diode cutoff at 0.7 volt during damping?

Dan

I think the circuit is similar to yours except N channel instead of P channel, so the voltages and feedback diode are reversed. I made R damp a little higher resistance than R input so the currents aren't the same. If R input went to the + input the current would stop when coil volts dropped below .7 volts. The output of the op amp tries to keep the + and - inputs equal with feedback. When the output can supply enough current, the - input will go to zero since the + input is at zero and the output will go to .7 volts due to the diode across the amp. The R input continues to draw current so the R damp and R input are in parallel. The spice circuit works if you remove R damp and change R input to a value equal to the parallel resistance. I've been using two 500 ohm 1/4 watt resistors in series for R input with no R damp for bench testing. My fet clamps at 450 volts instead of 200 volts. The on time effects the heating but I haven't burned any up yet.

[baum7154]

Very good Green. Have you tried just using about a 500 ohm resistor at R in or does that not give critical damping for the coil? I guess a better question would be what resistance is needed for critical damping of your coil In circuit?

[green]

Very good Green. Have you tried just using about a 500 ohm resistor at R in or does that not give critical damping for the coil? I guess a better question would be what resistance is needed for critical damping of your coil In circuit?

Most of my coils critical damp with 1000 ohms or higher. I presented some data in another thread a week or two ago with and without the series coil diode. Had to parallel the 1k with 400 ohms to critical damp without the diode.

[baum7154]

Yeah the low capacitance of that diode in series with the mosfet really brings the mosfet capacitance under control. So if you are running 1k as your input resistor with no damping resistor then how do you realize the low noise benefit of lower input resistance?

[green]

Yeah the low capacitance of that diode in series with the mosfet really brings the mosfet capacitance under control. So if you are running 1k as your input resistor with no damping resistor then how do you realize the low noise benefit of lower input resistance?

If the circuit critical damps with 1000 ohms and R damp and R input are in parallel I could make each 2000 ohms or make R input 1000 ohms with no R damp. Lower input resistor less resistor noise. Makes sense to me but I'm no expert so maybe it doesn't matter. Maybe it would be better to use the two 2k resistors. Maybe someone that knows could explain which is better. I think the main thing is R damp and R input are in parallel when connecting to the inverting input. Using the inverting input does shorten the delay time to sample. Don't know if there are any disadvantages.

[baum7154]

Maybe I'll pursue this later as I cannot sample earlier than 8us with the current version of CHANCE software.
My 8" coil critical damping calculates to 1111 ohms with the feed and 1547 ohms without the feed.

Dan

[Tepco]

You can simply trick it without software change using some “overclocking”. Use XTAL of somewhat higher frequency, 10-20% or even more, this will proportionally shift audio, filter constants and pulse width as well, not only sample time, but works just fine, I tried.

[green]

Maybe I'll pursue this later as I cannot sample earlier than 8us with the current version of CHANCE software.
My 8" coil critical damping calculates to 1111 ohms with the feed and 1547 ohms without the feed.

Dan

What is circuit resonance if you lay your scope probe near the fet (not connected) with R damping and the input resistor disconnected?

[baum7154]

You can simply trick it without software change using some “overclocking”. Use XTAL of somewhat higher frequency, 10-20% or even more, this will proportionally shift audio, filter constants and pulse width as well, not only sample time, but works just fine, I tried.

Excellent idea! I'll have to look into that. Does it shift he audio even higher? Silver is especially high in frequency already and I'd actually like to lower the audio frequency for it. In any event it is a good workaround. So a crystal in the range of 12.5mhz would be good to try.

Thanks,

Dan