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Damping adjustments
In this case 330 ohms should be pretty close.
I like to adjust the damping resistor by looking at both the output of the preamp AND the voltage across the coil at the same time. Some tips:
1 Make sure your scope probes can take the voltage of the fly back pulse which can be hundreds of volts. Most x10 probes can do this.
2. Ground both probes to the same point on the detector near the coil attachment point and use shortest leads possible.
3. Adjust the scope attenuators and offset/position controls so the waveforms fill the screen vertically as closely as possible.
4. Adjust the time base so once cycle of the transmit waveform fills the screen. Later you may want to zoom in and observe just the fly back pulse to tweak it the best.
5. Use bandwidth limiting. The lowest bandwidth is best as long as it is greater than 2MHz or so. Most hobbiest scopes have 20 MHZ bandwidth limiters and they should be left on most of the time for detector work.
6. Trigger on the start of the transmitter pulse using the -/+ slope and trig level controls. Also use normal triggering NOT auto.
Adjust trigger level so there is no jitter.
7. Do not move probes around or change setting at all unless the waveforms goes off the screen.
While looking at both waveforms adjust the damper r slowly until you see a smooth transition with no ringing and the narrowest ring back pulse at the same time. You will also notice the output , pin 6 , of the preamp at this same time interval will also become narrower and wider while you adjust dampening. You want the setting that produces the narrowest ring back (fly back) pulse and little or no overshoot And the narrowest preamp output. This will allow you decrease the sample delay to its minimum, but no less than 10-12 uS if you want fine gold. On photo 6 if the yellow waveform is the early sample pulse you can decrease the delay 4-5uS so the leading edge of the sample pulse coincides with the point where the preamp out just breaks from the lowest level. You can also shorten the sample pulses to 10-15 uS if you want maximum gold sensitivity.SMALL adjustments to the offset can also help eliminate the wrinkles you see in the preamp out and smooth the response of the detector.
These settings let me increased my depth on a small mans gold ring of 6 grams from 24 to 30 cm. so careful setting pays back with huge rewards.
One last thing, once you confirm these setting are good in the field. Then it is best to replace the adjustable pot with a fixed resistor or combination of resistors that is closest to the measure value of pot. Be careful to keeps the leads as short as possible. You have to remove the pot from the circuit before you measure it so be careful not to disturb the setting you have been so careful to make.
Good luck!
In this case 330 ohms should be pretty close.
I like to adjust the damping resistor by looking at both the output of the preamp AND the voltage across the coil at the same time. Some tips:
1 Make sure your scope probes can take the voltage of the fly back pulse which can be hundreds of volts. Most x10 probes can do this.
2. Ground both probes to the same point on the detector near the coil attachment point and use shortest leads possible.
3. Adjust the scope attenuators and offset/position controls so the waveforms fill the screen vertically as closely as possible.
4. Adjust the time base so once cycle of the transmit waveform fills the screen. Later you may want to zoom in and observe just the fly back pulse to tweak it the best.
5. Use bandwidth limiting. The lowest bandwidth is best as long as it is greater than 2MHz or so. Most hobbiest scopes have 20 MHZ bandwidth limiters and they should be left on most of the time for detector work.
6. Trigger on the start of the transmitter pulse using the -/+ slope and trig level controls. Also use normal triggering NOT auto.
Adjust trigger level so there is no jitter.
7. Do not move probes around or change setting at all unless the waveforms goes off the screen.
While looking at both waveforms adjust the damper r slowly until you see a smooth transition with no ringing and the narrowest ring back pulse at the same time. You will also notice the output , pin 6 , of the preamp at this same time interval will also become narrower and wider while you adjust dampening. You want the setting that produces the narrowest ring back (fly back) pulse and little or no overshoot And the narrowest preamp output. This will allow you decrease the sample delay to its minimum, but no less than 10-12 uS if you want fine gold. On photo 6 if the yellow waveform is the early sample pulse you can decrease the delay 4-5uS so the leading edge of the sample pulse coincides with the point where the preamp out just breaks from the lowest level. You can also shorten the sample pulses to 10-15 uS if you want maximum gold sensitivity.SMALL adjustments to the offset can also help eliminate the wrinkles you see in the preamp out and smooth the response of the detector.
These settings let me increased my depth on a small mans gold ring of 6 grams from 24 to 30 cm. so careful setting pays back with huge rewards.
One last thing, once you confirm these setting are good in the field. Then it is best to replace the adjustable pot with a fixed resistor or combination of resistors that is closest to the measure value of pot. Be careful to keeps the leads as short as possible. You have to remove the pot from the circuit before you measure it so be careful not to disturb the setting you have been so careful to make.
Good luck!
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