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Anonymous
Guest
Eric, I forgot to put a sensitivity control on my detector!!! Would you recommend varying the gain of the coil amplifier or varying the gain somewhere else? Thanks, Dave. * * *
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A
Anonymous
Guest
Dave,
Long time since I've heard from you! How did your trip to the UK with your detector go? I sure hope I get to try it out for myself in the Goldfields of the Mojave Desert as I know you've come up with some novel circuits which is sorely needed in this hobby. We need some fresh ideas as far as nuggetshooting goes, most have been found by metal detectorists in the past 20 or so years going over the same ground! Take care and good luck!
Randall Seden
Long time since I've heard from you! How did your trip to the UK with your detector go? I sure hope I get to try it out for myself in the Goldfields of the Mojave Desert as I know you've come up with some novel circuits which is sorely needed in this hobby. We need some fresh ideas as far as nuggetshooting goes, most have been found by metal detectorists in the past 20 or so years going over the same ground! Take care and good luck!
Randall Seden
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Anonymous
Guest
Hi Dave,
Thanks for the circuit you sent me last week. On the towtec metal detector designed by Eric, he varies the gain in a later stage (after filtering) not in the first stage.
Good luck,
Charles
Thanks for the circuit you sent me last week. On the towtec metal detector designed by Eric, he varies the gain in a later stage (after filtering) not in the first stage.
Good luck,
Charles
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Anonymous
Guest
Randy, We had fun in the UK. I never got to take the PI though. My best find there was a first century bronze Roman signet ring with a picture of a god on it. The ring is in good enough condition to wear! As to the new circuit ideas, we will have to wait until the GURU (Eric) checks the design for me to see if any of it is any good.
As to the nuggetshooters, sure they found some of the easy ones but they have only found a very small percentage of what is out there, Dave. * * *
As to the nuggetshooters, sure they found some of the easy ones but they have only found a very small percentage of what is out there, Dave. * * *
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Anonymous
Guest
Dave,
I would leave the preamp at max gain since any reason for reducing sensitivity (electrical interference, for example) won't show up here anyway. I don't think you will see any potential signal problems until after the integration. I guess you could vary a threshold level against which the integrator output is compared against, or use a post-integrator amp with variable gain. I have the extra amp.
- Carl
I would leave the preamp at max gain since any reason for reducing sensitivity (electrical interference, for example) won't show up here anyway. I don't think you will see any potential signal problems until after the integration. I guess you could vary a threshold level against which the integrator output is compared against, or use a post-integrator amp with variable gain. I have the extra amp.
- Carl
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Anonymous
Guest
N/T
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Anonymous
Guest
Hi Dave and all,
Historically the gain control in a PI has always been in the dc amplifier following the integrator. The reason for this was that the first IC front end was the venerable 709 (even till recently). Gain control was difficult because of the external frequency compensation and the likelihood of oscillation. For the majority of applications in a PI, having the gain control later in the circuit is no disadvantage, and this is what I would do in your first circuit. However, in future discriminating PI
Historically the gain control in a PI has always been in the dc amplifier following the integrator. The reason for this was that the first IC front end was the venerable 709 (even till recently). Gain control was difficult because of the external frequency compensation and the likelihood of oscillation. For the majority of applications in a PI, having the gain control later in the circuit is no disadvantage, and this is what I would do in your first circuit. However, in future discriminating PI
A
Anonymous
Guest
Eric, Thanks for the input. Placing the sensitivity control after the demodulators will make this first detector ready for use. With that said I am curious about some of the results I have had while bench testing. I have noticed some strange things going on with the coil amplifier. My amplifier is non inverting. The coil inputs to the + input of the op-amp through a resistor and back to back 1N4148 diodes. The coil is being pulsed with 100uS pulses and the receive waveform originaly took 13uS to decay with no target. I found that when I used two diodes in series in each direction instead of one diode each way that the receive waveform decayed in 10uS. Changing the value of the input protection resistor from 1K to 3.3K further reduced the decay to 9uS. My first thoughts were that I was halving the diodes capacitance but I am not sure if this is the reason? I tried adding a potentiometer in series with the - input resistor which goes to ground. The idea was to vary the gain. OK, the gain varied and the detector still worked fine without any oscillations. What was weird was that the received signals exponential decay varied with the gain setting? My RF background tells me to control the gain up front. An Automatic gain control or AGC as used in radio's would also seem to be a good idea. I will have to sit down and figure out what is going on with the coil amp. All this goes to show one that getting a PI design truly optimized is far from being a trivial task. Most engineers could design a PI which will work after a fashion. However, I believe that there are very few people who could really design a PI which approaches optimum unless they invest a tremendous amount of time and effort. This is why it is so great for us all to have you to help us as we stumble along in your footsteps. All the best, Dave. * * *
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Anonymous
Guest
Hi Dave,
One point to bear in mind is that the input resistor, even on the +input, acts as an additional resistor in parallel to the damping resistor. On switch off, the coil voltage rises to a few hundred volts causing one of the diodes to conduct heavily until the volts have fallen to below the conduction voltage. Only at this point is the input resistor effectively disconnected and the real damping resistor is on its own. Increasing the value from 1K to 3.3K reduces the overall damping and the switch-off will be faster. Another point to keep in mind is the peak diode current. Using a 1K, this would be 500mA if the coil voltage rises to 500V. The 1N4148
One point to bear in mind is that the input resistor, even on the +input, acts as an additional resistor in parallel to the damping resistor. On switch off, the coil voltage rises to a few hundred volts causing one of the diodes to conduct heavily until the volts have fallen to below the conduction voltage. Only at this point is the input resistor effectively disconnected and the real damping resistor is on its own. Increasing the value from 1K to 3.3K reduces the overall damping and the switch-off will be faster. Another point to keep in mind is the peak diode current. Using a 1K, this would be 500mA if the coil voltage rises to 500V. The 1N4148
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Anonymous
Guest
Eric,
Thanks for the explanation. I am going to try to find some high voltage, low capacitance, high speed diodes to replace the 1N4148's with. As to the gain control, I ended up placing it at the output of the sample and hold difference amplifier. This circuit follows the coil amplifier and has a voltage gain of only 2. I am going to be making the final adjustments to the circuitry tomorrow. Dave. * * *
Thanks for the explanation. I am going to try to find some high voltage, low capacitance, high speed diodes to replace the 1N4148's with. As to the gain control, I ended up placing it at the output of the sample and hold difference amplifier. This circuit follows the coil amplifier and has a voltage gain of only 2. I am going to be making the final adjustments to the circuitry tomorrow. Dave. * * *