Homemade Detector

[Smitty II]

Greetings,

While out using the home made PI detector that I posted a series of posts on I met Ken, another treasure hunter. He is also an engineer and we spent some time talking detectors. During the conversation he said now that you and built the one you have, if you built another one, would you do anything different. I told him, yes; I would add another gain stage and lower the gain in the other stages. He asked me why? I told him that even though the detector works great as it is and I have not experienced any instability, it has very high gain in the last two stages. The higher the gain is in any given stage the higher the probability is for that stage to become unstable under certain conditions. It is possible to build stable super high gain stages with proper compensation but they almost always require a PC board.

For any of you experimenters out there that are interested, I have attached the schematic of what I would do if I did it again. The total gain is the same as the one I am using now but the gain in the individual stages is reduced and there is less chance for things to become unstable.

God Bless and HH!!

Smitty II

 

[Sven]

Hi,
So are you going to build this version?
Inquiring minds want to know......

Thinking about giving it a shot.
After I figure out Eagle etc to get a board designed.

 

[Carl-NC]

The final gain stages are usually set up as integrators so high gain stability is not a problem. You would do better to split the preamp into 2 gain stages, that will make it respond faster and allow you to lower the sample delay.

 

[Sven]

Smitty can you do a revision of your schematic incorporating this?

 

[Smitty II]

Carl-NC

You are absolutely right and that is why I attached the schematic to my post. The preamp gain is about 200 and then I added an additional gain stage to amplify only the sampled signals, with a gain of 3.3 before the 1st integrator. This way, instead of amplifying the whole preamp output signal I am amplifying only the area of the wave form that I am interested in. You are also right in that high gain integrator stages are not likely to become unstable and start oscillating at high frequencies but under the right conditions they can oscillate at low frequencies and start motor boating. Also the higher the gain in the integrators the more drift there will be with temperature, power and environmental changes. Keeping the gain in the integrators reasonable is also helpful in keeping threshold of the PI operating smoothly.

Thanks for your input Carl, they are good ones and that is basically what I am doing, When I had the gain of the preamp at about 1,000, I could only sample in to about 18 us. With the preamp gain at around 200, I am now sampling in at 10us. It is now a lot more sensitive to small gold targets than it was with the preamp gain at 1,000. With an 8" coil, I can detect a 1.5 Grain flake of gold at about 1 inch in air.

Thanks again for your inputs.

God Bless and HH

Smitty II

 

[Smitty II]

Sven,

Actually the schematic that is attached to my post does what Carl was recommending. I think it does it in a little different way than he was thinking but I think the results are the same. It looks like our minds were running along the same track.

God Bless and HH!!

Smitty II

 

[Smitty II]

Sven,

Not right now anyway. The original circuit is working very well and I want to get some more experience with it in case I come up with another tweak or two.

God Bless and HH!!

Smitty II

 

[6666]

In one of Carls articles he talks about splitting the typical 5534 pre amp into two low gain stages (uses a max chip)
with a gain of about 33 each. 33k/1k =gain of 33

like this

 

[mikebg]

6666, please explain why the C24 has so large capacitance?

 

[6666]

Hi Mike
I have no idea, this is a carl design, I just just cut and pasted it

 

[Smitty II]

6666,

Carl's circuit is a good approach. I am assuming that the input resistor to the 1st amp is a 1K which would give the first amp a gain of 33. The second stage also has a gain of 33 so that makes the gain through both stages a total of 1,089. My circuit does a similar thing but I didn't split it up in the same ratio. My first stage has a gain of 212 and my second stage has a gain of 3.3 for a total gain of 699. I make up any remaining gain needed in the integrator stages. I like taking the target sample and second sample in between the two stages. It allows close in sampling and then I amplify only the samples in the second stage. I think Carl's circuit would sample in closer if he took the target and second sample between his two stages like I did.

I like kicking around ideas like this. It is interesting to see different approaches.

God Bless and HH!!

Smitty II

 

[Smitty II]

Mikebg,

I think I can answer your question. The large caps are to filter out any noise that might be on the power supplies that are used with the trimmer adjustment to set the zero output level on the second amplifer in Carl's circuit. They help keep any noise from getting on the target signal.

God Bless!!

Smitty II

 

[6666]

Added a bit more of the cct to see value

 

[mikebg]

I think I can answer your question. The large caps are to filter out any noise that might be on the power supplies that are used with the trimmer adjustment to set the zero output level on the second amplifer in Carl's circuit. They help keep any noise from getting on the target signal.

Smitty II

Smitty, my question is only for capacitor C24 connected in feedback of the second stage parallel to 33k resistor. At capacitance 0.47uF (shown in the circuit), it forms a timeconstant 15 ms or cutoff frequency 10Hz. However Carl showed that we can build an amplifier with timeconstant 3.3us or cutoff frequency 48 kHz.
http://www.geotech1.com/forums/showthread.php?t=18049

 

[Carl-NC]

You will gain more sampling speed by reducing the preamp gain, 212 is a bit high. That is the purpose of 2 low gain stages. With the gain split as 33-33 (or 26-26 if you choose) there should be no advantage in moving the second stage after the samplers.

The C24=0.47u in my schematic is probably a copy-paste remnant. You would normally use 100-1000pF.

- Carl

 

[Smitty II]

Carl,

You are right, the gain of 212 could be lower and I could sample in even closer. I reduces my Preamp gain to the point where I could sample at 10us from the end of the coil pulse and get good target data, see attached scope picture and target samples. I stopped there simply because I met my objective for the project. The detector will detect my 1.5 grain gold flake test target at a bit over an inch and a half in air with my 8" coil and it will detect my 3 grain test target at over 3" with the same coil. That was the objective I had for this PI detector.

I guess my thinking must be all wet but it seems to me that other than a bit faster recovery speed, with a total gain of 1,000 in the preamp, and taking a target sample at the output of the preamp, an input signal to the preamp of .005V should drive the output of the preamp into the + and - 5V rails whether it has 1 stage or 2 stages to get the gain to 1,000. That means that you can not sample in any closer than at the .005V level on the decay curve. It just seems to me that if you sample after the first low gain stage you can sample in further on the decay curve because the curve is not amplified as much, then amplify the target sample to where it is usable. Right or wrong, that is my thinking. When I had my preamp gain at 1,000 I could not sample in any closer that about 18us. I could not detect the 1.5 grain gold flake test sample at all and only detect the 3 grain when I was almost touching the coil. When I reduced the gain to 212, I was able to sample in at 10us very nicely and could detect both the 1.5 grain gold sample and the 3 grain one as indicated.

Right now, if I ever build another PI I would probably lower the preamp gain to 150, take my samples at the output and then amplify the samples about 6+ times and then pass them on to an integrator. I should be able to sample into around 8us or less that way and still have an amplification of 1,000 going into the integrator.

God Bless!!

Smitty II

 

[Smitty II]

Sorry about that, I was looking at the wrong caps. I am not sure why the value is so high. I would have not used one that large.

God Bless!!

Smitty II

 

[Smitty II]

Carl,

I didn't mention this in my other post but I agree with you, the speed of the preamp increases when you lower the gain. Since an op-amps max bandwidth is at unity gain, the higher the gain the slower the op-amps response time is. With lower gain in the preamp you get two benefits, faster speed ( helps sample in closer) and lower amplification so you can sample in further before you reach a decay voltage that is high enough to drive the preamp into its supply voltage rails. I would guess that the lower gain limit would be at the point where you don't have enough signal to noise ratio.

Just thinking, thanks for your thoughts.

God Bless!!

Smitty II

 

[Smitty II]

Carl,

I have been thinking more about your split amplifier approach to taking the target sample in closer to the end of the coil pulse. Correct me if my thinking is wrong. If I use a single amplifier with a gain of 1,000 and measure it rise time, then using the same amplifier and change the gain to 33 and measure its rise time. If I put 2 of those amps with a gain of 33 in series so I have basically the same gain at the output, the rise time at the output should be the square root of the sum of the square of the individual rise times. That would tell me how much actual speed improvement I am getting over the single amplifier.

A spice simulation should show that, your feeling that I don't need to sample where I am sampling is a good point. I think I can make a comparison between the two approaches.

Thanks for your input.

God Bless!!

Smitty II

 

[Carl-NC]

Yup, that's about right.