MPP Coil And Preamp Speed

[baum7154]

Hmmm, I'll try that when I get a chance, might be a couple days...

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I agree with Green on the diode as this technique is what the original CHANCE PI design did to try to keep saturation down. After looking at your scope trace posts for the amp outputs that flat section looks like the amps are going to rail voltage until they leave saturation and saturation is not our friend. Better horizontal resolution would also help in diagnosis, say 1us instead of 5us. Also a better description of each of the two traces would help to eliminate some guess work.

Just some thoughts.

Dan

[Old cart]

One other thing I just noticed, when making critical measurements involving the relative timing between two waveforms it is important that the trace rotation be set correctly. Easiest way to do this is to ground the input on both channels, position ch1 1 div from the top of the screen, ch2 1 div from the bottom of the screen, then carefully adjust trace rotation so that both traces run perfectly parallel to the graticule lines. Inexpensive scopes usually require this be adjusted any time the scope is rotated relative to the earth. Interestingly this is due to the earths magnetic field. Cheap scopes do not have mu metal shielding around the CRT to prevent this effect. Of course DSO's with LCDS are not affected. Sorry to be so anal about the measurements but you can not make good decisions using unknown data. 31 years of working for Tektronix did this to me! You should also check the probes, they should be carefully compensated and identical.

[Achillion]

The ADA4898 is available in both single and dual configuration. The opamps are pin compatible but you need one of these little adapters as the ADA4898 is only available in SMD (SOIC). They are also not cheap!

[green]

...and a fyi - for U5 I'm using the LM4562 instead of the NE5532

LM4562, 55Mhz, 20v/usec. Probably explains some of it.
Charted an exponential decay from 400volts with linear and log amplitude for an example. 300uH/1k=.3usecTC. The linear trace looks like it goes to zero at about 1.5usec. With a 5volt rail the coil volts has to decay 5/33=.15volts for amp#1, 5/1089=.0045volts for amp#2. Adds about 1usec for each stage with this example.

[KRinAZ]

LM4562, 55Mhz, 20v/usec. Probably explains some of it. Charted an exponential decay from 400volts with linear and log amplitude for an example. 300uH/1k=.3usecTC. The linear trace looks like it goes to zero at about 1.5usec. With a 5volt rail the coil volts has to decay 5/33=.15volts for amp#1, 5/1089=.0045volts for amp#2. Adds about 1usec for each stage with this example.

Thx, would the 5532 introduce less delay, or am I better off staying with the 4562? I'm under the impression the 4562 itself gives a bit more sensitivity - which is why I 'm using it.

[KRinAZ]

One other thing I just noticed, when making critical measurements involving the relative timing between two waveforms it is important that the trace rotation be set correctly. Easiest way to do this is to ground the input on both channels, position ch1 1 div from the top of the screen, ch2 1 div from the bottom of the screen, then carefully adjust trace rotation so that both traces run perfectly parallel to the graticule lines. Inexpensive scopes usually require this be adjusted any time the scope is rotated relative to the earth. Interestingly this is due to the earths magnetic field. Cheap scopes do not have mu metal shielding around the CRT to prevent this effect. Of course DSO's with LCDS are not affected. Sorry to be so anal about the measurements but you can not make good decisions using unknown data. 31 years of working for Tektronix did this to me! You should also check the probes, they should be carefully compensated and identical.

Thx for your suggestions, the scope only has one rotation control but the channels aren't perfectly parallel, & I had adjusted the probes for compensation. Personally, I would like to use my delay time base to view these signals at higher res but the delay module isn't working correctly...

[KRinAZ]

-----------------------------I agree with Green on the diode as this technique is what the original CHANCE PI design did to try to keep saturation down. After looking at your scope trace posts for the amp outputs that flat section looks like the amps are going to rail voltage until they leave saturation and saturation is not our friend. Better horizontal resolution would also help in diagnosis, say 1us instead of 5us. Also a better description of each of the two traces would help to eliminate some guess work.Just some thoughts.Dan

Thx Dan, I will try again to keep the signal on the screen at higher res. I tried a variety of triggers but at 2uS the signal was off the screen. I will also add as much description as I can think of. It may be a day or two or three before I get back to the scope - I'm primarily a prospector & I'm out at a claim... - Kyle

[green]

Thx, would the 5532 introduce less delay, or am I better off staying with the 4562? I'm under the impression the 4562 itself gives a bit more sensitivity - which is why I 'm using it.

The 4562 should be better.

[baum7154]

Thx Dan, I will try again to keep the signal on the screen at higher res. I tried a variety of triggers but at 2uS the signal was off the screen. I will also add as much description as I can think of. It may be a day or two or three before I get back to the scope - I'm primarily a prospector & I'm out at a claim... - Kyle

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Out at a claim, excellent... wish I were there too. Your claim wouldn't happen to be in the Superstition Mountains would it? Last week I had dinner with a fellow from down there who had a home near or in Gold Canyon. Sounded like a good place to go prospecting.

Big nuggets!

Dan

[baum7154]

Below is a repost of a 2 stage amp I worked up for the Barracuda. The purpose is to show the diodes used in the feedback loops of the 2 stages to keep the amps out of saturation. I think the same principles can be modified and applied to keeping the MPP amps from saturation. The number of 2nd stage series diodes could be increased from 3 to 6 in order to keep the 2nd stage output from reaching the 5volt rail by lowering gain on large signals exceeding about 4.2V.

Dan
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Attached is a schematic of the 2 stage pre-amp for the Barracuda that I have been working on. A quick run through of my thinking on this amp:

1. Two Non-Inverting amplifier stages based on the OP37 amp. The Non-Inverting amp matches the original design and the OP37 has better performance in virtually every respect compared to the original NE5534 amp. In addition the OP37 is available factory trimmed to 10uv which may be a benefit. That said the OP37 is a pin for pin substitute for the NE 5534 (with the exception of offset trim pin eight) so the original chip could be used as well.

2. First stage gain set at 28, Second stage set at 37. This pairing will give a gain of about 1036 to match/exceed the original design gain of 1000.

3. Single 'Trim' pot on the first stage should allow both stage offsets to be adjusted.

4. 1K interstage coupling resistor and 2ea 470pf in series (235pf effective) to ground create 677kHz 1st order LP filter allowing approximately 1.5us pulses or longer to pass.

5. 1st stage 38pf and 2nd stage 23pf capacitors in the feedback loops function with their respective feedback resistors to create approximately 677 kHz 1st order LP filters. These values take into consideration the added capacitance of the diodes in the same feedback loops.

6. Diodes in the feedback loops of both stages. The single 1N4148 diodes limit amplifier output of the positive signals to .7 volts. The double and triple diode strings limit amp out for the negative signals to -1.4v and -2.1v respectively, preventing stage saturation when large signals are present. -2.1 volt 2nd stage limiting was chosen to be just above the -1.4v operating level of the original/retained 2 diode limiter (D5 & D6). These limiters should allow desired small signals to be fully amplified while limiting the effects of large saturating signals.

7. Change of the original coil to 300uh from the original spec of 450uh is recommended for best small gold performance. Damping resistor would have to be set for critical damping with the coil change.

As I do not have a Barracuda detector I have not built this amplifier to see what other problems may need to be addressed and I welcome any one who builds it to provide feedback on it's performance. I hope this modification allows the Barracuda to operate at the short delay times necessary to detect small i.e. 1/2 gram gold, or better.


Best Regards,

Dan

Attached Images

• securedownload.jpg (7.4 KB, 105 views)

[Old cart]

Just trigger on the negative edge. You may have to adjust the trigger level up and down as many scopes do not have good trigger discrimination (between +and -). Delay time would be fine also but typically introduces additional jitter on the trace.

[Old cart]

Dan do you have a better quality copy of those mods. This one is hard to to read.

[baum7154]

Dan do you have a better quality copy of those mods. This one is hard to to read.

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Sorry Old Cart here it is

IMG_1625.jpg (976.2 KB, 107 views)

[Old cart]

Enhanced signal limiting

Thanks Dan. I also found this variation. See pg 11 figure 25. This allows further refinements in signal shaping.
http://www.analog.com/media/en/techn...80533AN106.pdf

[greylourie]

You could look at using a preamp with faster recovery. And maybe lowering each stage of preamp gain - just make up the difference at the integrator or after it. Less is more ?

Also look at power supply to preamp. bbsailor gave some useful tips in a cs6pi modification thread - change supply R's to lower value and maybe look at the caps.