PI noise

[green]

Any comment on the timing? I've posting things in the past that made sense at the time but looking at them latter I had to think for awhile to understand them. I could try to draw the timing if the scope pictures aren't clear or. Timing sequence: coil on 160usec, target delay 5.7usec,(sw#1) target sample 9.6usec, GEB delay 5.4usec, (sw#2)GEB sample 100usec, EF delay 604.3usec(1kHz sample rate changes with sample rate), (sw#2)EF sample 9.6usec, EF delay 5.4usec, (sw#1)EF sample 100usec. Repeat. (1C integrator, timing that I was using at the time, probably not optimum.) sw#1 preamp out normal, sw#2 inverted out

Some test data. Recorded integrator out change for different targets(target distance, 2 inches for the nuggets and 4 inches for the coins) with and without GEB. Target sample time was adjusted for no change in integrator out when a quart zip lock bag about 6x6x1.5 inches of California ground was placed flat on one of the figure 8 Rx coils. Post amplifier out was monitored with a scope, targets were lowered on a pendulum until a definite change could be seen in the noise trace. Integrator out mv and detection distance for each target with and without GEB was charted. Preamp feedback TC=.2usec. The nuggets might chart a little different with a different feedback TC. The US nickel looses some distance with GEB. US quarter shows an increase in signal(opposite polarity) with GEB but the noise is higher so the detection distance stayed the same. Not a big difference in detection distance except for the nuggets with the change in target delay time. Another test could be leaving the target sample at 10usec and adjusting GEB sample to balance ground or repeating above test at different GEB delay times. After looking at the test results don't think it would get much if any better. The shorter target delay time is needed for the 4 grain nugget and is a little better for the coin targets.

[green]

Can you draw a timing diagram that starts with the TX pulse (coil on) and then all sample pulses, delays and widths from that point. Just a typical setup will do. I want to see the relationship of each part of the cycle along to the next TX pulse, particularly how far out your GEB sample is from the 'off' edge of the TX.

Eric.

Any comment on the timing? I've posting things in the past that made sense at the time but looking at them latter I had to think for awhile to understand them. I could try to draw the timing if the scope pictures aren't clear or. Timing sequence: coil on 160usec, target delay 5.7usec,(sw#1) target sample 9.6usec, GEB delay 5.4usec, (sw#2)GEB sample 100usec, EF delay 604.3usec(1kHz sample rate changes with sample rate), (sw#2)EF sample 9.6usec, EF delay 5.4usec, (sw#1)EF sample 100usec. Repeat. (1C integrator, timing that I was using at the time, probably not optimum.) sw#1 preamp out normal, sw#2 inverted out

[BASEL]

see this video maybe can help you to figure where is noise come from

https://youtu.be/BFLZm4LbzQU

[Orbit]

Hi Eric !
Which logically brings less noise integrator ?

[green]

Lately I've been using an integrator and two post amp stages with a .01usec feed back TC each stage followed by a negative absolute value and a 2.3Hz 5pole butter worth filter. Recorded some scope pictures with the integrator and post amp feedback TC changed to .033usec without the abs. value and 5 pole filter. R7 and C4 are the feedback RC for the integrator, 10k input resistors in series with input switches(1C integrator). A photo cell was mounted 8 inches above the coil, a 200mm diameter disk was glued to the bottom of the water bottle swinging from a pendulum, US nickel stuck to the disk. 200mm/.2second=1meter/second. The nickel swings across one end or from end to end across both. Bottle was pulled back about 18 inches and let go then caught on the rebound, crosses coil twice, I think I like the circuit without the absolute value and 5 pole better.

reply#91

Some more scope pictures with 2N3906 matched pair. Integrator and post amplifier same as above. Nickel swinging end to end across both coils(Rx two 8 inch round coils figure 8, Tx oval surrounding Rx). Photo cell to show when target crosses coil. Target and EF sample, no ground sample.

[green]

[Tried the switch both places with the 1C integrator and didn't see a difference in the noise but wondered if I was missing something.]

I didn't notice a difference before. Yesterday I disconnected the leads from preamp out to the switch inputs and connected them to common to test integrator noise. Noticed if I touched common or either of the power supply pins I had a small change in output. Rewired the circuit with the switches to integrator -input, resistors to preamp. That problem fixed, don't think it changed the operating noise level. Ordered a different amplifier and switches to try to reduce noise level.

Using a dual transistor matched pair for the preamp input. Tried four different transistor pairs. I had tried the BCM 857 vs the 2N3906 in LT spice, the 857 was better. Wondering if I might be doing something wrong. Scope pictures of post amplifier out, preamp input shorted, target and EF sample(no ground sample). Recorded 5 scans with each transistor pair. Posting the lowest p-p(top row) and the highest p-p(bottom row) of the 5 scans for each pair. I had tried a different npn pair vs the 2n3904 awhile back that didn't test as good, don't have the data. Haven't tried the new switches yet. The test pendulum_9 reply #51 shows need S/N increase of about 3 times to gain a detection distance of 2 inches. Not much difference with 2N3904, 2N3906 and SSM2220 with the SSM being best.

[green]

The diff amp has less noise than the resistor input inverting input preamp for me. Requires a separate Rx, Tx IB coil. I had made a DOD coil awhile back. Connected it to the detector, the noise looked higher than I remembered the figure 8 Rx oval Tx looking. Ran a noise test comparing four different coils. Each connected to the diff amp, all circuits working except Tx command off. GEB switched off. Recorded 5 scans for each coil. Posting the lowest and highest p-p noise scan for each coil. Including some information I posted awhile back for the coils + the noise data from today. The 8 inch o0o is what I called figure 8 Rx oval Tx when I made the chart.

[green]

...going back to this post ... where you have a picture of the circuit boards built on veroboard (?) ... the main problem you have with noise is 80 percent due to this type of layout ... you could alleviate it possibly by placing all the boards on wire / capacitor standoffs on a blank sheet of PCB ... solder wire standoffs to ground and capacitor standoffs to power rails. The PCB forms a groundplane. The standoffs should not exceed 10mm in height.

The other 20% reason you have excessive noise is probably due to low CMMR on your diff integrator ... this can be fixed using the advice as per attached picture ...[ATTACH]36755[/ATTACH]

Thanks for the suggestions. The circuit boards are copper clad on one side and used as the - rail for the control circuits, common for the analog card. The circuit was built on the un clad side when I didn't have SMD adaptors. With the adaptors I can build on either or both sides. Was thinking the adaptors were acting as standoffs. Maybe not. Been using punched boards because that is what I had and the holes allow connecting circuits from side to side. I use a 1C integrator.

[moodz]

The circuit was put together as a learning platform. Didn't have SMD adapters for the control circuit and coil driver. They have made it a lot easier to wire the circuit.

...going back to this post ... where you have a picture of the circuit boards built on veroboard (?) ... the main problem you have with noise is 80 percent due to this type of layout ... you could alleviate it possibly by placing all the boards on wire / capacitor standoffs on a blank sheet of PCB ... solder wire standoffs to ground and capacitor standoffs to power rails. The PCB forms a groundplane. The standoffs should not exceed 10mm in height.

The other 20% reason you have excessive noise is probably due to low CMMR on your diff integrator ... this can be fixed using the advice as per attached picture ...

[green]

Can you draw a timing diagram that starts with the TX pulse (coil on) and then all sample pulses, delays and widths from that point. Just a typical setup will do. I want to see the relationship of each part of the cycle along to the next TX pulse, particularly how far out your GEB sample is from the 'off' edge of the TX.

Eric.

Got a PM from a member reminding me I didn't start the sequence at Tx on. All scope pictures are external triggered when Tx command turns off, T at top center of screen. Tx on starts when second EF(100usec, normal out) sample turns off. Using the upper picture, time between Tx off and first EF sample is around 750usec. Lower picture, GEB sample starts 20.7usec after turn off.

[green]

Can you draw a timing diagram that starts with the TX pulse (coil on) and then all sample pulses, delays and widths from that point. Just a typical setup will do. I want to see the relationship of each part of the cycle along to the next TX pulse, particularly how far out your GEB sample is from the 'off' edge of the TX.

Eric.

Some scope pictures. The 1kHz oscillator starts the control sequence. Takes EF samples before turning coil on. Target sample time was adjusted to cancel some hot ground. Times are what I've been using, not saying they are optimum. 1C integrator

[Ferric Toes]

Can you draw a timing diagram that starts with the TX pulse (coil on) and then all sample pulses, delays and widths from that point. Just a typical setup will do. I want to see the relationship of each part of the cycle along to the next TX pulse, particularly how far out your GEB sample is from the 'off' edge of the TX.

Eric.

[green]

[Tried the switch both places with the 1C integrator and didn't see a difference in the noise but wondered if I was missing something.]

I didn't notice a difference before. Yesterday I disconnected the leads from preamp out to the switch inputs and connected them to common to test integrator noise. Noticed if I touched common or either of the power supply pins I had a small change in output. Rewired the circuit with the switches to integrator -input, resistors to preamp. That problem fixed, don't think it changed the operating noise level. Ordered a different amplifier and switches to try to reduce noise level.

[green]

I would have to try and find my notes from many years ago for the 2C with hold. Not easy. Frequency cutoff is varied by the internal
loop gain, the more gain the higher the cutoff frequency. I seem to remember that the integrator gain remained the same.
How have you done GEB so far?

Eric.

Schematic I posted awhile back. There have been some changes but operation is the same. 1kHz oscillator starts sequence, take (A)target and (B)ground EF samples then turn coil on, then take (A)target and (B)ground samples. Switch to disable GEB samples if no ground problem. Target delay and target sample time adjustable, GEB delay and sample time fixed, adjust target sample time to cancel ground signal. GEB delay maybe 1 to 5usec. GEB sample 100usec.

[Orbit]

Thank you Eric !