Originally posted by Qiaozhi
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Originally posted by moorejl57 View PostOn stage 6, I am getting a short EFE pulse width (63us) at TP8. Is this an issue or within normal variation? TP9 and TP10 seem in the ballpark at 7.2us each.
[ATTACH]46355[/ATTACH] [ATTACH]46357[/ATTACH] [ATTACH]46356[/ATTACH]
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Originally posted by Qiaozhi View PostI assume you're building the REV-E version. In which case, TP9 and TP10 should be close to 10us (or maybe slightly higher due to the two diodes), and the EFE delay should be 100us.
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OK, found root cause on my TP8, 9, 10 timing issues. The parts I recently received in the MPP Beach kit for U7 and U10 are MM74HC4538N. Looking at the data sheet, the time pulse period IS NOT RxCx. Instead it is spec'd as 0.7RxCx. As such for the EFE delay of 100us, R32 needs to be more like 10K/0.7 ~= 14.3K. In the same vein, R35 needs to be 4.7K/0.7 ~= 6.7K. Hope this helps other folks building this kit.
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Originally posted by moorejl57 View PostOK, found root cause on my TP8, 9, 10 timing issues. The parts I recently received in the MPP Beach kit for U7 and U10 are MM74HC4538N. Looking at the data sheet, the time pulse period IS NOT RxCx. Instead it is spec'd as 0.7RxCx. As such for the EFE delay of 100us, R32 needs to be more like 10K/0.7 ~= 14.3K. In the same vein, R35 needs to be 4.7K/0.7 ~= 6.7K. Hope this helps other folks building this kit.
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Nice detective work.
The datasheet states that the MM74HC4538 is pin compatible with the CD4538. However that doesn't mean that it's compatible in other ways.
Both the CD4538 and the MM14538BCP have a pulse width output of T = R*C.
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Originally posted by Qiaozhi View Post
Nice detective work.
The datasheet states that the MM74HC4538 is pin compatible with the CD4538. However that doesn't mean that it's compatible in other ways.
Both the CD4538 and the MM14538BCP have a pulse width output of T = R*C.
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Originally posted by 6666 View PostYou get a direct ratio of the voltage IE: a 700 volt peak will give you 7 volts on the scope, so you dont blow your scope
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In my first pass testing of the completed board with a 7" bundle wound coil of 328uH and 2ohm resistance, damping resistor at 680ohm, SRS around 800kHz, I think I am not getting the best depth with it. It can air detect a US quarter at about 7" and a US penny at about 8-9". What are typical results for a well tuned MPP Rev. E so I know what to expect?
I would like to do some experiments to improve the detection depth for coins and rings etc... and would like some advice on what are the top Pareto items to change first.
a. Improve critical damping (I haven't been able to really dial this in yet due to jitter from TX pulse period messing with scope trigger)
b. Modify coil current with serial resistance to avoid MOSFET avalanche (do I just change the existing 3R3 2W resistor on the board?)
c. Use a different coil design (basket, spiral etc...)
d. Change current shielding from wrapped copper tape to something else?
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Originally posted by moorejl57 View PostIn my first pass testing of the completed board with a 7" bundle wound coil of 328uH and 2ohm resistance, damping resistor at 680ohm, SRS around 800kHz, I think I am not getting the best depth with it. It can air detect a US quarter at about 7" and a US penny at about 8-9". What are typical results for a well tuned MPP Rev. E so I know what to expect?
I would like to do some experiments to improve the detection depth for coins and rings etc... and would like some advice on what are the top Pareto items to change first.
a. Improve critical damping (I haven't been able to really dial this in yet due to jitter from TX pulse period messing with scope trigger)
b. Modify coil current with serial resistance to avoid MOSFET avalanche (do I just change the existing 3R3 2W resistor on the board?)
c. Use a different coil design (basket, spiral etc...)
d. Change current shielding from wrapped copper tape to something else?
b. If you are not looking for short TC targets with a fast coil(first delay time less than 10us)I would jumper MUR460 in series with coil. Adds some capacitance, requires a lower damping resistor, allows a higher peak coil current before avalanche.
c. Use a spiral or basket for a fast coil with the MUR460 not jumpered, fast coils require less peak current for the mosfet to not avalanche. Less peak current less signal if Tx shape stays the same. Bundle wound might be better if not looking for short TC targets?
d. Might try a different shield if the copper tape is acting as a target. Post a scope picture of amplifier out and coil hot lead insulation with no target.Attached Files
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Originally posted by green View Posta. To see coil decay shape. Clip probe to coil hot lead insulation, can't read volts but shape should be similar as hot lead. Can see if mosfet avalanches, flat at first part of decay. Including picture of my Tx circuit with hot lead and hot lead insulation(not MPP). I like to look at amplifier out and hot lead insulation on the scope when adjusting damping.
b. If you are not looking for short TC targets with a fast coil(first delay time less than 10us)I would jumper MUR460 in series with coil. Adds some capacitance, requires a lower damping resistor, allows a higher peak coil current before avalanche.
c. Use a spiral or basket for a fast coil with the MUR460 not jumpered, fast coils require less peak current for the mosfet to not avalanche. Less peak current less signal if Tx shape stays the same. Bundle wound might be better if not looking for short TC targets?
d. Might try a different shield if the copper tape is acting as a target. Post a scope picture of amplifier out and coil hot lead insulation with no target.
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Originally posted by moorejl57 View PostThanks green. Here is CH1 on coil hot lead insulation and CH2 on TP3 pre-amp out.
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Tried a spice simulation. middle trace(fast coil, critical damped, avalanched) bottom trace(fast coil, overdamped, reduced Rd to eliminate avalanche) top trace(added capacitance to make slow coil, same Rd as bottom trace, critical damped, no avalanche) slow coil critical damped decays faster than fast coil overdamped with same Rd.Attached Files
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