Electronics > Projects, Designs, and Technical Stuff
AC-coupling cap for AC inductor drive
ricko_uk:
PrehistoricMan a while back wrote in a reply above in this post:
"For detecting metals with a coil, you are expecting the inductance of the coil to change. To sense this, the drive frequency should be near the inductor's pole so that the output amplitude changes (phase will too). The area of interest (as seen in the graph) is around 30kHz - 100kHz. This region is where the inductor's impedance is increasing and therefore the amplitude of OUT is increasing."
Questions:
1) What does it mean to be "near the inductor's pole"?
2) How do I find where the inductor's pole (frequency?) is?
3) in his reply's attached graphs, where can I see the inductor's pole?
Thank you :)
Prehistoricman:
1)
In layman's terms, a pole is a frequency where the characteristic has greatest rate of change. Trust me, that's actually simplified compared to what a pole really is.
That's not a foolproof definition but it's something to use to identify a pole.
To be "near the pole" is to have a frequency close to the pole frequency.
2)
Calculations, experiments, or simulations. I would suggest the latter two :)
3)
If you look at the second image, upper graph, you see three poles. One is made by the capacitor and can be seen at 80Hz. You can see how the gain curve is changing from a diagonal line up to a horizontal line. The part of the curve where that change is 'fastest' is the pole frequency.
The second pole is due to the inductor and is about 6kHz. The third is also from the inductor, and is at about 200kHz.
However now I'm simulating this again, I can't reproduce those graphs. That's weird...
Also I wasn't really right in saying that you want your frequency close to the pole. What you want is the frequency to be in the middle of a steep part of the graph. A steep part that is made steep because of the inductor.
I've attached a new sim + results that's a bit easier to understand. It's also very intuitive when you think about how capacitors and inductors behave (in general) across frequency.
The idea frequency in this one would be something like 200kHz. High enough such that changes in inductance will result in large changes in amplitude, but small enough such that there is a big enough signal to observe.
It's also worth mentioning that my idea for this project was very different to Zero999's. I don't have enough practical knowledge to evaluate the two methods against each other. However mine's very easy to build and observe with a signal gen + scope in a few minutes.
ricko_uk:
Thank you PrehistoricMan, :)
could it be that the simulation does not come out the same because you inverted the inductor and the resistor compared to my initial schematic?
I can see the horizontal scale is logarithmic but not the values. I tried to work them out but if the first one is 1 then the 200KHz is outside the graph. What are they?
Thank you :)
Prehistoricman:
--- Quote from: ricko_uk on March 22, 2020, 04:06:15 pm ---Thank you PrehistoricMan, :)
could it be that the simulation does not come out the same because you inverted the inductor and the resistor compared to my initial schematic?
--- End quote ---
Yes, I know. I couldn't reproduce the original results for the original schematic. I tried for about half an hour.
--- Quote from: ricko_uk on March 22, 2020, 04:06:15 pm ---I can see the horizontal scale is logarithmic but not the values. I tried to work them out but if the first one is 1 then the 200KHz is outside the graph. What are they?
Thank you :)
--- End quote ---
Whoops haha! It starts at 10Hz, ends at 1MHz, just like the other graphs.
ricko_uk:
Thank you PrehistoricMan,
so when you say "What you want is the frequency to be in the middle of a steep part of the graph. A steep part that is made steep because of the inductor." you mean better frequencies would be slightly higher, perhaps 500KHz with reference to your graph?
How do I shift the inductor's down-going slope further to the left so that 200KHz is on the steep part of the downward curve like you suggested?
Thank you :)
Navigation
[0] Message Index
[#] Next page
[*] Previous page
Go to full version