lc resonance of LC meter - Page 1
Forum section: electronics / projects

author: electros6
author_rank: Regular Contributor
timestamp: December 19, 2013, 02:55:56 am
content: Hi guys ,I found lots of schematics for LC meter based on lc resonance . I attach the schematics published in the EPE magazine. when I build the circuit the pulse output scope shot is attached.Please tell me ,from the scope shots it is clear that the oscillation starts build as high frequency and produce a pulse when it reaches 2.5V at the +ve input of the opamp.As the microcontoller VIH is spec as 2.0V will the microcontoller starts count the high frequency stuff in between 2.0V and 2.5V.Is there any way to eliminate the high frequency stuff before it feed to the microcontoller

author: SArepairman
author_rank: Frequent Contributor
timestamp: December 19, 2013, 03:39:24 am
content: ****you might also consider the fact that the noise is not intrinsic to a good circuit design. Did you follow the design that they have to the key? Since you are using an comparator it might be possible that one of your parts is bad, or out of tolerance, and this is causing the circuit (comparator in this case) to misbehave and oscillate. Is the circuit you built soldered down? Did you decouple the op-amp and follow good board layout procedures? If the problem is with your design then filtering might not be necessary and it might not help you achieve your goals, however, if the design is sound and you still have a noise problem....What comparator did you use? Perhaps you chose a comparator which does not like the circuit that you built.however, assuming the design is fine,in order to eliminate high frequency interference you need to determine what frequency the interference is at and then choose the appropriate countermeasure. What frequency is your target signal at and what frequency is your noise at?This will be in the form of some kind of low pass filter.Appropriate solutions may include:passive filters such as:RC filterLC Filterrlc filterhttp://en.wikipedia.org/wiki/Electronic_filter#Passive_filtersactive filter (sallen key, multiple feedback, etc)http://www.ti.com/lit/an/sloa093/sloa093.pdfIC active filter (depending on your frequency there are various options with various cut off points. if you can find one that suits your application it might be a really great solution, because it is an ASIC designed to do what you want to do and it could offer really great performance, like 8th order filter in a single package. These filters tend to have some noise though, but it might be entirely acceptable.http://parametric.linear.com/filtershttp://www.maximintegrated.com/products/filters/and you might also want to look into ferrite beads, which are often extremely useful in eliminating HF interference:But, on a hunch, I think that maybe your problem could be cured by the right kind of ferrite bead, but why don't you try to make an appropriate RC filter? The passive filters are very easy to build (as little as two parts) and they may offer an immediate solution to your problem. Tell us more about the signal. Showing us the timebase and amplitude settings on your oscilloscope might be very useful towards determining a solution. I'm sure someone will know more about this particular application of a comparator, but if you are impatient try this:http://sim.okawa-denshi.jp/en/Fkeisan.htmthis is a nice website that can help you design passive and active filters if you choose to experiment.

author: KerryW
author_rank: Regular Contributor
timestamp: December 19, 2013, 02:55:05 pm
content: I have built this circuit, and it works. There are basically 2 oscillators an RC and an LC. The RC oscillator (~1-10 Hz) should only run on start up, to give the LC a "kick start". Then the LC should take over, at about 650 KHz.You don't provide a time scale for the scope picture, but I'm guessing that your "high frequency stuff" is the 650 KHz signal. If so, you need to figure out why the LC oscillator stops.