Re: Oscillator using AVR on chip comparator ?
From: Gert Baars (g.baars13_at_chello.nl)
Date: 01/30/05
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Date: Sun, 30 Jan 2005 23:19:25 GMT
A good quality L and C might take the AC swing in excess.
Putting 2 zenerdiodes in antiseries over the LC combination
will limit the AC voltage. Putting a resistor across will
also lower the effect of metal.
Regards,
Gert Baars
richard mullens wrote:
> richard mullens wrote:
>
>> The AT90S2313 has two pins which can be used as the inputs of an
>> analogue comparator.
>>
>> I propose to create an LC oscillator by placing a parallel resonant
>> tuned circuit across these pins and by supplying energy via a resistor
>> from an output pin (which would normally be tristated) when the
>> comparator switches state.
>
> Or at the expense of software complexity driving L & C from the same
> pins configured temporarily as outputs.
>
>>
>> I am thinking of a 20kHz frequency with a capacitor between 10nF and 1
>> micro Farad with a 3V supply.
>>
>> Do I need to protect the inputs from the stored energy in the tuned
>> circuit when power is removed ?
>> Would it be good (or bad) practice to include a resistor between the
>> tuned circuit and a comparator input.
>> The configuration I suggest leaves the comparator inputs floating - is
>> this a bad idea and if so what should I do about it ?
>>
>> I would like to achieve a stable frequency - as I would like to detect
>> changes in the frequency when metal comes into the vicinity of the
>> coil. Should I be employing a different oscillator configuration -
>> for example a "centre tapped" capacitor like in a colpitts oscillator.
>>
>> On a more theoretical note, presumably such an oscillator will
>> generate electromagnetic radiation - which I suppose one could model
>> by a resistor in series or across the tuned circuit. Is this a
>> misunderstanding on my part - or is it so negligable that that is why
>> one doesn't see it discussed in models of a tuned circuit.
>>
>> Many thanks for your comments.
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