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I need to create a circuit to generate a square wave with 50% duty at 32768 Hz +/- 5 Hz. The output of the circuit will drive an NPN transistor ( 0.2 A , 24VDC ).
The cost of the component is not a limit, I'm searching for the best way to have a reliable output.
What is the best way to do this? Crystal and inverter? A specific IC? I tried with a 555 IC, but it had very poor stability over time and over temperature changes.
You have basically two choices- buy a crystal and make an oscillator or buy an oscillator. If you buy an oscillator it should meet the specifications on the datasheet and if those match or exceed your requirements you'll be okay. Typically such oscillators use 1.8V, 2.5V or 3.3V and sometimes 5V power supplies. The duty cycle might be 40% or 60% rather than 50.00% (many are rated 45-55%).
If you make an oscillator, it could be as simple as an unbuffered CMOS gate, a couple of resistors and a pair of NP0 load capacitors. Small tuning fork crystals such as 32.768kHz are easy to overdrive so there's a balance between an oscillator that starts reliably under all conditions and one that overdrives the crystal causing frequency drift and even failure. One of the resistors controls that, the other is not so critical and is only to bias the gate. The load capacitors are calculated from the crystal datasheet, as you can find many references on. The duty cycle, as with some types of oscillator, may also not be all that close to 50.0% if you use a 32.768kHz crystal (the most common kind).
If you actually require a duty cycle very close to 50.0% you can source a crystal of 65.536kHz, make an oscillator with that, and divide it with a flip-flop, which will be very close to 50.0% if the power supply is stable. You may also be able to find an oscillator in that frequency range.
Some oscillators use a crystal inside operating at the output frequency, some use a higher frequency crystal and a circuit to derive the output frequency (not necessarily a simple divider) and some use a MEMS resonator and some circuitry. The latter tend to be a bit less stable than a crystal but orders of magnitude better than a 555.
You should peruse the cornucopia of datasheets for oscillators and crystals and compare with your stated and unstated requirements.
While you can certainly build or acquire a crystal oscillator, I'll offer a potentially very cheap solution based on the DS3231 RTC chip (not the DS3231M) which produces a 32768 Hz square wave clock output:
There are downsides too:
A retail DS3231 RTC clock comes with onboard BBU and typically costs less than $10
Note: You can't use a DS3231M because the chip does not produce an accurate clock (though it is reasonably stable). The time values (which is what you'd typically be using the RTC for) is set by adding or missing 32k clock cycles to keep accurate time.
A few years ago I used a DS32kHz in a clock. It is still available in various packages such as DS32KHZS#T&R ... it may satisfy your requirement.
