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xor gate oscillator?

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haxor5354

Tue Apr 26 2011, 09:21PM

Registered Member #2063 Joined: Sat Apr 04 2009, 03:16PM
Location: Toronto
Posts: 352

I need a simple oscillator that can oscillate in megaherts for a wireless power transmitter.
If I feed the output of a XOR gate back to it's B input while its A input is high, would it oscillate like a mad cricket on crack?
I already have the circuit built on a breadboard using 4 NAND gates to simulate a XOR gate, but I cant tell if its oscillating or not. too fast to see?
i'll test it with an o-scope at school tmr.
also woud feeding the output of a NOT gate back into its input cause it to oscillate?

Experimentonomen

Tue Apr 26 2011, 09:45PM

Registered Member #941 Joined: Sun Aug 05 2007, 10:09AM
Location: in a swedish junk pile
Posts: 497

Like that, i think it'd settle on Vcc/2, you likely need the rc circuit to make the oscillation work.

Maybe i should also add that these kinda oscillators generally only work with schmitt trigger input logic gates.

Mattski

Tue Apr 26 2011, 11:02PM

Registered Member #1792 Joined: Fri Oct 31 2008, 08:12PM
Location: University of California
Posts: 527

wrote ...
also woud feeding the output of a NOT gate back into its input cause it to oscillate?

It will oscillate. No need to complicate things with an XOR gate, a NOT gate (i.e. inverter) is the better way since a not gate is much simpler. If you put any odd number in a row with the last one feeding into the 1st one your frequency will be 1/(N*gate_delay) where gate delay is the delay and rise/fall time of each gate. You may want to add an RC delay between each to control the frequency depneding on the exact frequency you want.

Effectively you were using the XOR gate as an inverter anyway in your circuit.

It's called a ring oscillator

James

Tue Apr 26 2011, 11:19PM

Registered Member #3610 Joined: Thu Jan 13 2011, 03:29AM
Location: Seattle, WA
Posts: 506

It's easy to get a wide range of frequencies using just a single inverter gate, a resistor and a capacitor. It's simple, cheap and reliable. The other gates can be paralleled as a buffer, or another put in series to get complimentary outputs.

Arcstarter

Wed Apr 27 2011, 12:52AM

Registered Member #1225 Joined: Sat Jan 12 2008, 01:24AM
Location: Beaumont, Texas, USA
Posts: 2253

James wrote ...

It's easy to get a wide range of frequencies using just a single inverter gate, a resistor and a capacitor. It's simple, cheap and reliable. The other gates can be paralleled as a buffer, or another put in series to get complimentary outputs.

I would also like to add, you could use an extra cap and pot as well as another inverter gate to vary duty cycle, but im not sure that is what you are looking for. However, it is something to keep in mind, it is very useful!

haxor5354

Wed Apr 27 2011, 02:35AM

Registered Member #2063 Joined: Sat Apr 04 2009, 03:16PM
Location: Toronto
Posts: 352

where can I find a transistor thats fast enought to switch/amplify these outputs?

Arcstarter

Wed Apr 27 2011, 03:17AM

Registered Member #1225 Joined: Sat Jan 12 2008, 01:24AM
Location: Beaumont, Texas, USA
Posts: 2253

haxor5354 wrote ...

where can I find a transistor thats fast enought to switch/amplify these outputs?

Got any random circuit boards, from TVs or such? Also, we aren't sure what is fast enough, first tell use what Rt and Ct, or better yet, frequency, you will use. You will need a cap and resistor, direct feedback would not oscillate.

But anyway, the small 100ma or so signal transistors in TVs will switch into the MHz if you only need low power. If you need more power you can easily use typical mosfets (irf/irfp series) up to 500KHz (or even a few MHz if you really want to make a good gate driver) into the hundreds of watts.

Xray

Wed Apr 27 2011, 04:04AM

Registered Member #3429 Joined: Sun Nov 21 2010, 02:04AM
Location: Minnesota, USA
Posts: 288

True, it's cheap and easy, but frequency stability is generally horrible, especially since he wants to use it as a radio transmitter. I tried doing this same thing many years ago with a CMOS inverter. It oscillated just fine, but I had to retune my receiver every couple of minutes as the frequency was drifting with temperature. I ended up using an L/C tuned circuit which proved to be much more stable than the R/C circuit I started with. And of course if you want rock-solid stability (on a fixed frequency), then use a crystal oscillator.

Conundrum

Wed Apr 27 2011, 04:41AM

Registered Member #96 Joined: Thu Feb 09 2006, 05:37PM
Location: CI, Earth
Posts: 4061

Using a tuning diode with a reflex circuit also works.

Its just the tuning diode, choke, capacitor, two resistors, rectifier diode and a second tuned circuit which resonates at the frequency of interest,
as a result at resonance the discharge and charge speeds of the capacitor are identical and the circuit is stable.

-A

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