LC oscillator

[evandude]

I'm looking at this circuit for an RFID project:
https://www.electro-tech-online.com/custompdfs/2005/09/125kpowerosc4PDF.pdf

judging by the data on the PDF, it looks like it's been tested and everything, but it doesn't work in simulation for me... granted, I don't trust my simulator entirely, but still...

anybody care to offer any insight on the theory of operation? I want to at least get familiar enough with that to make changes if I can get it close to working...

 

[Roff]

Well, I can get it to oscillate in a sim with SwitcherCAD, but I don't have a model of 74HCU04. I did a best guess by using the behavioral inverter they provide.
I don't think the oscillation will be at the resonant frequency of the tank, because the phase shift at resonance is 90 deg. You would need another 90 deg. to add to the 180 deg of the inverter. I think it depends on the 470k-Cin time constant for some phase shift. The 5ns of the 74HCU04 adds a tiny bit. I think the tank will be off resonance. I got it to oscillate (simulated) at resonance by adding 50pF to GND on the input of the inverter.
One thing was baffling. If I used .IC v(out)=0, where out is the L-C junction, I got a different duty cycle from the inverter than if I omitted that initial condition.

 

[evandude]

what did you get for amplitude at the node between the cap and inductor? anything near the 100vpp indicated in the schematic?

 

[Optikon]

what did you get for amplitude at the node between the cap and inductor? anything near the 100vpp indicated in the schematic?

Interesting.. that doesnt seem correct. If you assume it is running at 125kHz at 100Vpp, compute the impedance on the feedback to the inverter input, you will see that it feeds back too many volts that the inverter would clamp to the supply rail. I think it is supposed to be 10Vpk-pk. thos enumbers look alot nicer..
:shock:

 

[evandude]

well actually, if you look at the page where I found the circuit,

http://www.discovercircuits.com/R/rfid.htm

there are several similar circuits, and all are marked 100Vpp... and 100vpp is the kind of amplitude i've seen tossed around when talking about RFID readers...

 

[Roff]

Here is what I simulated. I think 100 ohms is too low for the output resistance, but I used this value because it gave me the highest p-p output. With R1=1k, the output p-p was about 55V. Note the 50pF cap on the input. I had to add this to get the circuit to oscillate at the resonant frequency of the tank.

 

[evandude]

thanks! that should give me a good starting point

 

[Roff]

thanks! that should give me a good starting point

Do you use SwitcherCAD? If so, do you want the file?

 

[evandude]

Yeah, I've used switchercad a few times. If you want to send me the file at evandude AT gmail.com I'd appreciate it!