Nick said:
I am looking for a minimal hardware circuit to generate a 125 kHz
sinusoidal wave to be fed into the power stage that is in turn feeding
an antenna. The wave is used to generate a carrier frequency for an
RFID application.
I was thinking of using this [
http://www.ecsxtal.com/store/pdf/ecs-31.pdf
] crystal in a Pierce Oscillator configuration. However I am not sure
if this crystal will work in that configuration. Also, I can't seem to
figure out what the output would look like. Would it be a square wave
or a sinusoidal? I came across a diagram that pictured a sinusoidal at
the inverter input and a square wave at the output for the Pierce
configuration. Is that correct?
I would greatly appreciate any pointers on this.
If they don't have 125kHz as a standard frequency, you may have to get a
bazillion of them before you can get them at a reasonable price.
Assuming that you can get them* the traditional Pierce-ish inverter and
two caps oscillator could, indeed, be expected to give you something
like a sine wave at the input and something more like a square wave at
the output, with the crystal filtering things as they went through from
output back to input. You'd have to try this out on your circuit to see
how reliably it generated a nice sine wave.
Alternately, you could use (gasp!) discrete components to build a BJT or
JFET Colpitts oscillator. This gives you a heck of a lot more control
over the critical oscillator parameters, so if you're interested in
wringing the most out of your oscillator this may be the way to go.
With one of these oscillators you can use very light capacitive coupling
from the hot end of the crystal to get a nice sinusoid.
Whatever you do you'll have to make a tradeoff between power output,
circuit complexity, component sensitivity and the pureness of the
sinusoid. Anything that puts the oscillator pick-off at a point where
the signal is a nice pure sinusoid requires that you take little power
from the oscillator, which in turn means that you must follow your
oscillator with a healthy amplifier, which adds to the circuit complexity..
Were I doing this I would investigate using a cheesy DDS. This sounds
fancy, but all you do is use an oscillator running at some multiple of
125kHz followed by a divider driving a resistor network to make a
sorta-sinewave that drives an op-amp filter. Something like a 1.25MHz
oscillator driving a 4017 would let you hand-pick your ten resistor
values to really stomp down on the first five harmonics, which would
leave you only needing to filter above 500kHz or so.
* Check Digikey -- many folks have the rule of thumb that if Digikey
stocks it then it's probably a standard item. Beware of Digikey part
numbers that have no stock behind them -- those are either special buys
for someone, or they've been discontinued.
--
Tim Wescott
Wescott Design Serviceshttp://
www.wescottdesign.com
Do you need to implement control loops in software?
"Applied Control Theory for Embedded Systems" gives you just what it says..
See details athttp://
www.wescottdesign.com/actfes/actfes.html- Hide quoted text -
- Show quoted text -