crutschow
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What do you mean, they are 180° opposite each other? Do you mean the obvious -- that when the current is coming out of the inductor, it is going into the capacitor terminal connected to the same node?The currents in the L branch and the C branch of a parallel circuit are 180° opposite each other no matter what the voltage frequency is.
Here's a circuit outline to illustrate
A 90deg shift is easily possible by using an L & C in parallel, and which are tuned to the exact carrier frequency.
What do you mean, they are 180° opposite each other? Do you mean the obvious -- that when the current is coming out of the inductor, it is going into the capacitor terminal connected to the same node?
How is that relevant to achieving a 90° phase shift?
I can confirm this concept. Simulations show resonating action around the tank loop. Current through the coil and capacitor are 90 deg. from the incoming signal, when it is anywhere near the center frequency.
To tap for a signal, it should be done across a resistor inserted in the loop, because you want to read current through the resistor. It will not work to tap for volt level across either the capacitor or coil, because their volt reading is close to being in sync with the incoming signal.
The key L & C elements are in the center -- the 100nF & the 25uH in parallel.
Do try and pay attention Ratch. You're much smarter than this.
Thank you THANK you for pointing it out & bringing to my attention. I am now a better man. .
Seems that the All-pass circuit I posted which uses 3 resistors, 1 capacitor, and 1 op amp is a lot simpler. ;-)
I set this up in a simulation, and I applied a frequency sweep.
It works, and it yields 90 deg phase shift... .
Seems that the All-pass circuit I posted which uses 3 resistors, 1 capacitor, and 1 op amp is a lot simpler. ;-)
The phase-shift is 90 degrees at the -3dB frequency of the RC network. With a 1000kHz -3dB frequency (159Ω, not 300Ω, and 1µF) I simulated 127° shift @ 500Hz, 90° @ 1kHz, and 67° @ 1.5kHz, Don't know why you said it provides 90° over that range. :???:Yes. I have been running a simulation of this type as well. The diagram is a screenshot of my layout using Falstad's simulator. For convenience I am using a center frequency under 1000 Hz.
It provides a 90 degree phase advance. Looks as though it does so for a range of frequencies around 50% to 150% of the center frequency.
Up until today I am used to my simulations showing that the RC-to-ground yields reduced amounts of phase shift. So I suppose the op amp is key to the success in this case. Because usually we think of the series capacitor providing greater phase shift, and never more than 90 degrees.
(The all-pass filter can provide more than 90 degrees advance.)
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The phase-shift is 90 degrees at the -3dB frequency of the RC network. With a 1000kHz -3dB frequency (159Ω, not 300Ω, and 1µF) I simulated 127° shift @ 500Hz, 90° @ 1kHz, and 67° @ 1.5kHz, Don't know why you said it provides 90° over that range. :???:
Yes, that All-pass configuration with an op amp generates a phase-shift between 0° and 180° with frequency as compared to the 0° to 90° shift of a single RC filter. It also keeps the output constant with frequency, rather than rolling off past the -3dB frequency at the usual -6dB octave.
If you connect the two signals to generate a XY (Lissajous) display you can readily see the affect of phase-shift between the two.
Or superimpose the signals on top of each other.
You might try doing it in a Spice simulator such as LTspice which is a free download from Linear Technology. That has more options for observing and measuring the signals.
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