Invention of the day:  Time-Suspended LC Tank - Page 1
Forum section: electronics / general

author: RoGeorge
author_rank: Super Contributor
timestamp: January 01, 2025, 05:08:40 pm
content: Tadaaam!!! Proudly presenting ...  ... The Time-Suspended LC Tank!Each color shows the oscillations of the same LC tank, but under different ratios of "time-stretching".From the LC tank perspective, this circuit repeatedly stops the flow of time, then resumes it later.How's so?At first, let's imagine we have just a capacitor connected with a switch to a current source. If we close the switch, the capacitor starts to charge. If we open the switch, the capacitor remains charged at its last voltage. In theory, it will remain in that state indefinitely. If we close the switch later, the capacitor will resume charging as if nothing happened in between.From the capacitor perspective, this is no different than stopping the flow of time, then resuming it later. From the capacitor perspective, this is as if the time has stopped when the switch was open.Now, can we do the same with an entire LC tank? The answer is no, because the energy will "slosh" back and forth between C and L. An LC tank will continue to oscillate forever (in theory) if we disconnect the power.However, we can "pause" the time for an inductor, too, (for L alone, without C) if we suddenly short-circuit the inductor. In theory, an ideal inductor will preserve its last seen current forever, as long at we keep its terminals short-circuited.Capacitors and inductors are similar, their behavior is "complementary" in relation to voltage and current, in the sense that a disconnected C will preserve its last voltage forever, while a short-circuited L will preserve its last current forever.If we arrange these properties just right, we can "pause" the time for the entire LC tank with a single SPTD (Single Pole Double Throw) switch, like this:This is the idea implemented in the LTspice simulation above. Since we can now "pause" the oscillations in the entire LC tank (then resume from where they left), if we repeatedly do small pauses very, very fast, the period of a full oscillation will appear lengthen accordingly, by the amount of "paused time" we introduced when repeatedly toggling the SPDT switch.In the end, we get a wildly tunable analog LC tank, in which the frequency is controlled by a PWM (Pulse Width Modulation) that toggles the SPDT switch.

author: mawyatt
author_rank: Super Contributor
timestamp: January 01, 2025, 05:23:42 pm
content: It works!! See link:https://www.eevblog.com/forum/projects/reactive-clock-driver-circuit/Creating a time stopping open in the case of the inductor current at zero as shown in the reference is easier to implement in actual circuits than a short across the inductor when the capacitor voltage is zero. Of course if one has access to cryogenic inductors and switches, then the short becomes possible altho maybe not practical!!Happy New Year,Best

author: RoGeorge
author_rank: Super Contributor
timestamp: January 01, 2025, 06:56:43 pm
content: Wow, obviously seen that topic of yours, since I've even thanked back then, but didn't know it's the same idea as in my paused LC tank.The idea of a paused LC tank came to me after you teach me about the N-path filters. My initial thought from back then was to try to improve that by using an even steeper switched filter than the typical RC used in N-path filters, like a Quartz, or an LC with high Q factor.Tried with a 32768Hz quartz at first, and failed. Then I've realized that I can switch both C and L in an LC tank, so to get a paused/tunable tank. It was the next idea to try back then, when I was experimenting with N-path filters to learn more. It happens that I've mentioned the quartz idea here:https://www.eevblog.com/forum/rf-microwave/polyphase-or-n-path-mixer/msg3406382/#msg3406382(phew, I see now that I've wrote that post in 2021, and your link is from 2022. For a moment, I was afraid I might have seen the switched LC in your Reactive Clock Driver first, then later unconsciously reproduce a similar idea, and believe I come up with that. It is not about claiming any paternity, I am aware the papers you posted there are from many more years ago, but at least now I know that I didn't unconsciously plagiarized the idea.)Now, back to my experiment with the switched quart and the switched LC, got busy back then, some other things happened, and that project was parked for later. Until 2 days ago, when the questions from the topic about Tayloe mixer gain summoned the N-path filters theme again.So, TL;DR, did a simulation today, and was pleased to see the idea of the paused LC tank is working.This thing alone (paused LC-tank) can have a lot of applications. I didn't try yet to use this paused LC in an N-path filter. There are so many possible applications that come to mind (and easier to experiment than the N-path filter) that I don't know which one to try first.For today, I'll just go study your Reactive Clock Driver, thanks again.Happy New Year!

author: max.wwwang
author_rank: Frequent Contributor
timestamp: January 01, 2025, 10:19:36 pm
content: Happy NY and congrats on your invention!From the file extension .asc, it looks like you ran the simulation with LTSpice. I only recently became aware of this free and apparently powerful tool, which excited me. Will take time to learn and try to make use of it.[Also from your blog I had another great find, the unusualexcel youtube video and website. Very interesting. Thank you.]

author: mawyatt
author_rank: Super Contributor
timestamp: January 02, 2025, 02:49:19 am
content: Quote from: RoGeorge on January 01, 2025, 06:56:43 pmWe've mentioned somewhere about reviewing a young USC PhD student's IEEE ISSCC paper using the bilateral transform of the N-Path (PPM) Mixer. He created Quadratic low-pass functions with active transconductors at baseband which were then frequency translated by the bilateral transform the PPM imposes on the baseband transfer function up to the Rx input antenna port by the LO Effective Clock.These baseband Quadratic LPF functions then become symmetrical Bandpass Functions wrapped around the LO Effective Clock. The student utilized an 8 phase LO Clock and created a test chip in 65nm TSMC CMOS (popular CMOS process among academia because it was available thru MOSIS). His paper won the best student paper of the IEEE ISSCC that year (recall 2013~14).One needs to realize that this is entirely different than a typical heterodyne type Rx where the IF filter dictates the channel BW but has little influence on the Rx input port where the LNA and/or Preselect Filter create the effects "seen" at the Rx Input (Ant) Port. Whereas the PPM translates the "Baseband Entire Impedance Function" directly up to the Ant Port centered around the LO Effective Clock, which is what input signals "see"!! Anyway this is more for the PPM discussions than here, if interested we can move that over if you wish.So yes the "Paused Time LC Tank" and "Higher Order Filter PPM" work, and work well indeedBest

author: iMo
author_rank: Super Contributor
timestamp: January 03, 2025, 08:13:56 am
content: Do not forget that any inductance in the LTspice has got a series resistance of 0.001 ohm by default, unless you explicitly instruct the LTspice not putting the resistor there..So your LC will not oscillate indefinitely even at 0 Kelvin without the hack..