Why can't RC circuits oscillate in frequencies higher than RLC or LC circuits?

The resonant frequency of a tank circuit should be decreasing either with capacitance or inductance increasing according to the equiation given below.

It should be because, it will take longer time to charge a larger capacitance to unit amount of charge or start unit current against the "inertia" of a longer wire/inductor.

In a sense, a RC circuit also could be considered as a RLC circuit with negligible inductance (a much shorter lenght of wire than an inductor).

This makes me wonder why is it not possible to use only smaller capacitors and a short wire (so that their capacitive and inductive reactances match) to produce frequencies even higher than a LC or RLC circuit which has a larger indutance than RC?

What is the obstacle that practically limits building high frequency, low capacitance RC circuits?

• Point 1, a short wire is still an inductor. Point 2 resistors make tanks less efficient. Point 3 having no inductance doesn't make a tank circuit. Question: what are you talking about? Nov 21, 2016 at 11:08
• In a sense, a RC circuit also could be considered as a RLC circuit... If that is the case, it is not an RC circuit anymore, it becomes an RLC circuit. The L is important as you start using it as a part of the circuit. If you have an RLC circuit where the L is so small it can be neglected, we call that an RC circuit. Nov 21, 2016 at 11:08
• The main reason why (R)LC circuits can achieve higher oscillation frequencies than non-L circuits is that at resonance L and C "tune out". With an RC circuit, that is not possible so you're always stuck with parasitic capacitances which you cannot "tune out". Nov 21, 2016 at 11:11

This makes me wonder why is it not possible to use only smaller capacitors and a short wire (so that their capacitive and inductive reactances match) to produce frequencies even higher …

If you manage to build an oscillator out of a small capacitance and a smull inductance, no matter what shape they have (ceramic capacitors or just metal planes on the opposite sides of a PCB, wire wound around a metal core or just a zigtag line on a PCB, or a via), that is an LC circuit.

All your nomenclature aspects are really just distracting you from the core: The math doesn't care which shape your inductive and capacitive components have. All that matters are the physical properties with respect to electronics.

So, yes, of course, one can build very high-frequency (R)LC circuits with small values for L and C – why not? It's done, on purpose and accidentally, all the time - we even teach and model our radio frequency transmission lines as systems of couple L and C that can oscillate, as you might have noticed.

In reality, it's technologically harder with rising frequency to build an RLC that still works – there's simply too much losses in materia if you approach Gigahertzes if you do it in the classical "I take this C component here, and add the L component there" way.

But: If you look at LC filter circuits, you'll find out that for microwave circuits, it's very common to to build LC filters with very small values. However, for microwaves, the lines on a PCB are already complex components, and everything tends to get a little involved :) That also means that a bandpass resonant filter in microwave microstrip technology really just looks like this:

(image by BlackBird Engineering, CC-BY-SA)

You'll notice the parallel runs of line; they, by the mechanics of microwave technology, form L-C - ( L||C ) filter pairs (image excerpt from wikipedia):

This circuit in complete forms a filter centered at 1.090 GHz – and that's by far not the highest you can get with these types of circuitry (they get smaller, the PCB substrate gets a lot more expensive and manufacturing gets a lot more accurate the higher your frequency gets), but microstrip resonant LC components are state of the art since the early days of the cold war.

At the left and right ends of the filter picture, you can see stubs going up / down without a parallel other stub – these are really just open-end lines that are equivalent to a L||C combination to ground.

• I see that it's all a matter of ratio between the capacitance and inductance in a circuit that determines either it's called an (R)LC or RC. A high frequency "RC" circuit is already an (R)LC circuit. Nov 21, 2016 at 11:46
• As everything gets smaller, shapes and electrical properties have to get more precise to get a pure tone "ringing". And even those 45 degree cuts at the corners of those lines that look like tuning forks, maybe they were designed to preserve the waveform while the signal is reflected from corners. Thank you for your help sir. Nov 21, 2016 at 11:52
• NO!! stop it. Really. If you consider it to have an inductance, it's an RLC. Just because you can't "see" the component with the "L" label doesn't mean something is not containing any inductance. Please stop trying to name things like you want, and then claiming it should be named like something else. Nov 21, 2016 at 11:54
• @Xynon and your second comment really shows you're not familiar with microwave transmission lines. The whole point of my answer was to show that you need to look at things like they are, not like you idealize them to be (see my rant about your naming confusion). There's no "tuning forks" here – this is EE, not "dance my name until my filter works". If you want to do high-frequency stuff, there's no way around learning the basics of electronics, then RF,then transmission line theory,then microwave engineering. You're constantly mixing dangerous half-knowledge with naming confusion and claims. Nov 21, 2016 at 11:56
• I see the point. Of course there is no such thing as a RLC circuit which could be a high frequency RC circuit. I understood the difference now. Nov 21, 2016 at 12:01

Passive RC filters have no frequency limits but may have some L, C parasitics.

A resonance within the gain bandwidth limits of an active inductor using with a negative impedance converter (NIC) cct.in an Op Amp or video amp is possible without an L.

e.g. LC passive filters and Microwave Circulators, isolators with passive NIC waveguides.

The practical limits of ANY passive parts are specified by parasitic values and Q at some test frequency.

Parasitics are the other 2 variables out of 3 for RLC, because every passive component has an equivalent Rp-L-C-Rs value or more.

• Um, this is a bit hard to read, even for an EE. Maybe you could expand the abbreviations? I doubt Xynon will have an idea what most of them mean... Nov 21, 2016 at 12:02
• I really don't mean to offend, but I'm not a native speaker, and I've learned EE in Germany, so what is a) cct.in b) the conv. in NIC? And your last sentence seems to be missing a verb! Nov 21, 2016 at 12:07
• Thanks for clearing that up! I expanded frequency and gain bandwidth, too. Still confused about cct.in, though! Nov 21, 2016 at 12:14
• Thanks @MarcusMüller Forgive me for my gransdon spilled coffee on my wireless keyboard , so I was typing with thumbs on a Rii touchpad. I just installed my old E-machine PS2 keyboard in the meantime. . Often my iPad makes auto spell fixes wrong at other times... Nov 21, 2016 at 12:27
• Say "hello" to your grandson. My keyboard sees a serious coffee spill about three times a year, and I don't even have kids :) keep up the great work! Nov 21, 2016 at 12:29