LC oscillators

Hi,
I'm trying to get a basic idea of the use of LC oscillators in simple RF transmitters, and I have a few questions for you guys. First off, I understand how an inductor and a capacitor connected in parallel can act as an oscillator if an instantaneous voltage is applied then removed. The capacitor is charged up and then the inductor starts to discharge the capacitor, but since the change in current through an inductor cannot be instantaneous, the inductor acutally forces all of the charge to the opposite plate of the capacitor, and then the process repeats in reverse. What I dont understand is how one could apply a continuous signal or dc voltage to an oscillator to make it work. I'm guessing this would require some extra components...I've seen the basic hartley and colpitts oscillators (with one active device), but they dont make total sense to me. Also, can these oscillators work as frequency modulators, or would that require some extra work? Thanks...
-Mike

This seems an odd place to ask that question, and odd that the question has to be asked at all since L-C oscillators are pretty well-covered in the literature. Do I detect an engineering student working on an assignment? :wink:

Since real-world inductors and capacitors have dissipation, and a tank circuit made of a real capacitor and a real inductor will display a damped (gradually dying off) oscillation in response to a stimulus, you need to wrap an amplifier around the tank circuit to keep "pushing the flywheel", as it were. The net gain of the amplifier minus the loss in the tank circuit must be unity or greater. Since all you need is to provide a pulse at the proper instant during the oscillation, a class-C amplifier is usually used. There must be positive (aiding) feedback in order to maintain oscillation.

An L-C oscillator can be frequency-modulated. There are several ways to do this. Popular approaches include adding a voltage-variable capacitance (a varactor diode) across the tank circuit and using signal voltage to modulate this capacitance. Applying a signal to the base of a transistor used in, say, a Colpitts circuit with collector-emitter feedback is another way to do it. The frequency can be shifted either through altering the resonant frequency of the tank circuit, or somehow changing the "loading" on the circuit.

The basic idea of FM is that the modulating signal causes the frequency of oscillation to shift to either side of its no-signal (unmodulated carrier) state. Matter of fact, depth of frequency modulation is usually referred to as deviation. Whereas in AM, we would speak of so-many percent modulation, in FM, we talk about so-many kilohertz of deviation.

Those are the stripped-down basics. There's much more to the subject, but it would be less trouble to look it up online or in the library than it would be for me to re-hash it in my own words.

> I understand how an inductor and a capacitor connected in parallel can act as an oscillator if an instantaneous voltage is applied then removed.

Same as a mass and spring. Give it a smack, and it shakes at its preferred frequency.

But everything has friction, so the shake gets smaller and stops. Try to take power out of it, and it stops faster.

Stick an amplifier on it. Take a small part of the shake, amplify it, and put signal back into the L-C tank in the right way. Now instead of the shake getting smaller, it gets larger.

If an amplifier has positive feedback, it is pretty sure to oscillate. The L-C tank just influences the frequency of oscillation.

Hartley and Colpits are just common ways to get positive feedback, when a single tube mostly wants to do negative feedback (because it is inverting). The Harley takes output at the cathode, and steps it up in the coil. The tube is unity voltage gain, the coil has voltage gain of 2 or 3, and that's all it takes.

A more obvious way is the "tickler": the L-C drives the grid, the plate feeds a small coil wrapped around the main L. If it won't oscillate, reverse the connections to the tickler coil.

How do you start it? While Dave mentions Class C oscillators, they won't self-start without a big bang. Most oscillators start in Class A. And since they are amplifying their own output, and there is always some random noise, they start-up fine.

With an ideal amplifier, the output would get bigger and bigger to infinity. In real life, if you don't do anything else, the output rises until the amplifier can't make any more power, until its effective gain is less than the losses in the L-C tank.

For very best frequency stability, you add some kind of gain control to keep the amp in class A. That's what the H-P patent does (or pretends to do).

With tubes, for many situations it works well enough to rise through Class A to the edge of either Class B or more often Class A2. Get the grid current to flow through a resistor (47K usually works) and it will drive the grid negative, amplifier gain drops without large distortion. This trick works good for a few milliwatts of output.

For high power from a single tube, just let it rise through Class A and B into Class C, and adjust the feedback so it sits near optimum Class C conditions. When you whack the L-C tank that hard, you push it off of its preferred frequency, and it will drift with voltage and tube-swaps. If you want big RF to melt metals, the exact frequency isn't critical: I know a place that melts tons of ultra-pure (no fuel soot) metals with 200KW single-tube oscillators. And years ago it was legal-enough to stick a telegraph key under a Class-C oscillator for radio transmission. Very distinctive chirp on each dit-dah. It was even done with speech, though the resulting frequency shift must have trashed-up the audio in the receiver.

Frequency modulation is usually done with a variable reactance in the L-C tank. Today the vari-cap diode is universal. Before that, we added a second tube with a small reactance from plate to grid. The input reactance, as shown by Miller, is the small reactance times the gain. The gain of a tube can be shifted by varying its current, most easily by shifting the grid bias. That was how FM radios did AFC.

FM transmitters almost universally started with phase modulation, not frequency modulation. There were even special tubes for this, but the reactance tube would work too. A classic FM transmitter modulation system was a huge affair.

A crude frequency modulator was the wobble generator. In the L-C tank, you had a fixed (or manually adjusted) C, and a tuning capacitor modified to spin 360 degrees, and a motor to spin it. Say you had 100pFd fixed and 0pFd-5pFd variable spining 3600RPM: you could sweep the IF passband of an FM radio 60 times a second, so a very simple o-scope hookup would let you trim the passband.

The Theremin is another FM oscillator. Fixed L-C plus the variable C of your hand near a metal plate or loop.

There are RF condenser mikes where the C in the L-C tank is the capacitance of the capsule. Maybe 30pFd static, varying +/-0.01pFd on loud sounds. This controls a 10MHz oscillator, and you can use a fairly long cable (no tube inside the mike). A frequency discriminator recovers the audio.

[quote author="NewYorkDave"]This seems an odd place to ask that question, and odd that the question has to be asked at all since L-C oscillators are pretty well-covered in the literature. Do I detect an engineering student working on an assignment? :wink: [/quote]

Guilty as charged, although this is more of an extension of the ideas I need to be portraying for an independent study that I am working on. I have a need to understand (to satisfy my own curiosity) the basic electronic ideas behind the theoretical signals stuff for FM...otherwise I don't feel like im getting what I want out of this work. I was also hoping for some real world answers...ones that some of my professors can't seem to give me sometimes...

I guess I have a few other questions...
So, in a basic sense, is the LC tank is still what we use today for RF oscillation (with the use of the varactor diode for the variable capacitance)? Have there been any advancements that make the LC oscillator obsolete? I know we have quartz oscillators, but I havent read about them being used. I guess I need to be reading more... :shock:

[quote author="PRR"]With an ideal amplifier, the output would get bigger and bigger to infinity. In real life, if you don't do anything else, the output rises until the amplifier can't make any more power, until its effective gain is less than the losses in the L-C tank.[/quote]

Are we talking about a system with poles on the right hand side of the complex plane, characteristically unstable? Sorry, I'm really trying to make connections here between class and life...

[quote author="PRR"]The Theremin is another FM oscillator. Fixed L-C plus the variable C of your hand near a metal plate or loop.[/quote]
Thats really cool...I never realized that.

Thanks a lot guys, I'm gonna read this like 50 bazillion more times, and I should understand it by then.
-Mike

quote: "Are we talking about a system with poles on the right hand side of the complex plane, characteristically unstable?"

We're talking (for a feedback system) about that pole pair being right on the j-axis, for lowest distortion---adding just enough energy in a timely fashion to make up for the dissipation losses and the energy extracted to actually do things. Otherwise any real system with poles in the right half-plane quickly go spiraling off in amplitude until you run into hard limits on the voltages and currents.

As far as FM generation: the direct approach uses a voltage-controlled oscillator, perhaps with the varactor diode(s) as the variable C(s) of the LC oscillator. But such oscillators don't have fantastic stability, so often we wrap them into a phase-locked loop with a crystal oscillator at the center frequency. Then we have a long lowpass time constant on the output of the phase detector and drive our audio signal into a summer with the phase detector output. The slow loop keeps the average frequency constant, equal to the ref oscillator.

It is possible to make a voltage-tuned crystal oscillator, but pulling that crystal the requisite deviation either side of center with good linearity is not terribly easy, and when you do, usually stability and phase noise suffer.

> some real world answers...ones that my professors can't seem to give

You're trying to learn radio from professors????

Get an old ARRL Handbook and read it; you'll learn more real-radio than any school knows.

> poles on the right hand side of the complex plane,

Huh? I dunno. Marconi and de Forrest and Armstrong didn't know either. Get a gut-level understanding of radio gizmos. Save the complex planes for complex problems, or to decorate academic papers.

> any real system with poles in the right half-plane quickly go spiraling off in amplitude until you run into hard limits on the voltages and currents.

Which shift that pole pair leftward (averaged over a cycle) until it sits right on the j-axis. The art is in making it shift gently and repeatably.

> Have there been any advancements that make the LC oscillator obsolete?

LC tanks are basic tools in RF; they give high impedance at the desired frequency, low everywhere else, have "flywheel effect", and can also absorb stray capacity. Golly, that was basic EE stuff not so many decades ago; have curriculums "progressed" so far that EE101 doesn't spend a day where you "assume all tanks have Q of 3 or more"?

> I know we have quartz oscillators

Quartz resonators are very old and more stable than Ls and Cs. Your first tank may be quartz to set the frequency, but all later stages use LC coupling for best efficiency. (Some trend to use cheap RC coupling in intermediate stages.)

Carso reminds me why broadcast FM used phase-shift. Broadcast frequency stability requires a quartz crystal for reference. You can't bend quartz linearly more than a few Hz. But you can shift the phase of a fixed-frequency signal. A guitarist's wah-wah pedal can do the job. You can generate a short-term frequency shift by constantly increasing the phase shift. But you can't stay off-frequency: when you run out of phase-shift you will be at the original frequency. If you low-pass (integrate) your audio and phase-shift the reference, you get a signal that is the same as FM, minus the ability to stay off-frequency long enough to get a citation.

These days I suppose they use a vari-cap and two PLLs. One applies feedback to correct vari-cap nonlinearity for low audio distortion; the other feedsback to keep the average frequency equal to a reference quartz. But this and newer techinques are really getting away from basics into economic cleverness.

Bad Bad move!

Do not knock the professors!

They may actually answering your questions.

I have a really good paper on oscillators from my High School electronics class written by a guy who could explain a Pentium 3 to a pototo. Let me see if I can dig it out.

[quote author="CJ"]I have a really good paper on oscillators from my High School electronics class written by a guy who could explain a Pentium 3 to a pototo. Let me see if I can dig it out.[/quote]

Thanks, that would be much appreciated!

[quote author="adrianh"]Bad Bad move!

Do not knock the professors!

They may actually answering your questions.[/quote]

Well, I didn't mean to be knocking any professors. They certianly know more than I do and are a valuable source of information. Hell, someday I hope to be one... What I meant by "can't answer my questions" was more like "I need someone who has hands on experience with this stuff, and can relate to idiots like me". Unfortunately a lot of professors at my school just talk at a level thats too over my head for me to get a huge amount out understanding out of it...not all, but most of them. At least this I can read over and over until I understand... :wink:

[quote author="PRR"]Huh? I dunno. Marconi and de Forrest and Armstrong didn't know either. Get a gut-level understanding of radio gizmos. Save the complex planes for complex problems, or to decorate academic papers.[/quote]

Yes sir!

> Well, I didn't mean to be knocking any professors.

I was. I do technical support for professors, so I know they need knocking.

> They certianly know more than I do and are a valuable source of information.

This is true. Though many professors "know too much" to have meaningful conversations with newbies. They know answers that you don't yet have questions for.

And their job isn't really teaching. MANY of the students admitted really won't make good EEs. Some don't have the right kind of technical mind. Some are "bright" but "brain-scattered" and need to grow up. Many college students drink their brain to death before Junior year.

So the job of the professors is to give exams to weed-out the students who are not going to make it into an EE life. To make the exam "fair", the instructors must spend 4 months before each exam explaining the questions. And while the ultimate goal may be to turn out partially trained EEs (most real EE training happens on the job), the real job is to collect some tuition money from wannabes, and then kick them out (most leave quietly) and spend that money focusing on the few who don't wash-out. Until your senior year, or even into grad school, you are just meat for the mill. And many of the concepts quickly taught and tested in an EE program are mathematical abstractions, not "how it works". How-it-works changes every few years, and the math stays good forever. Many students are OK at "how THIS works", but will be lost facing a new device. For the business of running a school, it is apparently better to avoid much practical detail, get the abstract concepts hammered-in, wash-out the weaklings, and hope the survivors manage to pull it together on the job.

PRR: OMG that is so accurate. I can imagine that you are regarded with some trepidation by the omniscient faculty, however indispensable you are to them.

One guy I know who designs amps all day long said he would only consider hiring an engineer fresh out of school if he/she had studied with either of two (!) people: Meyer at UC Berkeley or Blesser at MIT. Probably both of them have retired by now.

I've had indirect evidence of a few others who can provide a compelling mix of theory and practice, based on their writings. But you are right: it's a weeding process. There is never enough time, and it's only the talented and obsessed students that will pursue the subject with adequate intensity.

Sadly, there are hardly any experimenter/hobbyists coming in as undergrads anymore either, which used to give a leg up.

For the record:

Meyer is still teaching, evidently: http://www.ieee.org/organizations/pubs/newsletters/sscs/apr03/meyer.html

Barry Blesser has been a consultant for a while now: http://cartchunk.org:8080/consult.htm

Speaking of radio: a really great book, despite the inevitable errata, is Paul Nahin's The Science of Radio (Springer/AIP Press, 2nd Ed. 2001). I damn near didn't even pick it off the shelf at the bookstore, but once I started to browse I was won over swiftly.

I emailed Prof. Nahin to see if there was an errata list, but he has not responded. He may have good reason not to, or he may have a tendency like so many academics of not responding to any email lacking an .edu extension.

Robert Meyer ... distinguished teaching and mentoring methods... known worldwide for the distinction of his graduate students...

Grad, not undergrad. Undergrads are beneath everybody's notice. They pay the rent, but we hardly say "thank you".

Since... 1968, Meyer has supervised more than 20 doctoral students and more than 60 master's students who form a notable group of today's leading radio frequency integrated circuit designers.

OK, 2003-1968= 35 years, 20+60= 80 distinguished students, not even 2.5 distinguished students per year. For this he was paid ??? average $40K/year, and (at 15 students/faculty, $10K/year tuition) he was worth $150K/year. You can argue the exact numbers, but roughly: 2 or 3 students got $40K education, a dozen students got a sheepskin and a hasty handshake.

Meyer received his... degree{s} ... from the University of Melbourne, Australia.

Lot of good electronics-thinking comes from down under.

[quote author="PRR"]Lot of good electronics-thinking comes from down under.[/quote]

:green:
this is where I should jump in and give a list of important Audio Australians
some of them were involved with some or our favourite things in audio and DIY

:green:
but I won't

there is many great audio persons from all around the world and I hope in your research you may find and learn about people that have done and are currently doing good things in audio right in your neighbourhood.
Perhaps even your garage.
:thumb:

I have a really good paper on oscillators from my High School electronics class written by a guy who could explain a Pentium 3 to a pototo. Let me see if I can dig it out.

Click to expand...

Let's make an oscillator out of a potato......oh wait, someone already did. They call it VODKA! :green:

Dig it out....potato......ha, ha.

Why is the word potato so funny to me this morning?