Hello the most knowledgeable electronics group on earth!
I am trying to build a few LC oscillators for experimenting with mixers. I do not want to use Crystals, Frequency Synthesis, DDS, etc. Let's say 2 MHz and 100 MHz. I don't know where to start!
I have a few RF books and the treatment of oscillator can be one of the following:
1- Too much theory on the analysis of oscillator circuits, phase noise, IMD, etc. No circuits to build. Just demo circuits for illustration, sometimes only the small signal model.
2- Circuits to build but designed at very specific frequencies with very specific components. Very little on how to generalize things into different frequencies. (The ARRL handbook 2010 and Experimental Methods fall under this).
3- Books that give me a design procedure with S parameters but I can't find any S parameter files for the devices I'm interested in using.
4- Cookbooks that have the same problem as (2).
Why is it so hard to build an oscillator at an arbitrary frequency?
Idea 1: find a DIP meter schematic. They typically cover a fairly wide range using plug-in coils i.e. you just need to change the inductor. And it's often a single FET circuit.
Idea 2: You could cover quite a wide range using an unbuffered CMOS inverter (4069UB) with: cap from input to ground; cap from output to ground; inductor from input to output; maybe a resistor in series with the output. You should get a sinusoid at the inverter input. It'll need buffering to drive your mixer.
In both cases you should be able to cover quite a wide frequency range with suitable choices of L and C. You can lash it up on a breadboard.
But the upper frequency limit for the above circuits will be well below 100 MHz. You will need a different circuit for VHF.
Because oscillator design isn't easy, if you're going to go about it from an engineering perspective, then you need lots of theory. If you're going to go about it from a technician's perspective, then you need a lot of working examples to start with, and a lot of patience.
If you really want to wrap your head around the theory, then grab some books from (2) and (4), and apply what the books from (1) and (3) teach you to analyze the circuits.
I found that "Oscillator Design and Computer Simulation" by Rhea was very helpful. When you get right down to the nitty-gritty, Rhea's book shows you how to break the loop in an oscillator circuit, put it into a simulator (such as LTSpice), and do a sweep on it to analyze its characteristics.
I've also found that whatever magic LTSpice does to be efficient at analyzing switching power supplies must work for oscillators, because it's been very fast and accurate and simulating oscillator start-up.
My liberal friends think I'm a conservative kook.
My conservative friends think I'm a liberal kook.
Building an oscillator is easy. Keeping it from drifting around is a different story, but if all you need is something that emits radio frequency most schematics will work if you recalculate the tuning part according to the needed frequency.
I'm a big fan of the negative resistance oscillator made using two jfets, one N and one P channel, or a N jfet and a PNP BJT, called "lambda diode oscillator". It will work from audio frequency to the UHF just by changing the LC circuit. I made my first GDM using that circuit. Look for "lambda diode" in google images.
Well, assuming your building a receiver, you're starting your design in the wrong place. Mixers have very specific input level range requirements. Some of the higher level mixers, needed to obtain decent overload and intermodulation performance, require substantial amounts of power to operate. There are also many different types of mixers, ranging from simple D-type flip-flops, to messy double balance diode ring mixers. Mixers also like to see their favorite input and output impedances on every port and over the entire frequency range of operation. When almost the entire performance of the receiver is tied into the mixer, some effort should be directed in that direction.
In short, you need to select a mixer type and design before you can design the local oscillator. That will determine the output level, output impedance, and frequency range of the oscillator. Note the effort by the designer of this board to make sure all 3 ports see 50 ohms: Also, few designs drive a mixer directly from an oscillator because of "pulling" effects, where the mixer load affects the frequency of the oscillator. There is usually an isolation amplifier in between.
What I suggest is that you produce a better description of what you're trying to accomplish. Do the whole system on paper or computah before you begin designing sub-sections. A useful tool is AppCAD: where you can work out the gain distribution, levels, signal handling requirements, and noise figure requirements, before you build something. Incidentally, oscillator noise can be an important specification. If you're building an FM broadcast receiver, with a huge bandwidth, local oscillator noise is not very important. If you're building a SSB (single side band) receiver, close in oscillator noise is critical.
By the time you have the overall design worked out, the individual components can be more easily specified. Your oscillator will have a more specific frequency range, stability requirement, noise requirement, output level requirement, and DC power requirement. You may be able to get away with something plagiarized from a commercial receiver (plenty of schematics online), or you may want to design it from scratch.
Because oscillators are usually designed to a specification, which is seriously lacking. Also, because the oscillator is part of a receiver system, which is also lacking. Once you narrow down the type of oscillator and its approximate characteristics, things will be easier to design, analyze, or copy. Designing a universal oscillator, that works at all frequencies, into any impedance, with any power supply, and has superior noise performance, is just not going to happen.
Jeff Liebermann email@example.com
150 Felker St #D http://www.LearnByDestroying.com
** A free running oscillator is as accurate as YOU set it.
The real issue is *stability over time* or drift and that is nearly all due to temperature variations.
** With most AM receivers, one tunes the LO to the desired station by ear or the uses of an optical indicator. The allowed error is in the hundreds of Hz. Over time the LO may drift, then one retunes it.
** Communications on VHF, UHF and above pretty much depends on the use of crystal locked oscillators at both ends.
Operating in that frequency range was a challenge 50 years ago with germanium transistors. The parameter spread was very large, so making a reliable oscillator was hard. To sustain oscillation, the _power_ gain of the active element(s) must be larger than the losses in the resonator and other passive components. This gets hard, when the typical fT for an RF germanium transistor was 40-200 MHz.
These days transistors with much higher fT are available. Even some cheap MMICs have considerable gain up to 5-10 GHz. If the gain is not enough, cascading an other MMIC does not cost much. Of course, at frequencies at UHF and above, lumped LC resonators can not be used, stripline and microstrip constructions are often used.