Oscillator Fun! - Page 1
Forum section: electronics / projects

author: vxp036000
author_rank: Regular Contributor
timestamp: May 04, 2012, 10:08:09 pm
content: So, I have two oscillators designed to operate at exactly the same frequency, but with vastly different topologies. Which would you choose and why? I'm still learning, so there are no right answers :-)

author: nctnico
author_rank: Super Contributor
timestamp: May 04, 2012, 11:40:21 pm
content: Quote from: vxp036000 on May 04, 2012, 10:08:09 pmNeither if you don't tell what you need an oscillator for

author: vxp036000
author_rank: Regular Contributor
timestamp: May 05, 2012, 12:00:32 am
content: Lol! That's part of the fun. I'm thinking along the lines of a general purpose RF source with clean spectral output. Something like a UHF VCO (I'm going to ignore dedicated ICs for the time being).Quote from: nctnico on May 04, 2012, 11:40:21 pm

author: jimmc
author_rank: Frequent Contributor
timestamp: May 05, 2012, 01:06:50 pm
content: I've never liked the common base circuit as shown, the circulating current in the tuned circuit has to flow through the power supply leading to poor performance. Simply moving the lower end of C1 from ground to the supply rail improves things.I can't see the second circuit as a Colpitts.My own favourite Colpitts is the common collector eghttp://how-to.wikia.com/wiki/How_to_build_an_oscillator_circuit.This has the advantage of having one end of the inductor earthed which makes replacing it with a resonant line or adding varicap tuning much easier.See fig 7 here for an example of a Colpitts running at UHFhttp://www.skyworksinc.com/uploads/documents/200316A.pdfJim

author: EmbeddedEric
author_rank: Contributor
timestamp: May 05, 2012, 02:25:27 pm
content: Cool timing, I actually just finally got my JFET based Oscillator up and running last night and wrote about it this morning. Another topology for you to evaluate.http://embeddederic.blogspot.com/2012/05/rf-isnt-black-magic.html

author: nctnico
author_rank: Super Contributor
timestamp: May 05, 2012, 03:23:13 pm
content: Quote from: vxp036000 on May 05, 2012, 12:00:32 amAFAIK the biggest trick is to get an oscillator compensated for temperature changes. An interesting experiment is connecting 2 identical crystal oscillator modules to an oscilloscope and use the X-Y mode to show the phase and frequency difference. Just blowing some air on one on them is enough to see the changes.

author: vxp036000
author_rank: Regular Contributor
timestamp: May 05, 2012, 04:48:09 pm
content: I guess I should have clarified that in any practical implementation, both of these circuits would have a blocking cap from Vcc to ground and possibly even an RFC to provide isolation from the supply. No signal path through the supply line.Your probably right about the second oscillator not being Colpitts, closer to Hartley. The downside I see in the second architecture is that there is a spurious resonance at sqrt(2) w0. I'll let you figure out whySo much for a clean output waveform.Quote from: jimmc on May 05, 2012, 01:06:50 pm

author: vxp036000
author_rank: Regular Contributor
timestamp: May 05, 2012, 05:18:05 pm
content: That's an interesting concept, buffering the feedback through J2. The one thing I don't like is that this oscillator requires two transistors. Is there an advantage to this over a conventional single transistor design?Quote from: EmbeddedEric on May 05, 2012, 02:25:27 pm

author: vxp036000
author_rank: Regular Contributor
timestamp: May 05, 2012, 09:53:24 pm
content: Here is another common textbook oscillator. We'll see how it stacks up; haven't finished evaluating the open loop phase and gain yet.

author: vxp036000
author_rank: Regular Contributor
timestamp: May 05, 2012, 10:11:09 pm
content: What do you know, another poor topology. Plotting the open loop phase and gain clearly shows a stray resonance at w0 / sqrt(2). It will be interesting to see if the Hartley configuration for a common collector oscillator has better spectral purity. Here is a bode plot, so you can see what I'm talking about.

author: vxp036000
author_rank: Regular Contributor
timestamp: May 12, 2012, 12:16:06 am
content: In case anyone is interested, here is the general approach I use in evaluating oscillator configurations. I'm working through a few Hartley oscillators to see if they're any better with respect to spurious outputs and phase stability.

author: jimmc
author_rank: Frequent Contributor
timestamp: May 12, 2012, 04:53:49 pm
content: Quote from: vxp036000 on May 05, 2012, 04:48:09 pmYou misunderstand me a little, if you feed a current (I) to a simple parallel LC network at its resonant frequency then the current in both L and C will be Q times I.Where Q is the quality factor of the tuned circuit. (Energy is exchanged between them during each cycle)Take an example...For a particular tuned circuit at resonance XL= XC= 10ohmsIf the Q of the inductor is 100 and the Q of the Capacitor is very high (>>100)then at resonance the tuned circuit will look like 1k resistor (Q.XL= Q.XC= 100x10)Stick a volt across the tuned circuit an the current will be 1mA but the current through the inductor or capacitor will be 1v/10ohms = 100mA.Again if you try injecting a small voltage (V) in series with the capacitor you will find the voltage across the tuned circuit is Q.VMy point was that moving C1 as I suggested would reduce the current passing through the supply decoupling by a factor of Q and also reduce the susceptibility to ripple at the resonant frequency by the same factor.JimOops...Forgot to add that as well as moving C1 to the positive rail, C3 should also be moved. ie all refer all RF paths to the positive rail.If you want thee RF ground to be the negative rail, switch to a PNP transistor and swap the rails or use the common collector circuit.

author: vxp036000
author_rank: Regular Contributor
timestamp: May 12, 2012, 06:06:58 pm
content: Hmm, that's an interesting point. I'm tempted to build it up and see if practically speaking it makes much difference.Quote from: jimmc on May 12, 2012, 04:53:49 pm

author: zeino
author_rank: Newbie
timestamp: January 07, 2016, 10:00:17 am
content: Do you know why in * OscillatorFundamentals2.pdf" shared by vxp036000 when breaking the loop of the common emitter Oscilator it includes the RFC in the analysis? From which book or document is this analysis from. I am a little confused on how it breaks the feedback and includes the RFC in the frequency analysis. Can anyone help me on understanding it.