Anthony Fremont wrote:

Tim Wescott wrote:


The secret to a beautiful waveform is -- you usually don't need it
straight from the oscillator.


Okay..


There are a lot of things that you want out of an LC oscillator. Low
phase noise, frequency stability, consistently strong oscillation,
pure tone, etc. Of these, the only two that you can't clean up later
in the following amplifier chain is low phase noise and frequency
stability. Concentrate on those, & don't sweat the nice waveform.


Okay, that certainly explains why all the sample circuits I find don't
expend any great effort at creaing a nice sine wave, and none at explaining
why. What you say certainly makes sense, especially if there are no really
negative consequences of having the oscillator make a "less than perfectly
shaped" wave.


Frequency stability and phase noise performance are often achieved by
intentionally designing the amplifier so the active element operates
in class C, without ever going into voltage saturation. This keeps
it's drain (or collector) impedance high, yet delivers a large
voltage swing to the gate (or base) to keep phase noise low. It also
gives you a more or less consistent standing voltage in the tank,
which helps the design of the following buffer stages.

If you absolutely positively must tap the World's Most Beautiful Sine
Wave off of the oscillator section, consider a parallel-tuned tank
that's loosely coupled to the active element. Then loosely couple
your output tap to that -- it's your best chance.


Ok, thanks for the information. :-) I did allot of googling but found
nothing that explained it like this. I was thinking of building a little
single conversion superhet WWV receiver for 10MHz, if I continue with that
I'll just concentrate on cleaning it up in another stage.

Some material I read suggested keeping Xl of L1 at ~300Ohms, the series Xc
(C3) at ~200Ohms and Xc of C1/C2 at 45Ohms. Do you have any thoughts on
that? Right now I have way too much inductance for 3.5MHz by that theory,
and judging from other circuits I've seen. 10uH seems to be the going thing
for around 4MHz?


That sounds more or less right. With a Clapp oscillator the main tank
is isolated by the series cap, so more of the energy is kept in the coil
and C3, and less of it shows up in C1, C2, and the transistor.

If you're driving a balanced mixer you want to have an LO signal that
doesn't have much even-harmonic (2nd, 4th, etc.) energy in it, but for a
casual receiver that's the least of your worries. Since you're
operating at a fixed frequency it may be a good idea to just feed the
oscillator output into a single-tuned resonant circuit to clean it up,
then send it on to the mixer.

--

Tim Wescott
Wescott Design Services
http://www.wescottdesign.com

Posting from Google? See http://cfaj.freeshell.org/google/

"Applied Control Theory for Embedded Systems" came out in April.
See details at http://www.wescottdesign.com/actfes/actfes.html