LC Oscillator Questions

[Anthony Fremont]

Glad it worked out. By the way the feedback path is through the
capacitive network between source and gate. That would be more
obvious in the configuration that uses a tapped inductor, but works

I had to play around with this quite a bit to get it running near 10MHz. It
seemed to be more picky about the cap ratios than their actual values.

the same way. Leaving out the diode was the right thing to do; it
just adds to the noise. I don't know what kind of stability and

I figured that it would cause horrid clipping the way it was installed. I
thought maybe the designer intended it to be the other way around to protect
the transistor from reverse voltages on the gate, hmm.... always something
to keep you guessing. ;-)

linearity you need, but if that is important, do not use the common
type of ceramic capacitors that are meant for bypassing. They are
lossy, and their value varies with applied voltage. Use mica, NPO
ceramic, or Mylar and similar for larger values.

I don't really "need" anything in particular, it's just an exercise to try
and learn something. I just used the ceramic caps because they were handy.
I didn't have a pile of NPO caps laying around. ;-) I d

Time to add a buffer now since the scope probes load it down so badly now.
I'm getting 1.1Vpp into the apparent 5M load of two scope probes. It's
probably pulled a ton off frequency as well. Let's see.....(removes one
probe).....yep, 90KHz rise in frequency after taking one probe off.

 

[Tim Shoppa]

Pictures available in ABSE

The top trace (yellow) is taken between C4 and R2. The bottom trace (cyan)
is taken at the base of the transistor. There is a switchercad file, but
the simulation will show allot of distortion that really isn't present in
the prototype circuit, because of lots of circuit capactance I suspect. R1
was something I was playing with to try and tame the voltage across L1/C3
being applied to the base.

Hello all,

I was tinkering with this LC oscillator (Colpitts/Clapp) this weekend. I
arrived at the values of C1 and C2 empirically after starting with a crystal
oscillator circuit. The values in the original circuit created a horrid
waveform that looked allot like the simulation. After much tinkering around
and simulating, I come to the conclusion that getting a perfect waveform is
nearly impossible, especially with big swing. It seems that the transistor
likes to take a bite out of the right half of the peak of the wave.

What is the secret to beautiful waveforms? Do I need another LC resonator
on the output to fix it up? I mean, I'm getting a pretty nice wave now, but
there is still some distortion that you can just see at the top of the peaks
on the yellow trace.

How do you control the peak voltages of an LC resonattor without mangling
the waveform? The waveform at the junction of L1/C3 is of course quite
beautiful, how do I get it from there to the output? ;-)

Oscillators have to have gain greater than one at the frequency of
oscillation.

When turned on, the amplitude builds up until something in the circuit
cuts back the gain. In simple oscillators, that "something that cuts
back the gain" is almost always the active device saturating and
distorting its output.

The higher your gain, the more reliable the oscillator starting up,
but also the higher the distortion.

If you take the output not from the output of the active device, but
from a lightly-coupled tank, then you'll see something much more like
the sine wave you were expecting. This is what you see at the L1/C3
junction. But still you'll get lower distortion there if the active
device isn't driven so far into saturation/distortion. And by
definition you cannot suck much power out of the L1/C3 junction
without decreasing the Q of the tank and making distortion there too.

You can add a few more active devices and not only buffer things but
also put a fairly linear AGC in the loop. This still has distortion,
but this is done intentionally in a rectifier to derive the AGC
control voltage, which is then filtered. The intentional distortion
does not have to appear in the output!

Clever use of devices can make the AGC loop quite beautiful. Look at
the Wien Bridge or Meacham Bridge oscillators that use a light bulb in
the bridge to not only be the loop-control device but also do
filtering (thermal time constant of the filament).

Tim.

 

[Anthony Fremont]

Anthony Fremont wrote:

In some LC oscillators, the amplitude of the oscillation is
controlled by a feedback loop. For example, a rectifier can be used
to create a DC voltage proportional to the oscillation amplitude on
the LC tank, and then an op-amp can be used to compare the rectifier
output signal to a reference voltage. The output from the op-amp can
be filtered and then used to control the current in the oscillator
core. It is difficult to do all of this in a way that keeps the
phase noise low, but given the right simulation tools (e.g. SpectreRF
which is rather expensive), good results can be obtained. In
particular, a well-defined oscillation amplitude can help to keep the
KVCO well controlled, which is useful in PLLs.

Thanks for the reply. Sounds pretty slick. Way out of my league though.
;-)

 

[Anthony Fremont]

The prettiest waveforms come from balanced oscillators. Distortion
then turns up as 3rd 5th 7th etc harmonics which are far less ugly
than the 2nd 3rd 4th 5th etc generated by the single ended types.
Balanced ALC is also easier and more effective.

The "conversation" that L1 and C3 sure looks nice on the scope.

My own experience says that 'prettier' is better. Those oscillators
offering gross distorted outputs also seem to suffer badly in other
areas and gross distortion always causes problems further down the
line.

Procuring good quality is a classic black art, one aspect is to
allow the LC just an occasional vague glimpse of the maintaining
amplifier. Another is to cause limiting by use of an amp having a
gentle gain change (eg Fet v bipolar) and the other is ALC. (Or all
three together).

Well there sure isn't much talk about it out there. Material I find is
like, "here's a schematic, pick a coil and cap and your done. No one seems
to care what the result looks like. Seems like you can make a reasonably
decent wave _and_ still have the oscillator start reliably.

Failing that, there is always the cop-out of an output filter

Seems to be the way people like to do it. ;-)

 

[Anthony Fremont]

Oscillators have to have gain greater than one at the frequency of
oscillation.

When turned on, the amplitude builds up until something in the circuit
cuts back the gain. In simple oscillators, that "something that cuts
back the gain" is almost always the active device saturating and
distorting its output.

The higher your gain, the more reliable the oscillator starting up,
but also the higher the distortion.

If you take the output not from the output of the active device, but
from a lightly-coupled tank, then you'll see something much more like
the sine wave you were expecting. This is what you see at the L1/C3
junction. But still you'll get lower distortion there if the active
device isn't driven so far into saturation/distortion. And by
definition you cannot suck much power out of the L1/C3 junction
without decreasing the Q of the tank and making distortion there too.

It seams reasonable that if I can look at the junction with a scope and the
wave looks good, I should be able to tap it with a secondary JFET without
destroying it. Yet I see no examples of that being done. I guess it's just
easier to accomplish the waveform repair by using a tank on the output of
the oscillator and not loading down the primary tank circuit.

You can add a few more active devices and not only buffer things but
also put a fairly linear AGC in the loop. This still has distortion,
but this is done intentionally in a rectifier to derive the AGC
control voltage, which is then filtered. The intentional distortion
does not have to appear in the output!

This sounds like what Chris Jones was talking about. Do you have a link so
I could check it out?

Clever use of devices can make the AGC loop quite beautiful. Look at
the Wien Bridge or Meacham Bridge oscillators that use a light bulb in
the bridge to not only be the loop-control device but also do
filtering (thermal time constant of the filament).

Clever stuff.

 

[Tony Williams]

In message <[email protected]>, Anthony Fremont

What is the secret to beautiful waveforms? Do I need another LC resonator
on the output to fix it up? I mean, I'm getting a pretty nice wave now, but
there is still some distortion that you can just see at the top of the peaks
on the yellow trace.

Symmetry is a nice word to use where LC resonance is concerned.
Symmetry for the drive and clipping mechanisms.

Have a look at a long-tail transistor pair with cross-coupled
collector-base feedback resistors, and collectors driving a
centre-tapped LC resonant circuit.

 

[Tim Shoppa]

This sounds like what Chris Jones was talking about. Do you have a link so
I could check it out?

A "classic" oscillator with AGC is the Sulzer Oscillator. Very nice
pics and schematics at

http://leapsecond.com/museum/sul25-1/

Clever stuff.

For really nitty-gritty stuff about low-distortion oscillators, see
Jim Williams' examples in Linear Technologies appnote AN-43. (Go to
http://www.linear.com/ and do a search for "AN43" without the hyphen).
Most of the examples there are bridges, and distortions in the
sub-0.1% category are achieved AND THEN IMPROVED UPON BY FACTORS OF
HUNDREDS! Also google "Meacham Bridge" and "Wien Bridge".

Tim.

 

[Tim Shoppa]

This sounds like what Chris Jones was talking about. Do you have a link so
I could check it out?

In addition to the low-noise-low-distortion-high-stability URL's I
pointed you towards in my other followup, recent ARRL Handbooks have
some really clever low-noise VFO circuits using a multitude of
approaches, including explicit AGC circuitry. I have been slowly
working my way through the cookbook examples and every example has its
merits.

In typical ham use, for better or worse, stability and reliability to
start-up are often the most important criteria. What you are
complaining about when you see a distorted output, is something that
is actually a design goal of oscillators that are followed by
multipliers.

One very common method over the years of decoupling the frequency-
determining tuned circuits from other frequencies generated in a radio
is to run the oscillator grid tank at half the output frequency and
depend on distorition to make the desired output frequency. In the
simplest case a balanced or push-pull oscillator is a "No-No" because
you WANT the second harmonic. The electron-coupled oscillator that was
in the 50's/60's/early 70's handbooks is a classic design.

Tim.

 

[K7ITM]

Well there sure isn't much talk about it out there. Material I find is
like, "here's a schematic, pick a coil and cap and your done. No one seems
to care what the result looks like. Seems like you can make a reasonably
decent wave _and_ still have the oscillator start reliably.

Some people worry a whole lot about it. What they worry about,
typically, is phase noise first and stability second. Lack of
harmonics in the waveform are generally lower on the list. That's
because filtering out harmonics is relatively easy, compared with
cleaning up phase noise and stability problems. You can find lots of
articles on minimizing phase noise, but there's also quite a bit of
trade-secret sorts of knowledge whose owners aren't particularly
interested in sharing, understandably. It's far from trivial to get
the phase noise and spurious performance you'll find in the good
commercial signal generators. Good as they are, though, I know of
none that's good enough to be used without harmonic filtering to do
low-level harmonic distortion measurements.

Cheers,
Tom