Re: Yo! RF dudes!

Mike Monett wrote:

John Larkin <jjlarkin@xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx> wrote:

[...]

> The oscillator is coarse-tuned by the flecap at powerup, and then
> the varicap takes over. We need to keep the varicap pull range
> fairly small to meet our noise/jitter requirements. What we don't
> like is that the inherent TC of the oscillator was tuned to near
> zero, and a minor layout change messed it up enough to lose lock
> at temperature extremes. Again, I have a trim dac and a
> temperature sensor on board, and the firmware is tweaking the
> baseline varicap voltage to mostly null out the temperature drift,
> but we don't understand what's going on.

Sounds like the poor varicap is pushed to the limit. How about a
slightly larger one if it would still allow you to meet the phase
noise. Or maybe several in parallel. Called Massively-Parallel.
Rohde shows examples in figures 5 and 8 of

http://www.synergymicrowave.net/articles/PDF/Voltage-Controlled-
Oscillators.pdf (132KB)

(sorry for the wrap)

Also, if I'm not mistaken, the flecap has pretty low Q. This might
also be worth looking at to try to reduce the effect. Other circuit
arrangements might improve the phase noise by reducing the loading
on the tank.

I found using a simple constant current source in place of the
emitter resistor in a Colpits really improves the tank Q under load.

It also helps align the emitter current pulse with the peak of the
sine wave from the tank, which improves the phase noise. Hajimiri
and Lee discuss this in

http://www.chic.caltech.edu/Publications/phase_tutor.pdf

Also, Rohde patented a simple bias stabilization technique shown in
figure 15 of

http://www.synergymicrowave.net/articles/PDF/Oscillators.pdf (915KB)

I believe Matjaz Vitmar, S53MV, also described it long before Rohde
applied for the patent.

I've tried it, and it works very well. Other feedback arrangements
to stabilize the bias also help improve the phase noise.

So there's lots to look at that might help.


Ulrich Rohde's book "Communications Receivers" is one that I find indispensable for such noise critical jobs. If it just didn't have such an ugly green cover after the dust cover fell apart.


[...]

> But who can "Build, debug document and ship in an afternoon."?

Depends on how much you can cut and paste from previous projects.
Betcha Joerg does it all the time:)


I try not to ;-)

The only true race I did was when a client's service technician got seriously sick and almost a dozen big machines had piled up before I got there. That was true pit stop action. The worst was one machine that I had to repair over the lunch hour because the trucker was waiting outside the building to take it back, with his Diesel running.

--
Regards, Joerg

http://www.analogconsultants.com
.

Relevant Pages

  • Re: Yo! RF dudes!
    ... > The oscillator is coarse-tuned by the flecap at powerup, ... We need to keep the varicap pull range ... Also, if I'm not mistaken, the flecap has pretty low Q. This might ... which improves the phase noise. ...
    (sci.electronics.design)
  • Re: VCO control with AVR tiny/mega?
    ... Phase noise and jitter of the VCO - I don't have the figures, ... the oscillator of the SA612 in a LxColpitts configuration. ... the oscillator is tuned with a varicap ...
    (comp.arch.embedded)
  • Re: Yo! RF dudes!
    ... > caps, a Maxim flecap (coarse tune), and a varicap ... > I'd rather fix the inherent oscillator TC. ... > linear-with-temperature compensation voltage to the varicap, ... > nasty TC that varies with capacitance, so this compensation winds ...
    (sci.electronics.design)
  • Re: Yo! RF dudes!
    ... > caps, a Maxim flecap (coarse tune), and a varicap ... > I'd rather fix the inherent oscillator TC. ... > linear-with-temperature compensation voltage to the varicap, ... > nasty TC that varies with capacitance, so this compensation winds ...
    (sci.electronics.design)
  • Re: Yo! RF dudes!
    ... linear-with-temperature compensation voltage to the varicap, ... nasty TC that varies with capacitance, so this compensation winds ... There has to be some crystal oscillator nearby. ... is why my linear oscillator TC, compensated by a linear temperature ...
    (sci.electronics.design)