[time-nuts] Microstepper

[Dana Whitlow]

Thanks, Bob, for clarifying what Magnus means by "microstepper".  To me,
the term has always referred to operation between the discrete steps defined
by the polepieces and windings.  One can do it (albeit poorly) with many
stepper
motors.  There is a class of servo motor in which the designers of the
magnetics
went to great lengths to avoid salient poles (think magnetic detent-like
effects),
which are meant to operate with I & Q controlled currents.

BTW, the "phase noise" associated with a DDS is not entirely random.  In
fact
most is usually from systematic effects and can be eliminated or at least
dodged
by restricting one's choice of frequencies.

Dana


On Wed, Dec 25, 2019 at 8:00 AM Bob kb8tq <kb8tq at n1k.org> wrote:

> Hi
>
> Different sort of micro stepper. The 3D printing version runs a stepping
> motor
> to improve the angular resolution. As you mention, the poles in the motor
> limit
> just how well you can do. There are dedicated chips out there these days
> that
> get down below the limits on any motor I’ve ever seen. They are available
> on pre-assembled boards for not a lot of money per board.
>
> The micro stepper Magnus is after is one to adjust the phase of the output
> of
> his shiny new (to him) maser. Same name, different beast.
>
> Bob
>
> > On Dec 24, 2019, at 9:03 PM, Dana Whitlow <k8yumdoober at gmail.com> wrote:
> >
> > Magnus,
> >
> > Why not just clock a good DDS (AD9854) with the reference frequency, and
> > run its
> > I & Q outputs into the motor via suitable LP filters and some power gain?
> > You might
> > need to periodically alternate between two different output frequencies
> to
> > get the desired
> > rotation speed (as with a fractional N divider), but with the 48-bit DDS
> > I'd think that this
> > would work well.  After all, the magnetics in a motor are not all that
> > accurate anyway-
> > one cannot get perfection in avoiding salient pole effects etc.
> >
> > Dana
> >
> >
> > On Tue, Dec 24, 2019 at 6:02 PM Magnus Danielson <magnus at rubidium.se>
> wrote:
> >
> >> Hi,
> >>
> >> I realize that I lack a microstepper. Consider that I have a stable and
> >> low-noise 5 or 10 MHz but I want to resynthesize to correct frequency
> >> and do phase-steps, and doing so without too much loss of noise.
> >>
> >> This has traditionally been done using a variation of techniques, but if
> >> we would use some of the things that happened lately, pretty OK
> >> performance should be possible to achieve without too much hardware.
> >>
> >> OK, so after a discussion with Bob, here is one sketch for a
> >> possibility, just to toss one proposal to crush into pieces and propose
> >> improvements or better versions.
> >>
> >> So, consider using a modern Silabs chip clocked from an oscillator,
> >> producing a offset generator with I/Q and then do an I/Q mixdown to a
> >> beat frequency, which is digitized by a pair of ADCs. A pair of DACs
> >> produces I/Q which is used to mixed to produce the output signal using
> >> the signals from the Silab. The ADCs/DACs can be either be fed to some
> >> CPU platform or FPGA. With this platform one can choose to servo the
> >> reference oscillator, or just modify the beat received. Maybe use a
> >> Raspberry Pi as platform. Rotating the vector and steering the rate of
> >> rotation should not be extremely hard to do. The input vector can be
> >> added internally or used to steer the oscillator. Either way, the in
> >> loop noise from the Silabs will be fairly well suppressed since it only
> >> acts as a transfer oscillator.
> >>
> >> So, suggestions, thoughts and improvements?
> >>
> >> God Jul and Merry Christmas!
> >>
> >> Cheers,
> >> Magnus
> >>
> >>
> >>
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