[TowerTalk] Steppir tuning noise

[Jim Lux]

----- Original Message -----
From: <K3BU at aol.com>
To: <towertalk at contesting.com>
Sent: Friday, February 20, 2004 5:23 AM
Subject: Re: [TowerTalk] Steppir tuning noise


> In a message dated 2/20/04 8:09:41 AM Eastern Standard Time,
> smidtca at sprint.ca writes:
> >>Not if you are lucky enough to have a SteppIR, but for the other beams,
> sure. Two seconds of low level hash and then 1:1 VSWR, boy am I glad that
I
> dumped my old fashioned, fixed tuned, beam.<<
>
>
> I wonder where is hash coming from. Stepping motors do not have brushes,
they
> are driven by pulses, but that is VLF. Could it be static from tape moving
> inside fiberglass tubes?
> Jus' wondering.

Pulses with fast edges and ringing.  Could easily have noticeable spectrum
up into HF, especially since it's fairly high power, and it's coupled pretty
well to the feed line (capacitance from windings to element, if nothing
else)


Also, it could be that fluidmotion is using a PWM stepper driver, which also
puts out a fair amount of HF noise. Steppers are very inductive loads, and
driving them from a constant voltage source has all sorts of problems.  You
need high voltage to get the current flowing quickly, but without doing
something special, the current keeps rising, and will overheat the windings.
Low buck drivers use a big resistor in series (which works, but limits your
max step rate because of the LR time constant, and also results in
disspating most of your powersupply power in the resistor).  Some
controllers synthesize a waveform with a leading edge spike in voltage,
followed by a long back porch (usually done with a combination of a resistor
and a capacitor).  State of the art, these days, is to use a nifty driver
chip from Allegro (and others) which is a constant current source (or even
fancier, a shaped pulse current source) made with a fairly standard PWM
controller.

>
> Yuri, K3BU.us
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