[TowerTalk] Steppir tuning noise

[Jim Lux]

----- Original Message -----
From: <K3BU at aol.com>
To: <Towertalk at contesting.com>
Sent: Friday, February 20, 2004 4:53 PM
Subject: Re: [TowerTalk] Steppir tuning noise


> In a message dated 2/20/04 8:55:32 AM Eastern Standard Time,
> jimlux at earthlink.net writes:
>
> > >>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)<<
> >
> >
>
> I am surprised that they would live with noise generated by the drive
system.

It's just one of those design decisions, I suspect.. Spend years working out
all the last little foibles, or get the product on the market where it
accomplishes 99.99% of the ideal and get some practical field experience.

>
> There should be no capacitive coupling between windings and elements.

The elements are physically close to the motor case. The windings are close
to the case as well. There's also the lead wires.  There are motors with
shielded multipin connectors running into shielded multiconductor cables,
etc., but they're very expensive, and might not help anyway.  Inexpensive
run of the mill steppers (with decent temperature ranges, etc.) are probably
around $50, brand new, in big quantities.  Shielded steppers with the fancy
connectors are more like $500 or $1000.  Fluidmotion would have a hard time
selling $10,000 antennas, and I suspect that folks would balk at spending
dollars/foot for high performance shielded control cables as well.


Consider that the antenna is feeding a receiver that has a sensitivity of
something like  -140 dBm. At the 50 ohm nominal impedance, that's a few
nanovolts.  If the transients on the stepper leads are 12 or 24 volts, you
need some pretty impressive shielding and isolation. (100 dB might not be
anywhere near enough).

What's a trivial amount of transient and ringing in the digital world (say,
0.5 Volt on a 5V TTL/CMOS signal) is a huge amount when talking about
sensitive receiver front ends with submicrovolt sensitivity.   I've spent a
good number of months over the years trying to keep digital noise out of low
level RF, and it's always a battle.  That EMI filter connector may seem
impressive at 50 dB attenuation, until you run the numbers.

For the momentary noise when tuning, it's a decent tradeoff to not worry
about it.

> Windings are completely enclosed. By using shielded cables the crosstalk
between the
> cables should be minimized.

I don't think it's crosstalk between cables.  I would speculate that it's
radiated EMI from the wiring, picked up by the antenna.

 I have not seen SteppIR antennas, so just
> speculating and surprised that was not eliminated.
>
> Yuri, K3BU


Jim, W6RMK