Hi David, thanks for that succinct explanation. The SSC is a "fudge" to
defeat the measurement technique, which has nothing to do with the
psychological effect of the interference, or the BER effect on the
legitimate user. In fact it is often easier for the legitimate user of
the frequency to avoid a fixed clock harmonic than deal with one
spluging all of the band. It legitimises poor design!
Alan Melia G3NYK
RSGB EMC Committee mmember
On 21/05/2019 18:00, Dave B via time-nuts wrote:
Date: Mon, 20 May 2019 20:41:51 -0500
From: "Bill Byrom" <[email protected]>
To: [email protected]
Subject: Re: [time-nuts] timing properties of "spectru spreading"
clocks
Message-ID: <[email protected]>
Content-Type: text/plain;charset=utf-8
SSC (spread spectrum clocking) is widely used in both commercial (servers etc.)
and consumer products. The big advantage is at harmonics of the spread clock,
where the higher frequency energy can more easily escape the product case,
shielding, and cables. CISPR (EU) standards in general specify a receiver
bandwidth of 120 kHz for signals below 1 GHz and a receiver bandwidth of 1 MHz
for signals above 1 GHz.
<Snipped>
------------------------------------------------------------------------
Hi.
"Spread Spectrum" clock's were designed as a quick and dirty fix for out
of specification emissions from all sorts of digital based products (and
some non digital!)
It does *not* reduce (or increase) the level of leakage, it just spreads
it over a wider spectrum. (Never mind the quality, feel the width!)
It was specifically designed to "Fool" the Quasi Peak (QP) detectors
used in the EMC Emissions test receivers, so that a lower "Under the
limit" reading is produced.
Such emissions tests are usually done in two stages. A fast pre-scan
with a Peak detector. If the product passes that, job done. If any
signals extend over the Peak threshold limit, then those frequencies are
again tested with the QP detector, that is quite slow to respond. If
the signal is still over the limit, it's a fail. If it's now under the
QP limit (as it often is for SS signals) it's a pass.
The limits are there to protect broadcast signals, not to limit
interference to other systems, or small signal communications links.
In reality, it's a "bean counter" induced fix to poor technical design.
"Short term cost saving".
Just look up the history of the QP detector, it's utterly inappropriate
for what it's now being used for. It's original purpose was to get some
sort of quantitative and repeatable measure, for pulse interference to
AM broadcast signals, such as that experienced from poorly suppressed
spark ignition systems in passing vehicles. That of course go past and
away after a short spell.
Declaration:
I work in the EMC "Industry".
I'm also a government licensed Radio Amateur, who's utterly P'd off with
the RF pollution from poorly designed and/or implemented digital systems
and switched mode power converters.
Such levels of wide band interference is already starting to bite the
hand that feeds it. Some LED lighting systems can and do wipe out
terrestrial DAB reception! Of course, it's not the LED's themselves,
but the SMPS based "drivers".
The EMC rules are lax, and they are not monitored, let alone enforced in
the marketplace. A lot of the industry is fed up with it all too.
The soap box is creaking.
73.
Dave B G0WBX.
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