There are two reasons for spread spectrum techniques.
One contributed to by the lady movie star, for avoiding detection and
interference and which is used by all sorts of digital communications today as
directed by several standards.
The other, patented by IBM, is used deliberately to reduce QP measurements
within the QP defined parameters of nominally steady state emissions, such as a
clock.
The second was originally implemented in an analog fashion using a version of
FM, as in radio. The not so famous "hershey kiss" shape for time spend per
frequency unit.
The first was done digitally jumping among a list of random frequencies for
brief moments.
A hybrid came about with analog FM being implemented digitally by a list of
frequencies describing a type of shape over time.
A common interpretation by a test house is looking at the peak hold of a QP
detector over time and frequencies and comparing to the limits.
However, a QP measurement is done at single frequencies, each one individually,
at the defined BW, and time dwell. ps. if the excursion over the limit is
seldom, rarely, an argument that it is a "click" can used. Some devices have a
brief overshoot when jumping among frequencies. Get another device, but by
time this is discovered, one may be somewhat committed to that device. That
sucks.
A "hershey kiss" shape will give a flat top smooth shape on the peak or QP hold
detector.
Analog FM will give a smooth shaped top where a digital modulation will give an
indication of the individual frequencies in the list by the bumpy top.
A common appearance of any mod but the "hershey kiss" looks like batman ears.
The frequency high and low ends of the modulation will be a bit higher than the
middle due to added dwell time within the BW. Those two ears are often the
failing frequencies.
Now all the debating and teeth gnashing begin.
Remember it is the manufacturer, not the test house, that is responsible.
And here is a real life story.
Long ago, worked on a system that operated at 58 kHz +/- 200 hertz with about
15 kW of energy.Was informed by an outside agency, that it was well over the
limit around 5.8 GHz.
Never previously measured there as not required nor anticipated.
Put the system in a reverb chamber with the receiver, detector in the diagonal
corner.
Saw nothing initially. BUT on peak hold and a long scan, overnight, some
measurements over the limit were detected. Eventually the bandwidth detected
displayed an amazing range of frequencies for nominal 400 hz. The combination
of brief transmissions with the scan rate of the spec an, made detection rather
rare.
Asked agency what standard is required to meet?
No answer. They went away.
On Tuesday, December 7, 2021, 2:21:43 AM EST, Amund Westin
<[email protected]> wrote:
Thanks guys.
I checked out the actual case yesterday and was told that the reason for
observing QP for a longer time was that the product had used a spread spectrum
technique.
Best regards Amund
Fra: Paasche, Dieter
Sendt: 6. desember 2021 16:35
Til: [email protected]; Amund Westin <[email protected]>
Emne: RE: [PSES] Max Hold Quasi-peak ?
Hi,
I agree with the statements mentioned. It depends on the nature of the
disturbance and you would have to find out what is happening during that period
of time. I would also suggest to review CISPR 16-2-1, specifically to the
definition of continuous disturbance.
Section 3 continuous disturbance
RF disturbance with a duration of more than 200 ms at the IF-output of a
measuring receiver,
which causes a deflection on the meter of a measuring receiver in quasi-peak
detection mode
which does not decrease immediately
Section 6.5 Interpretation of measuring results.
6.5 Interpretation of measuring results
6.5.1 Continuous disturbance
The following steps shall be applied when interpreting the results for
continuous disturbance
measurements:
a) At each frequency for which the level of disturbance is close to the limit
and not steady,
the reading on the measuring receiver is observed for at least 15 s for each
measurement;
the highest readings shall be recorded. Some product standards allow the
exclusion of
isolated clicks, which shall be ignored (e.g. CISPR 14-1).
There are more conditions in this sections.
Sincerely,
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Dieter Paasche
Senior Product Developer
Electrical / EMC Compliance
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[email protected]
Work: (519) 744-8005 Ext. 7211
www.christiedigital.com
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809 Wellington St. N.
Kitchener, Ontario
N2G 4Y7 Canada
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From: Charles Grasso <[email protected]>
Sent: Monday, December 06, 2021 10:08 AM
To: [email protected]
Subject: Re: [PSES] Max Hold Quasi-peak ?
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Interesting topic!
Completely agree with Brent!.
I would only add the "quality" of the signal can also contribute to the
variation - by that I mean if there
is any kind of modulation on the signal then that will contribute to the
measurement difficulties.
It is also not unusual for a lab to sit on a signal if the signal has a noted
time variance
(this is from experience).
On Sun, Dec 5, 2021 at 11:46 AM Amund Westin < wrote:
This message originated outside of DISH and was sent by:
[email protected]
Some weeks ago I went to a test lab as an observer on a CE marking project (IT
product).
I participated on the radiated emission testing and some frequencies were above
the limit line during the scan (peak measurement). Later on, the Quasi-Peak
measurement lowered the values significant and therefore with the result Passed.
Later on, I was told that they continued the testing by 10-15 minutes and was
logging the maximum Quasi Peak value (repetitive QP meas) on spike with the
highest value. During these 15 minutes, one or two QP measurement was above the
limit and the result was changed from Passed to Failed.
I have not seen this test procedure before. I’m familiar with just one QP
measurement on each frequency and not Max Hold QP on each for 10-15 minutes.
Anyone who have experienced the same? Is it a CISPR procedure or just some
overzealous lab engineers?
Best regards
Amund
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