spread spectrum cheating

John Larkin <jlarkin@highlandSNIPMEtechnology.com> writes:

...
> So one idea is to spread-spectrum, wobulate the clock frequency or
> phase to smear the spectral peak below the CE limits.
>
> Has anyone done this? I wonder how wide a frequency sweep we'd need
> but more important is what the equivalent FM modulation frequency
> would have to be so the spectrum analyzer never sees the peak spectral
> line. Imagine a sawtooth frequency modulation, which turns the
> spectral spike into a nice flat plateau. What sort of sawtooth
> frequency would work?
>
> So, how wide and how fast should I sweep?

Sure, standard trick in the arsenal. I can't give specific numbers, as
those depend on how much above the limits you are, what kind of
spreading you'll be using. One important thing to keep in mind is that
many standards specify quasi peak detector with specific EMC bandwidths
that are available in test receivers, but not that often in normal
spectrum analyzers.

My most horrible work in the specman field was using slightly filtered
PWM output to spread non-locked VCO used to downconvert a band limited
signal to a log amp level controller. You should have seen the reaction of
an RF engineer when he saw my LO implementation. But in EMC sense the
thing was below detection limit in the lab!

-- mikko

On Thursday, 10 November 2022 at 06:24:21 UTC, Ricky wrote:
> On Wednesday, November 9, 2022 at 11:17:04 PM UTC-4, John Larkin wrote: 
> > On Thu, 10 Nov 2022 02:45:40 -0000 (UTC), Mike Monett VE3BTI 
> > <spa...@not.com> wrote: 
> > 
> > >Clifford Heath <no_...@please.net> wrote: 
> > > 
> > >> On 10/11/22 10:47, John Larkin wrote: 
> > >>> "Avoid ground loops" makes no sense for fast stuff, and doesn't even 
> > >>> make sense for audio or thermocouples. 
> > >> 
> > >> Says the man who's obviously never operated an audio system in a hall 
> > >> where you can get >1VAC between the grounds of different outlets. 
> > > 
> > >Don't they make transformers for that situation? 
> > > 
> > >Of course, nowadays everything is probably WiFi. Look at the stage 
> > >microphones used by Taylor Swift, Madonna, Rihanna and others. 
> > RF doesn't have ground loops! Fiber doesn't either.
> No, but shields connected at both ends along with other ground connections do. 
> 

Which is why a suitable series capacitor in one end of the grounded shield
can be a good idea.  It blocks 50/60Hz but is almost a short circuit at RF.

John

On 09/11/2022 16:42, Ricky wrote:
> On Thursday, November 3, 2022 at 10:37:53 AM UTC-4, Joe Gwinn wrote:
>> On Wed, 02 Nov 2022 20:00:00 -0700, John Larkin 
>> <jla...@highlandSNIPMEtechnology.com> wrote:
>> 
>>> So, how wide and how fast should I sweep?
>> Wobbling the clock frequency to reduce EMI is in fact a standard
>> trick going back decades, with commodity chips to do just that.
>> 
>> .<https://www.eetimes.com/isscc-spread-spectrum-clocks-mitigate-emi/>
>
>> 
> I've always wondered if frequency wobbling is a method to reduce the
> interference impact of the emissions, or if it is just a way to
> impact the measurement.  The same amount of power is being emitted at
> a given bandwidth at the time the sweep passes that range, swept or
> not.  I would image there are victim devices that would still be
> impacted in the same way, even with the frequency sweeping.

It is a bit of both. If the radiated power at f0 is reduced by spreading 
it out a bit you might hit other resonances.

But a spectrum analyser is the worst case seeing all of it.

A 10 fold reduction at any single frequency might still be good enough 
to pass even if more frequencies are now affected by its influence.

Conservation of energy says that it has to go somewhere.

-- 
Regards,
Martin Brown

On Thursday, 10 November 2022 at 11:02:55 UTC, Martin Brown wrote:
> On 09/11/2022 16:42, Ricky wrote: 
> > On Thursday, November 3, 2022 at 10:37:53 AM UTC-4, Joe Gwinn wrote: 
> >> On Wed, 02 Nov 2022 20:00:00 -0700, John Larkin 
> >> <jla...@highlandSNIPMEtechnology.com> wrote: 
> >> 
> >>> So, how wide and how fast should I sweep? 
> >> Wobbling the clock frequency to reduce EMI is in fact a standard 
> >> trick going back decades, with commodity chips to do just that. 
> >> 
> >> .<https://www.eetimes.com/isscc-spread-spectrum-clocks-mitigate-emi/> 
> > 
> >> 
> > I've always wondered if frequency wobbling is a method to reduce the 
> > interference impact of the emissions, or if it is just a way to 
> > impact the measurement. The same amount of power is being emitted at 
> > a given bandwidth at the time the sweep passes that range, swept or 
> > not. I would image there are victim devices that would still be 
> > impacted in the same way, even with the frequency sweeping.
> It is a bit of both. If the radiated power at f0 is reduced by spreading 
> it out a bit you might hit other resonances. 
> 
> But a spectrum analyser is the worst case seeing all of it. 
> 
> A 10 fold reduction at any single frequency might still be good enough 
> to pass even if more frequencies are now affected by its influence. 
> 
> Conservation of energy says that it has to go somewhere. 

A 10-fold reduction isn't enough.  JL needs to reduce the level by 30dB.
The bandwidth of the EMC measurement receiver will almost certainly be
120kHz, so the frequency modulation would need to be extreme to achieve
that by FM alone.  The quasi-peak detector has an attack time of 1ms
and a decay time of 550ms.
With a uniform frequency sweep of +/-50% I estimate that the measurement will
drop by about 27dB which is not enough.

John

On Thursday, November 10, 2022 at 5:55:47 AM UTC-4, John Walliker wrote:
> On Thursday, 10 November 2022 at 06:24:21 UTC, Ricky wrote: 
> > On Wednesday, November 9, 2022 at 11:17:04 PM UTC-4, John Larkin wrote: 
> > > On Thu, 10 Nov 2022 02:45:40 -0000 (UTC), Mike Monett VE3BTI 
> > > <spa...@not.com> wrote: 
> > > 
> > > >Clifford Heath <no_...@please.net> wrote: 
> > > > 
> > > >> On 10/11/22 10:47, John Larkin wrote: 
> > > >>> "Avoid ground loops" makes no sense for fast stuff, and doesn't even 
> > > >>> make sense for audio or thermocouples. 
> > > >> 
> > > >> Says the man who's obviously never operated an audio system in a hall 
> > > >> where you can get >1VAC between the grounds of different outlets. 
> > > > 
> > > >Don't they make transformers for that situation? 
> > > > 
> > > >Of course, nowadays everything is probably WiFi. Look at the stage 
> > > >microphones used by Taylor Swift, Madonna, Rihanna and others. 
> > > RF doesn't have ground loops! Fiber doesn't either. 
> > No, but shields connected at both ends along with other ground connections do. 
> >
> Which is why a suitable series capacitor in one end of the grounded shield 
> can be a good idea. It blocks 50/60Hz but is almost a short circuit at RF. 

What is the advantage of using the cap?  If there is a good ground between the systems, are you suggesting it's not an RF ground?  Anyway, for a shield connected to a connector shell, grounding through a cap is an awkward thing to do.  They are designed to provide a ground through mounting.  Where exactly does the cap go?  The cable itself? 

I would think any RF design is going to pay attention to details, rather than using a cookbook approach. 

-- 

Rick C.

-+ Get 1,000 miles of free Supercharging
-+ Tesla referral code - https://ts.la/richard11209

On Thursday, 10 November 2022 at 16:12:16 UTC, Ricky wrote:
> On Thursday, November 10, 2022 at 5:55:47 AM UTC-4, John Walliker wrote: 
> > On Thursday, 10 November 2022 at 06:24:21 UTC, Ricky wrote: 
> > > On Wednesday, November 9, 2022 at 11:17:04 PM UTC-4, John Larkin wrote: 
> > > > On Thu, 10 Nov 2022 02:45:40 -0000 (UTC), Mike Monett VE3BTI 
> > > > <spa...@not.com> wrote: 
> > > > 
> > > > >Clifford Heath <no_...@please.net> wrote: 
> > > > > 
> > > > >> On 10/11/22 10:47, John Larkin wrote: 
> > > > >>> "Avoid ground loops" makes no sense for fast stuff, and doesn't even 
> > > > >>> make sense for audio or thermocouples. 
> > > > >> 
> > > > >> Says the man who's obviously never operated an audio system in a hall 
> > > > >> where you can get >1VAC between the grounds of different outlets. 
> > > > > 
> > > > >Don't they make transformers for that situation? 
> > > > > 
> > > > >Of course, nowadays everything is probably WiFi. Look at the stage 
> > > > >microphones used by Taylor Swift, Madonna, Rihanna and others. 
> > > > RF doesn't have ground loops! Fiber doesn't either. 
> > > No, but shields connected at both ends along with other ground connections do. 
> > > 
> > Which is why a suitable series capacitor in one end of the grounded shield 
> > can be a good idea. It blocks 50/60Hz but is almost a short circuit at RF.
> What is the advantage of using the cap? If there is a good ground between the systems, are you suggesting it's not an RF ground? Anyway, for a shield connected to a connector shell, grounding through a cap is an awkward thing to do. They are designed to provide a ground through mounting. Where exactly does the cap go? The cable itself? 
> 
> I would think any RF design is going to pay attention to details, rather than using a cookbook approach. 

This seems to be more about politics than RF design, as all the good options have been
rejected, either by the customer or by JL for various reasons.
-Coding the data and clock in a way that massively broadens the spectrum seem
to have been rejected because too much development and testing has been done
on the existing design.  Manchester coding is self-clocking, dc balanced and
spread spectrum so it would be ideal.
-Grounding the shield at both ends would help a lot but has been rejected by the
customer because they don't want ground loops.
-Transformer coupling seems to have been rejected.  In any case the data would probably
need to be modified to ensure that lack of dc balance doesn't saturate the transformer.

A series capacitor in the ground shield is hardly a cookbook approach if it is
chosen carefully to be self resonant at the clock frequency so that it will only add
a few tens of milliohms impedance at that frequency.
For example, a GCM2195C1K123GA (12nF, COG, 80V, 0805) has a self resonant frequency
of 65MHz and an ESR of 20mohm.
At 62MHz the magnitude of the impedance is 30mohm.
At 60Hz the impedance is about 265kohm.  Therefore at mains frequency there is effectively
no connection to the ground shield, but at the clock frequency almost a short circuit.
How can this not help? (Of course grounding at both ends would be much better!)

John

On Thursday, November 10, 2022 at 1:55:47 AM UTC-8, John Walliker wrote:
> On Thursday, 10 November 2022 at 06:24:21 UTC, Ricky wrote: 
> > On Wednesday, November 9, 2022 at 11:17:04 PM UTC-4, John Larkin wrote: 

> > > RF doesn't have ground loops! Fiber doesn't either. 

> > No, but shields connected at both ends along with other ground connections do. 

> Which is why a suitable series capacitor in one end of the grounded shield 
> can be a good idea. It blocks 50/60Hz but is almost a short circuit at RF. 

Or, in conjunction with a ground loop, makes a sharp LC resonance... which
can pick up and deliver signals inductively to any OTHER loops in the vicinity.
For AM suppression, resistor-capacitor snubbers are recommended; not
sure what the right thing is, for other frequency ranges.

On Thu, 10 Nov 2022 06:21:38 -0800 (PST), John Walliker
<jrwalliker@gmail.com> wrote:

>On Thursday, 10 November 2022 at 11:02:55 UTC, Martin Brown wrote:
>> On 09/11/2022 16:42, Ricky wrote: 
>> > On Thursday, November 3, 2022 at 10:37:53 AM UTC-4, Joe Gwinn wrote: 
>> >> On Wed, 02 Nov 2022 20:00:00 -0700, John Larkin 
>> >> <jla...@highlandSNIPMEtechnology.com> wrote: 
>> >> 
>> >>> So, how wide and how fast should I sweep? 
>> >> Wobbling the clock frequency to reduce EMI is in fact a standard 
>> >> trick going back decades, with commodity chips to do just that. 
>> >> 
>> >> .<https://www.eetimes.com/isscc-spread-spectrum-clocks-mitigate-emi/> 
>> > 
>> >> 
>> > I've always wondered if frequency wobbling is a method to reduce the 
>> > interference impact of the emissions, or if it is just a way to 
>> > impact the measurement. The same amount of power is being emitted at 
>> > a given bandwidth at the time the sweep passes that range, swept or 
>> > not. I would image there are victim devices that would still be 
>> > impacted in the same way, even with the frequency sweeping.
>> It is a bit of both. If the radiated power at f0 is reduced by spreading 
>> it out a bit you might hit other resonances. 
>> 
>> But a spectrum analyser is the worst case seeing all of it. 
>> 
>> A 10 fold reduction at any single frequency might still be good enough 
>> to pass even if more frequencies are now affected by its influence. 
>> 
>> Conservation of energy says that it has to go somewhere. 
>
>A 10-fold reduction isn't enough.  JL needs to reduce the level by 30dB.
>The bandwidth of the EMC measurement receiver will almost certainly be
>120kHz, so the frequency modulation would need to be extreme to achieve
>that by FM alone.  The quasi-peak detector has an attack time of 1ms
>and a decay time of 550ms.
>With a uniform frequency sweep of +/-50% I estimate that the measurement will
>drop by about 27dB which is not enough.
>
>John

I've seen some suggestions that we might pick up around 8 dB
improvement in EMC qualification with a reasonable spread spectrum
clock. Every 8 dB helps.