spread spectrum cheating

On 2022-11-03 15:37, Joe Gwinn wrote:
> On Wed, 02 Nov 2022 20:00:00 -0700, John Larkin
> <jlarkin@highlandSNIPMEtechnology.com> wrote:
>
>> We make a bunch of boxes that go into a semi fab tool. One measures an
>> optical waveform and shoots it to a bigger box, over three twisted
>> pairs (clock, data, data) using shielded RJ45 ethernet type stuff.
>>
>> When we originally did it, they told us we were exempt from ROHS and
>> EMI standards, but now we aren't. ROHS is no big deal, but the little
>> box makes a continuous 62 MHz clock, differential at 5 volt swings,
>> and radiates too much.
>>
>> We can't lowpass filter the fundamental of course. We can't drop the
>> amplitude much. A common-mode balun might help some.
>>
>> 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?
>>
>> My options are to add a modulated phase shifter in the clock path, or
>> to replace the main XO with a VCO and apply some waveform to the VCO
>> input to FM the whole FPGA clock and everything. Clock and data would
>> sweep together, which is kind of nice.
>>
>> 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/>
>
> Joe Gwinn
>

I was surprised to discover that spread-spectrum wasn't only an electronics
subject. I came across a youtube video by Steve Mould, explaining that the
grooves in car tyres are unevenly spaced to reduce whining noises.

<https://www.youtube.com/watch?v=ock8v7-IG7I>
   
Check! He's right! I never noticed before.

Jeroen Belleman

torsdag den 3. november 2022 kl. 16.50.49 UTC+1 skrev Jeroen Belleman:
> On 2022-11-03 15:37, Joe Gwinn wrote: 
> > On Wed, 02 Nov 2022 20:00:00 -0700, John Larkin 
> > <jla...@highlandSNIPMEtechnology.com> wrote: 
> > 
> >> We make a bunch of boxes that go into a semi fab tool. One measures an 
> >> optical waveform and shoots it to a bigger box, over three twisted 
> >> pairs (clock, data, data) using shielded RJ45 ethernet type stuff. 
> >> 
> >> When we originally did it, they told us we were exempt from ROHS and 
> >> EMI standards, but now we aren't. ROHS is no big deal, but the little 
> >> box makes a continuous 62 MHz clock, differential at 5 volt swings, 
> >> and radiates too much. 
> >> 
> >> We can't lowpass filter the fundamental of course. We can't drop the 
> >> amplitude much. A common-mode balun might help some. 
> >> 
> >> 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? 
> >> 
> >> My options are to add a modulated phase shifter in the clock path, or 
> >> to replace the main XO with a VCO and apply some waveform to the VCO 
> >> input to FM the whole FPGA clock and everything. Clock and data would 
> >> sweep together, which is kind of nice. 
> >> 
> >> 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/> 
> > 
> > Joe Gwinn 
> >
> I was surprised to discover that spread-spectrum wasn't only an electronics 
> subject. I came across a youtube video by Steve Mould, explaining that the 
> grooves in car tyres are unevenly spaced to reduce whining noises. 

and turbine engines have a different number of blades and stators in each stage so 
it doesn't turn into an air sirene loud enough to make buildings collapse

John Larkin <jlarkin@highlandSNIPMEtechnology.com> Wrote in message:r
> We make a bunch of boxes that go into a semi fab tool. One measures anoptical waveform and shoots it to a bigger box, over three twistedpairs (clock, data, data) using shielded RJ45 ethernet type stuff.When we originally did it, they told us we were exempt from ROHS andEMI standards, but now we aren't. ROHS is no big deal, but the littlebox makes a continuous 62 MHz clock, differential at 5 volt swings,and radiates too much.We can't lowpass filter the fundamental of course. We can't drop theamplitude much. A common-mode balun might help some.So one idea is to spread-spectrum, wobulate the clock frequency orphase to smear the spectral peak below the CE limits.Has anyone done this? I wonder how wide a frequency sweep we'd needbut more important is what the equivalent FM modulation frequencywould have to be so the spectrum analyzer never sees the peak spectralline. Imagine a sawtooth frequency modulation, which turns thespectral spike into a nice flat plateau. What sort of sawtoothfrequency would work?My options are to add a modulated phase shifter in the clock path, orto replace the main XO with a VCO and apply some waveform to the VCOinput to FM the whole FPGA clock and everything. Clock and data wouldsweep together, which is kind of nice.So, how wide andnub how fast should I sweep?


-- 
300khz resolution band width comes to mind. The SA's used use
 quasi peak measuring of the emissions. I never  looked into
 defeating it.  But if your wobbling more than 300khz, there's a
 good chance to fool the detector.

Cheers


----Android NewsGroup Reader----
https://piaohong.s3-us-west-2.amazonaws.com/usenet/index.html

On Thu, 03 Nov 2022 07:51:04 -0700, John Larkin
<jlarkin@highlandSNIPMEtechnology.com> wrote:

>On Thu, 3 Nov 2022 03:45:02 -0700 (PDT), John Walliker
><jrwalliker@gmail.com> wrote:
>
>>On Thursday, 3 November 2022 at 05:28:19 UTC, Jan Panteltje wrote:
>>> On a sunny day (Wed, 02 Nov 2022 20:00:00 -0700) it happened John Larkin 
>>> <jla...@highlandSNIPMEtechnology.com> wrote in 
>>> <sha6mhlu5664pfrgu...@4ax.com>:
>>> >We make a bunch of boxes that go into a semi fab tool. One measures an 
>>> >optical waveform and shoots it to a bigger box, over three twisted 
>>> >pairs (clock, data, data) using shielded RJ45 ethernet type stuff. 
>>> > 
>>> >When we originally did it, they told us we were exempt from ROHS and 
>>> >EMI standards, but now we aren't. ROHS is no big deal, but the little 
>>> >box makes a continuous 62 MHz clock, differential at 5 volt swings, 
>>> >and radiates too much.
>>> Google double shielded RJ45 cable?
>>
>>Are you testing to class A or B?  As this is an industrial application you should be
>>able to go for class A which gives you an extra 9.6dB.
>>The measurement receiver will almost certainly have a bandwidth of 120kHz plus
>>a bit extra for the finite slope of the filters, so if you want a 3dB reduction in the
>>received signal you will need the clock to be uniformly spread over at least a 250kHz
>>range.  That way, the unwanted signal is within the measurement bandwidth for
>>only half the time.  This may of course be too much for your system to cope with.
>>Spread spectrum clocks are much more effective at dealing with high harmonics
>>than the clock fundamental as the deviation gets higher for the harmonics but the
>>receiver bandwidth stays the same (up to 1GHz).
>
>250K is well inside the sweep range (some do an octave) but outside
>the modulation range of most little commercial VCOs, but maybe I can
>find one, to replace the 125 MHz XO in the box.
>
>The two knobs to turn, to fool the spectrum analyzer, is how wide a
>range to sweep over, and what is the sweep rate? An SA has both a
>front-end bandwidth and "video bandwidth", essentially lowpass
>filtering after the detector.
>
>Another option is a circuit that phase modulates the 62.5 MHz clock
>after the FPGA generates it. Shades of the old "phasing method" of FM
>generation. 
>
>
>>How is the box powered?  If there is a separate power supply, then adding some
>>common mode inductance such as a ferrite ring or clamp to the power cable
>>should help a lot.
>
>The customer provides 24 volts DC, and they insisted I not ground the
>low side so I have an isolating dc/dc converter. The claim is that
>they are not seeing EMI from the power cable.
>
>>If the box only connects to one other device plus a power supply,  then I don't
>>understand how grounding the shield at one end only can make any sense.
>
>It never made sense. One fix is to change the RG45 connector and the
>box end plate to get a good shield ground, but for some reason the
>customer says that's not quite good enough.
>
>Of course, customers have been known to be wrong before.
>
>
>>It
>>should be solidly grounded at the main equipment end and be connected to the
>>internal ground reference in the small box.  Any break in the ground structure
>>to avoid "ground loops" should be obtained by having a floating power supply
>>if possible.
>
>
>The PCBs in both boxes are hard grounded to our grounded metal
>enclosures. The "ground loop" concept was stupid. There are giant
>metal pipes between the relevant parts of this machine and they make
>"ground loops" too. Not to mention all the electrical power conduits
>and the building structure steel.

The point is to avoid ground loops at power frequency and harmonics.
One can ground the floating end for RF with a capacitor.

Joe Gwinn

Am 03.11.22 um 15:51 schrieb John Larkin:

> My real question remains, what sweep width and modulation do I need to
> spread the EMI spectrum enough to fool a CE-test-class spectrum
> analyzer?
> 
> If the SA RF bandwidth is 125K, I might sweep 1 MHz p-p to help. The
> digital stuff might tolerate that.

The idea is not to betray the SA, it is more like playing with
the specs. There is a noise / energy density that must not
be exceeded. It's better to have just-so-results over a wide
frequency region than having too much on one spot.
I think there are clock chips just for this purpose. I did not
remember them, my problem is usually the opposite:
Minimizing the phase noise.

cheers, Gerhard

On Wednesday, November 2, 2022 at 11:00:11 PM UTC-4, John Larkin wrote:
> We make a bunch of boxes that go into a semi fab tool. One measures an 
> optical waveform and shoots it to a bigger box, over three twisted 
> pairs (clock, data, data) using shielded RJ45 ethernet type stuff. 
> 
> When we originally did it, they told us we were exempt from ROHS and 
> EMI standards, but now we aren't. ROHS is no big deal, but the little 
> box makes a continuous 62 MHz clock, differential at 5 volt swings, 
> and radiates too much. 
> 
> We can't lowpass filter the fundamental of course. We can't drop the 
> amplitude much. A common-mode balun might help some. 
> 
> 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? 
> 
> My options are to add a modulated phase shifter in the clock path, or 
> to replace the main XO with a VCO and apply some waveform to the VCO 
> input to FM the whole FPGA clock and everything. Clock and data would 
> sweep together, which is kind of nice. 
> 
> So, how wide and how fast should I sweep?

Can you get away with a differential sine wave? It would easier to achieve balance, I would think, with a split phase coupling transformer. 
https://www.minicircuits.com/WebStore/Transformers.html
The amplitude and phase imbalances, which are actually pretty good at your frequency, should be 40dB down, then an SMT common mode choke should  provide an additional 15dB minimum. A 55dB reduction is  pretty good starting point.
https://www.murata.com/en-us/search/productsearch?cate=cgsubChipCommoModeChokeCoil&partno=DLW21*

On Thu, 3 Nov 2022 09:03:42 -0700 (PDT), Lasse Langwadt Christensen
<langwadt@fonz.dk> wrote:

>torsdag den 3. november 2022 kl. 16.50.49 UTC+1 skrev Jeroen Belleman:
>> On 2022-11-03 15:37, Joe Gwinn wrote: 
>> > On Wed, 02 Nov 2022 20:00:00 -0700, John Larkin 
>> > <jla...@highlandSNIPMEtechnology.com> wrote: 
>> > 
>> >> We make a bunch of boxes that go into a semi fab tool. One measures an 
>> >> optical waveform and shoots it to a bigger box, over three twisted 
>> >> pairs (clock, data, data) using shielded RJ45 ethernet type stuff. 
>> >> 
>> >> When we originally did it, they told us we were exempt from ROHS and 
>> >> EMI standards, but now we aren't. ROHS is no big deal, but the little 
>> >> box makes a continuous 62 MHz clock, differential at 5 volt swings, 
>> >> and radiates too much. 
>> >> 
>> >> We can't lowpass filter the fundamental of course. We can't drop the 
>> >> amplitude much. A common-mode balun might help some. 
>> >> 
>> >> 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? 
>> >> 
>> >> My options are to add a modulated phase shifter in the clock path, or 
>> >> to replace the main XO with a VCO and apply some waveform to the VCO 
>> >> input to FM the whole FPGA clock and everything. Clock and data would 
>> >> sweep together, which is kind of nice. 
>> >> 
>> >> 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/> 
>> > 
>> > Joe Gwinn 
>> >
>> I was surprised to discover that spread-spectrum wasn't only an electronics 
>> subject. I came across a youtube video by Steve Mould, explaining that the 
>> grooves in car tyres are unevenly spaced to reduce whining noises. 
>
>and turbine engines have a different number of blades and stators in each stage so 
>it doesn't turn into an air sirene loud enough to make buildings collapse

I didn't know that, but it certainly makes sense.  I thinks some
cooling fans do that as well.

Joe Gwinn

On Thu, 03 Nov 2022 16:50:42 +0100, Jeroen Belleman
<jeroen@nospam.please> wrote:

>On 2022-11-03 15:37, Joe Gwinn wrote:
>> On Wed, 02 Nov 2022 20:00:00 -0700, John Larkin
>> <jlarkin@highlandSNIPMEtechnology.com> wrote:
>>
>>> We make a bunch of boxes that go into a semi fab tool. One measures an
>>> optical waveform and shoots it to a bigger box, over three twisted
>>> pairs (clock, data, data) using shielded RJ45 ethernet type stuff.
>>>
>>> When we originally did it, they told us we were exempt from ROHS and
>>> EMI standards, but now we aren't. ROHS is no big deal, but the little
>>> box makes a continuous 62 MHz clock, differential at 5 volt swings,
>>> and radiates too much.
>>>
>>> We can't lowpass filter the fundamental of course. We can't drop the
>>> amplitude much. A common-mode balun might help some.
>>>
>>> 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?
>>>
>>> My options are to add a modulated phase shifter in the clock path, or
>>> to replace the main XO with a VCO and apply some waveform to the VCO
>>> input to FM the whole FPGA clock and everything. Clock and data would
>>> sweep together, which is kind of nice.
>>>
>>> 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/>
>>
>> Joe Gwinn
>>
>
>I was surprised to discover that spread-spectrum wasn't only an electronics
>subject. I came across a youtube video by Steve Mould, explaining that the
>grooves in car tyres are unevenly spaced to reduce whining noises.
>
><https://www.youtube.com/watch?v=ock8v7-IG7I>
>   
>Check! He's right! I never noticed before.
>

I had noticed that on my car, but didn't think much about it.  Makes
sense, though.

Endmills used on vertical milling machines often have the cutting
edges arranged slightly irregularly around the circumference, so the
bit won't sing at the edge-passing frequency.  Same idea.

Joe Gwinn

torsdag den 3. november 2022 kl. 18.48.58 UTC+1 skrev Joe Gwinn:
> On Thu, 03 Nov 2022 16:50:42 +0100, Jeroen Belleman 
> <jer...@nospam.please> wrote: 
> 
> >On 2022-11-03 15:37, Joe Gwinn wrote: 
> >> On Wed, 02 Nov 2022 20:00:00 -0700, John Larkin 
> >> <jla...@highlandSNIPMEtechnology.com> wrote: 
> >> 
> >>> We make a bunch of boxes that go into a semi fab tool. One measures an 
> >>> optical waveform and shoots it to a bigger box, over three twisted 
> >>> pairs (clock, data, data) using shielded RJ45 ethernet type stuff. 
> >>> 
> >>> When we originally did it, they told us we were exempt from ROHS and 
> >>> EMI standards, but now we aren't. ROHS is no big deal, but the little 
> >>> box makes a continuous 62 MHz clock, differential at 5 volt swings, 
> >>> and radiates too much. 
> >>> 
> >>> We can't lowpass filter the fundamental of course. We can't drop the 
> >>> amplitude much. A common-mode balun might help some. 
> >>> 
> >>> 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? 
> >>> 
> >>> My options are to add a modulated phase shifter in the clock path, or 
> >>> to replace the main XO with a VCO and apply some waveform to the VCO 
> >>> input to FM the whole FPGA clock and everything. Clock and data would 
> >>> sweep together, which is kind of nice. 
> >>> 
> >>> 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/> 
> >> 
> >> Joe Gwinn 
> >> 
> > 
> >I was surprised to discover that spread-spectrum wasn't only an electronics 
> >subject. I came across a youtube video by Steve Mould, explaining that the 
> >grooves in car tyres are unevenly spaced to reduce whining noises. 
> > 
> ><https://www.youtube.com/watch?v=ock8v7-IG7I> 
> > 
> >Check! He's right! I never noticed before. 
> >
> I had noticed that on my car, but didn't think much about it. Makes 
> sense, though. 
> 
> Endmills used on vertical milling machines often have the cutting 
> edges arranged slightly irregularly around the circumference, so the 
> bit won't sing at the edge-passing frequency. Same idea. 

some lathes and mills have the option to vary the spindle speed to avoid chatter

https://youtu.be/kR6KUsh-jg4

Phil Hobbs <pcdhSpamMeSenseless@electrooptical.net> wrote:

> John Larkin wrote:
>> We make a bunch of boxes that go into a semi fab tool. One measures an
>> optical waveform and shoots it to a bigger box, over three twisted
>> pairs (clock, data, data) using shielded RJ45 ethernet type stuff.
>> 
>> When we originally did it, they told us we were exempt from ROHS and
>> EMI standards, but now we aren't. ROHS is no big deal, but the little
>> box makes a continuous 62 MHz clock, differential at 5 volt swings,
>> and radiates too much. 
>> 
>> We can't lowpass filter the fundamental of course. We can't drop the
>> amplitude much. A common-mode balun might help some.
>> 
>> 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?
>> 
>> My options are to add a modulated phase shifter in the clock path, or
>> to replace the main XO with a VCO and apply some waveform to the VCO
>> input to FM the whole FPGA clock and everything. Clock and data would
>> sweep together, which is kind of nice.
>> 
>> So, how wide and how fast should I sweep?
>> 
> How badly did you miss the limit?
> 
> Cheers
> 
> Phil Hobbs

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-- 
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