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Phil Hobbs <pcdhSpamM...@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
Cat 8 cable might help. Each twisted pair is foil shielded and the entire
cable has an outer aluminum shield. The cable supports 40 Gbps for 30
meters. Note also the connectors have to be shielded and grounded. An
example from Amazon:
\"CAT8 Ethernet Cable,20FT (6.1 Meters) Shielded Network Cable, 26AWG Cat8
LAN Cable 40Gbps 2000Mhz Internet Cable, High Speed Gaming Ethernet Cable,
Weatherproof, Heavy Duty RJ45 Cable, Router, Switch, Modem\"
$19.99
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
Cat 8 cable might help. Each twisted pair is foil shielded and the entire
cable has an outer aluminum shield. The cable supports 40 Gbps for 30
meters. Note also the connectors have to be shielded and grounded. An
example from Amazon:
\"CAT8 Ethernet Cable,20FT (6.1 Meters) Shielded Network Cable, 26AWG Cat8
LAN Cable 40Gbps 2000Mhz Internet Cable, High Speed Gaming Ethernet Cable,
Weatherproof, Heavy Duty RJ45 Cable, Router, Switch, Modem\"
$19.99
On 03/11/2022 03:00, 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?
Mock it up on the bench with a signal generator and a spectrum analyser.
(any old frequency will do you can scale the results)
Try a triangle wave at say 1% of f0 to get a feel for it. That should
give nicer behaviour than a sawtooth. Discontinuities always hurt.
I seriously doubt if you can modulate it in such a way that will be a
recognisable clock signal that will get the fundamental down by 30dB.
Board redesign so the RF hot traces are between the PSU planes might
help. Is something pulling serious current at that clock frequency?
On Thu, 3 Nov 2022 15:27:59 +0000, Martin Brown
\'\'\'newspam\'\'\'@nonad.co.uk> wrote:
On 03/11/2022 03:00, 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?
Mock it up on the bench with a signal generator and a spectrum analyser.
(any old frequency will do you can scale the results)
I can Spice/FFT too, but I need to model the spectrum analyzer set up
for the specific EMI test.
I was just wondering if anyone had done ss to pass EMI tests.
Try a triangle wave at say 1% of f0 to get a feel for it. That should
give nicer behaviour than a sawtooth. Discontinuities always hurt.
I meant triangle, to get a flat spectrum. Schmitt oscillator or
something.
I seriously doubt if you can modulate it in such a way that will be a
recognisable clock signal that will get the fundamental down by 30dB.
Board redesign so the RF hot traces are between the PSU planes might
help. Is something pulling serious current at that clock frequency?
I\'m driving the clock twisted pair antiphase from some CMOS buffers
run from a 4 volt supply.
On Thursday, 3 November 2022 at 18:38:52 UTC, John Larkin wrote:
On Thu, 3 Nov 2022 15:27:59 +0000, Martin Brown
\'\'\'newspam\'\'\'@nonad.co.uk> wrote:
On 03/11/2022 03:00, 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?
Mock it up on the bench with a signal generator and a spectrum analyser.
(any old frequency will do you can scale the results)
I can Spice/FFT too, but I need to model the spectrum analyzer set up
for the specific EMI test.
I was just wondering if anyone had done ss to pass EMI tests.
Try a triangle wave at say 1% of f0 to get a feel for it. That should
give nicer behaviour than a sawtooth. Discontinuities always hurt.
I meant triangle, to get a flat spectrum. Schmitt oscillator or
something.
I seriously doubt if you can modulate it in such a way that will be a
recognisable clock signal that will get the fundamental down by 30dB.
Board redesign so the RF hot traces are between the PSU planes might
help. Is something pulling serious current at that clock frequency?
I\'m driving the clock twisted pair antiphase from some CMOS buffers
run from a 4 volt supply.
The quasi-peak detector has an attack time constant of 1ms and a decay
time constant of 550ms for your frequency range. If an average detector
is used for testing, then the low-pass corner frequency of the averaging
filter will probably be 100Hz. So if your sweep repetition rate is at least
10kHz you should get reasonably good smoothing.
John
On Thu, 3 Nov 2022 11:45:29 -0700 (PDT), John Walliker
jrwal...@gmail.com> wrote:
On Thursday, 3 November 2022 at 18:38:52 UTC, John Larkin wrote:
On Thu, 3 Nov 2022 15:27:59 +0000, Martin Brown
\'\'\'newspam\'\'\'@nonad.co.uk> wrote:
On 03/11/2022 03:00, 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?
Mock it up on the bench with a signal generator and a spectrum analyser.
(any old frequency will do you can scale the results)
I can Spice/FFT too, but I need to model the spectrum analyzer set up
for the specific EMI test.
I was just wondering if anyone had done ss to pass EMI tests.
Try a triangle wave at say 1% of f0 to get a feel for it. That should
give nicer behaviour than a sawtooth. Discontinuities always hurt.
I meant triangle, to get a flat spectrum. Schmitt oscillator or
something.
I seriously doubt if you can modulate it in such a way that will be a
recognisable clock signal that will get the fundamental down by 30dB.
Board redesign so the RF hot traces are between the PSU planes might
help. Is something pulling serious current at that clock frequency?
I\'m driving the clock twisted pair antiphase from some CMOS buffers
run from a 4 volt supply.
The quasi-peak detector has an attack time constant of 1ms and a decay
time constant of 550ms for your frequency range. If an average detector
is used for testing, then the low-pass corner frequency of the averaging
filter will probably be 100Hz. So if your sweep repetition rate is at least
10kHz you should get reasonably good smoothing.
John
Cool. A lot of varicap VCXOs and VCOs have about a 10 KHz modulation
bandwidth.
I could almost drive one with a 10 KHz square wave!
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.
A simple double insulated wall wart would achieve this.
An alternative way of breaking round loops if the customer really insists is to do
what ethernet often does - put a small capacitor in series with the shield so that
there is minimal 60Hz coupling but the impedance is low at 62MHz. A ceramic
capacitor selected to be self-resonant at close to 62MHz would be ideal. Murata
and some others are good about providing such data.
If there is a separate power supply, then keep its cable as close as possible to the
data cable during testing to make the dipole antenna created by the two cables as
small as possible.
John
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.
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
\"John Miles, KE5FX\" <jmiles@gmail.com> wrote:
On Wednesday, November 2, 2022 at 8:00:11 PM UTC-7, 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.
Probably a dumb question, but those clock and data signals are source-
terminated to drive ~100 ohms, right? If you have standing waves
on the wire pairs, I can see them exciting the ungrounded shield at the
current points, making a very effective antenna. Radiation from the
cable could easily be 30 dB worse than expected.
-- john, KE5FX
Cat 8 requires shield grounding. Example:
LINKUP [40Gbps Certified] Cat8 Ethernet Patch Cable Double
Shielded?2000MHz (2GHz) CAD$21.96
https://www.amazon.ca/LINKUP-Ethernet-Screened-Stranded-Structure/dp/B08GCV
59L7/
Signal cables
The best way to wire shielded cables for screening is to ground the shield
at both ends of the cable.[6] Traditionally there existed a rule of thumb
to ground only the source end of the shield to avoid ground loops. Best
practice is to ground at both ends, but there is a possibility of ground
loops. In airplanes, special cable is used with both an outer shield to
protect against lightning and an inner shield grounded at one end to
eliminate hum from the 400 Hz power system
https://en.wikipedia.org/wiki/Shielded_cable
JL likes to leave things ungrounded to increase the fun of
troubleshooting.
\"Avoid ground loops\" makes no sense for fast stuff, and doesn\'t even
make sense for audio or thermocouples.
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.
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.
On Wednesday, November 2, 2022 at 8:00:11 PM UTC-7, 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.
Probably a dumb question, but those clock and data signals are source-
terminated to drive ~100 ohms, right? If you have standing waves
on the wire pairs, I can see them exciting the ungrounded shield at the
current points, making a very effective antenna. Radiation from the cable
could easily be 30 dB worse than expected.
-- john, KE5FX
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.
Clifford Heath <no_spam@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.
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.
On Wed, 9 Nov 2022 11:11:06 -0800 (PST), Lasse Langwadt Christensen
langwadt@fonz.dk> wrote:
torsdag den 3. november 2022 kl. 04.00.11 UTC+1 skrev John Larkin:
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.
if it is balanced why do you need such a massive swing?
The original design, about 10 years ago, used a big swing and a
receive-end attenuator to give a lot of common-mode rejection against
ground loops. This system is spread over floors of a big expensive
building.
We were exempt from EMI standards then.
On Thu, 10 Nov 2022 12:33:07 +1100, 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.
Critical signals should be differential. And with audio, you can
transformer isolate.
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.
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?