spread spectrum cheating (2023 Update)

How badly did you miss the limit?

Cheers

Phil Hobbs

When we did the box, the customer insisted that all cable shields be grounded on one end only "to prevent ground loops" ! So we didn't ground the cat5 shield at the ESM, the little box that transmits the clock and data. That fails CE test by about 30 dB. The crazies are gone so we can ground the shield now and fail by maybe 10.

Grounding, and some baluns, might just fix it, but spread spectrum would help a lot too.

Strangly, we fail at 62 MHz, the clock frequency, and we can't lowpass filter that.

On a sunny day (Wed, 02 Nov 2022 20:00:00 -0700) it happened John Larkin snipped-for-privacy@highlandSNIPMEtechnology.com wrote in snipped-for-privacy@4ax.com:

Google double shielded RJ45 cable?

On Thursday, 3 November 2022 at 05:28:19 UTC, Jan Panteltje wrote:

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). 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. 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 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. 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

You probably shouldn't sweep at all, but make the box and it's cables radiate less.

A balanced clock and balanced data shouldn't radiate much at all, and shielded twisted pair ribbon cable with a solid foil shield might well radiate less than RJ45.

Forcing balance with a clamp-on ferrite common mode choke - balun - would help too, as you say.

A 5V swing is probably more than you need. ECL and LVDS both swing through only a volt or so and put a lot less hash on your power supply rail and through your ground returns.

Wobbling the clock frequency to reduce EMI is in fact a standard trick going back decades, with commodity chips to do just that.

.

Joe Gwinn

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.

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.

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.

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.

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.

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 a sunny day (Thu, 03 Nov 2022 07:51:04 -0700) it happened John Larkin snipped-for-privacy@highlandSNIPMEtechnology.com wrote in snipped-for-privacy@4ax.com:

You mentioned 62 MHz, the 74HC4046A at 5 V goes that high. Modulate F with VCO pin 9 So modudate with a gaussian ;-)

But I do not like the idea of frequency changing clocks, screening is possibly better.

62 MHz? RTL_SDR stick and xpsa:

How do they make the real spread spectrum stuff? Noise generator, low pass, VCO?

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.

Check! He's right! I never noticed before.

Jeroen Belleman

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

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:

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

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

Joe Gwinn

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:

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

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.

I meant triangle, to get a flat spectrum. Schmitt oscillator or something.

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