On Thursday, November 3, 2022 at 2:06:34 PM UTC-4, Mike Monett VE3BTI wrote:> 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 >Made in China All they know how to do is lie about their product specifications, performance, and quality.> MRM
spread spectrum cheating
Started by ●November 3, 2022
Reply by ●November 3, 20222022-11-03
Reply by ●November 3, 20222022-11-03
On Thu, 3 Nov 2022 18:06:27 -0000 (UTC), Mike Monett VE3BTI <spamme@not.com> wrote:>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" > >$19.99I did a lot of cable testing when we designed the box. I recommended Cat6 S/FTP individually-shielded pairs with overall shield, which should be armor plating to EMI... if one grounds the shields. Maybe they didn't use that. It can be a little tricky using Ethernet cables and connectors in non-equalized, non-Ethernet applications, but they are sure handy.
Reply by ●November 3, 20222022-11-03
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.
Reply by ●November 3, 20222022-11-03
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
Reply by ●November 3, 20222022-11-03
On Thu, 3 Nov 2022 11:45:29 -0700 (PDT), John Walliker <jrwalliker@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. > >JohnCool. 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!
Reply by ●November 3, 20222022-11-03
On Thursday, 3 November 2022 at 19:56:46 UTC, John Larkin wrote:> 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!You will not get the full 30dB improvement you need just by sweeping the clock, but you might get close if you go for an extreme sweep range. Assume that the maximum frequency deviation that you can use is +/-50% and that the measuring receiver has brick-wall filters: Then the gain from sweeping is about 10*log(2*0.5*62/0.12) = 27dB Check whether the results you have been given are for domestic (class B) or industrial (class A) environments. If the classification is wrong and you can use class A rather than class B you gain another 9.6dB, in which case the problem is solved! John
Reply by ●November 3, 20222022-11-03
John Larkin 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. > >> 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. >The video bandwidth is also relevant. To get N dB improvement, your P-P sweep range needs to be > (10**N/10)*125 kHz, but your sweep rate can be slower by a factor (Video BW)/(Resolution BW). It also matters what order the analyzer does things in. Most of us old timers are probably familiar with the problem of measuring noise with an average-reading AC voltmeter such as an HP 400EL whose dial is calibrated for sine wave input, where you have to add 1 dB to the reading to get the right answer. Old-timey SAs and many scope FFTs take the log first and do the video BW filter afterwards. That makes the noise read low by 2.5 dB. (The HP/Agilent/Keysight app note AN150 is a super good read on this point, among many other things.) Cheers Phil Hobbs -- Dr Philip C D Hobbs Principal Consultant ElectroOptical Innovations LLC / Hobbs ElectroOptics Optics, Electro-optics, Photonics, Analog Electronics Briarcliff Manor NY 10510 http://electrooptical.net http://hobbs-eo.com
Reply by ●November 6, 20222022-11-06
On Thu, 3 Nov 2022 10:53:01 -0700 (PDT), Lasse Langwadt Christensen <langwadt@fonz.dk> wrote:>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 >Yeah, and it does work. But it won't be widely used until the patents run out. I bet that there are VFDs than can be programmed to vary the speed, if one knows the magic codes. I don't, and it wouldn't work with a big heavy chuck on an old manual lathe. Joe Gwinn
Reply by ●November 9, 20222022-11-09
On 2022-11-03, John Larkin <jlarkin@highlandSNIPMEtechnology.com> wrote:> 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?30-33kHz modulation frequency seem to be standard. There are lots of ready-made SSC clock generators, and even 5*7mm-style crystal oscillators with SSC output. For CPU clocks etc., I like the SI5351A clock generator (if you can get them since Skyworks bought the parts). You can either order them pre-programmed, or you can set the registers via I2C after each power-on (that even works on pre-programmed parts). PLLA can do SSC with configurable parameters, and 125MHz is in the range of supported outputs. cu Michael
Reply by ●November 9, 20222022-11-09
On 9 Nov 2022 10:32:55 GMT, Michael Schwingen <news-1513678000@discworld.dascon.de> wrote:>On 2022-11-03, John Larkin <jlarkin@highlandSNIPMEtechnology.com> wrote: >> 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? > >30-33kHz modulation frequency seem to be standard. There are lots of >ready-made SSC clock generators, and even 5*7mm-style crystal oscillators >with SSC output.That's interesting. We could just drop one into our board. Cool. We do have a PLL inside our FPGA that multiplies the 125 MHz clock up, to clock a 250 MHz ADC. A little jitter wouldn't bother me (the signal is grossly oversampled) but we need the PLL to still work.> >For CPU clocks etc., I like the SI5351A clock generator (if you can get them >since Skyworks bought the parts). You can either order them pre-programmed, >or you can set the registers via I2C after each power-on (that even works on >pre-programmed parts). PLLA can do SSC with configurable parameters, and >125MHz is in the range of supported outputs. > >cu >MichaelGreat. Thanks.
