On Sunday, August 7, 2022 at 5:06:06 PM UTC-4, lang...@fonz.dk wrote:> søndag den 7. august 2022 kl. 22.52.07 UTC+2 skrev John Larkin: > > On Sun, 7 Aug 2022 13:27:44 -0700 (PDT), Lasse Langwadt Christensen > > <lang...@fonz.dk> wrote: > > >sųndag den 7. august 2022 kl. 21.48.51 UTC+2 skrev John Larkin: > > >> On Sun, 7 Aug 2022 11:09:02 -0700 (PDT), whit3rd <whi...@gmail.com> > > >> wrote: > > >> >On Sunday, August 7, 2022 at 10:57:17 AM UTC-7, John Larkin wrote: > > >> > > > >> >> My question was, why make a sine wave if the final result is a digital > > >> >> clock? > > >> > > > >> >Do you want the digital clock edges to be synchronous with an existing source, or > > >> >asynchronous? Mathematically, the creation of an asynchronous clock is > > >> >not gonna happen in clocked logic circuitry, it has to have an analog component. > > >> Of course. The analog components are dac, filter, comparator. > > >> > > >> I want a programmable internal trigger rate for a pulse generator. > > >> > > >> A 48-bit DDS will make a frequency of Fclk * N / 2^48 for arbitrary N, > > >> up to Nyquist. But it gets messy at low frequencies where the dac is > > >> incremented infrequently and the filter doesn't do much. > > > > > >if there is no more timing or amplitude steps to use, the only thing you can do it lower the filter cutoff > > > > > That has problems too. > > > > We were thinking that you could gain-up and clip the sine wave to > > increase the zero-cross slope. The logical end of that is to make a > > trapezoid with a steep rise. > keep decreasing the rise time and you get back to a squarewave > a sine is probably some kind of optimumIt is an optimum in that it is most easily filtered to give lowest jitter.> > The DAC lsb increments rarely at low frequencies, so magically include > > some lower phase accumulator bits to effectively increase the DAC > > sample rate on that steep slope. Digitally interpolate. > but if the DAC can't run any faster or have any more bits, how?He's trying to intuit a solution by pushing thoughts around, rather than reading the knowledge of others. None of this is new stuff and he is unlikely to find any "magical" solutions as he keeps referring to. In the end, his enemy is jitter. The effect of various spurs on jitter is known. The ones that are hardest to filter are close in spurs. Those mostly come from truncation of the phase accumulator. This is not the same thing as truncation of the sine value/DAC resolution. Anyone who wishes to research DDS design will find this. -- Rick C. --- Get 1,000 miles of free Supercharging --- Tesla referral code - https://ts.la/richard11209
DDS questions
Started by ●August 7, 2022
Reply by ●August 8, 20222022-08-08
Reply by ●August 8, 20222022-08-08
On Sun, 7 Aug 2022 14:06:02 -0700 (PDT), Lasse Langwadt Christensen <langwadt@fonz.dk> wrote:>s�ndag den 7. august 2022 kl. 22.52.07 UTC+2 skrev John Larkin: >> On Sun, 7 Aug 2022 13:27:44 -0700 (PDT), Lasse Langwadt Christensen >> <lang...@fonz.dk> wrote: >> >s?ndag den 7. august 2022 kl. 21.48.51 UTC+2 skrev John Larkin: >> >> On Sun, 7 Aug 2022 11:09:02 -0700 (PDT), whit3rd <whi...@gmail.com> >> >> wrote: >> >> >On Sunday, August 7, 2022 at 10:57:17 AM UTC-7, John Larkin wrote: >> >> > >> >> >> My question was, why make a sine wave if the final result is a digital >> >> >> clock? >> >> > >> >> >Do you want the digital clock edges to be synchronous with an existing source, or >> >> >asynchronous? Mathematically, the creation of an asynchronous clock is >> >> >not gonna happen in clocked logic circuitry, it has to have an analog component. >> >> Of course. The analog components are dac, filter, comparator. >> >> >> >> I want a programmable internal trigger rate for a pulse generator. >> >> >> >> A 48-bit DDS will make a frequency of Fclk * N / 2^48 for arbitrary N, >> >> up to Nyquist. But it gets messy at low frequencies where the dac is >> >> incremented infrequently and the filter doesn't do much. >> > >> >if there is no more timing or amplitude steps to use, the only thing you can do it lower the filter cutoff >> > >> That has problems too. >> >> We were thinking that you could gain-up and clip the sine wave to >> increase the zero-cross slope. The logical end of that is to make a >> trapezoid with a steep rise. > >keep decreasing the rise time and you get back to a squarewave >a sine is probably some kind of optimumMaybe. But it's worth thinking about. The optimum DDS waveform is entangled with the filter response. The sawtooth is interesting. It could be Spiced, in some number of hours. Or days. We can design the schematic and do a board layout and futz with DDS shapes and filters and dividers later.> >> The DAC lsb increments rarely at low frequencies, so magically include >> some lower phase accumulator bits to effectively increase the DAC >> sample rate on that steep slope. Digitally interpolate. > >but if the DAC can't run any faster or have any more bits, how?It would run at the XO rate of course, but one might generate a very slow trigger rate by doing something smarter that generating a very slow sine wave. A 1 Hz synthesized sine wave, filtered and stuffed into a comparator, is going to have a lot of jitter. Just thinking. That's often not popular. -- John Larkin Highland Technology, Inc trk The cork popped merrily, and Lord Peter rose to his feet. "Bunter", he said, "I give you a toast. The triumph of Instinct over Reason"
Reply by ●August 8, 20222022-08-08
Phil Hobbs <pcdhSpamMeSenseless@electrooptical.net> wrote:> Phil Hobbs wrote:[...]>> Just using fully differential stages (a la ECL) fixes the ground bounce >> problem pretty well.> I should add that it's important that the limiter be fully differential, > because otherwise you get a bunch of AM-PM conversion.ECL helps as long as both outputs are equally loaded. For example, higher capacitance on one output can introduce switching transients. However, it is difficult to find differential sources. Double balanced mixers and XOR gates are single-ended. If you are trying to achieve high gain, small effects can add up.> It's also quite feasible to mix down, limit, filter, and mix back up > again. With ideal mixers, this reduces the limiter's phase noise power > by a factor > > (f_RF/f_IF)**2. > > The LO doesn't have to be as stable as the desired signal, because its > phase gets subtracted and then added again.I'm not so sure about cancellation. The propagation delay through the filter will change the phase. The group delay around cutoff of a butterworth filter can have enormous phase shift. High frequencies may even add instead of subtract.> Cheers > > Phil Hobbs >-- MRM
Reply by ●August 8, 20222022-08-08
Mike Monett wrote:> Phil Hobbs <pcdhSpamMeSenseless@electrooptical.net> wrote: > >> Phil Hobbs wrote: > > [...] > >>> Just using fully differential stages (a la ECL) fixes the ground bounce >>> problem pretty well. > >> I should add that it's important that the limiter be fully differential, >> because otherwise you get a bunch of AM-PM conversion. > > ECL helps as long as both outputs are equally loaded. For example, higher > capacitance on one output can introduce switching transients. However, it > is difficult to find differential sources. Double balanced mixers and XOR > gates are single-ended. If you are trying to achieve high gain, small > effects can add up. > >> It's also quite feasible to mix down, limit, filter, and mix back up >> again. With ideal mixers, this reduces the limiter's phase noise power >> by a factor >> >> (f_RF/f_IF)**2. >> >> The LO doesn't have to be as stable as the desired signal, because its >> phase gets subtracted and then added again. > > I'm not so sure about cancellation. The propagation delay through the > filter will change the phase. The group delay around cutoff of a > butterworth filter can have enormous phase shift. High frequencies may even > add instead of subtract. >The filter phase stays reasonably still, though, so the LO phase fluctuations remain highly coherent between the down- and up-conversions. 'T'ain't perfect, but it can really help sometimes. 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 ●August 8, 20222022-08-08
On Tuesday, August 9, 2022 at 10:36:56 AM UTC+10, Mike Monett wrote:> Phil Hobbs <pcdhSpamM...@electrooptical.net> wrote: > > > Phil Hobbs wrote: > > [...] > >> Just using fully differential stages (a la ECL) fixes the ground bounce > >> problem pretty well. > > I should add that it's important that the limiter be fully differential, > > because otherwise you get a bunch of AM-PM conversion. > ECL helps as long as both outputs are equally loaded. For example, higher > capacitance on one output can introduce switching transients. However, it > is difficult to find differential sources. Double balanced mixers and XOR > gates are single-ended. If you are trying to achieve high gain, small > effects can add up. > > It's also quite feasible to mix down, limit, filter, and mix back up > > again. With ideal mixers, this reduces the limiter's phase noise power > > by a factor > > > > (f_RF/f_IF)**2. > > > > The LO doesn't have to be as stable as the desired signal, because its > > phase gets subtracted and then added again. > > I'm not so sure about cancellation. The propagation delay through the > filter will change the phase. The group delay around cutoff of a > butterworth filter can have enormous phase shift. High frequencies may even > add instead of subtract.So you don't use a Butterworth filter, but a Bessel linear phase shift filter, or one of the variations on that that comes close enough. Finite impulse response filters (built around a tapped delay line) can be linear phase. A filter design handbook - Williams and Taylor is well thought of - can be helpful. https://www.google.com.au/books/edition/Electronic_Filter_Design_Handbook_Fourth/2CBGAQAAIAAJ?hl=en -- Bill Sloman, Sydney
Reply by ●August 8, 20222022-08-08
Phil Hobbs <pcdhSpamMeSenseless@electrooptical.net> wrote:> Mike Monett wrote:[...]>> I believe it was Bruce Griffiths who championed low gain stages driving >> back-to-back diodes between stages. This would alleviate the ground >> bounce problem and allow slew rates down to DC.> The wideband noise both adds and intermodulates with the desired signal, > causing phase noise. In the high-SNR limit, the RMS phase noise > deviation (rad/sqrt(Hz)) due to additive noise can be found from the > small-angle approximation: > > <delta phi> = 1/sqrt(2 * SNR ). > > As long as the intermodulation is small, I agree that the last stage is > most of what matters, but not 100%. > > Noise intermodulation will shift not just the zero crossings, but also > the times when the amplifier goes in and out of clipping. The next > filter will turn that into a zero-crossing shift.I'm not sure I understand what you mean. The noise is symmetrical. It can add jitter to the zero crossings, but that's what noise does. You show this in your equation. In the Griffiths approach, the limiting is done by back-to-back diodes. There is no amplifier going in and out of clipping, so it's not clear how there can be a shift in the zero crossing.> Cheers > > Phil Hobbs >-- MRM
Reply by ●August 8, 20222022-08-08
Mike Monett wrote:> Phil Hobbs <pcdhSpamMeSenseless@electrooptical.net> wrote: > >> Phil Hobbs wrote: > > [...] > >>> Just using fully differential stages (a la ECL) fixes the ground bounce >>> problem pretty well. > >> I should add that it's important that the limiter be fully differential, >> because otherwise you get a bunch of AM-PM conversion. > > ECL helps as long as both outputs are equally loaded. For example, higher > capacitance on one output can introduce switching transients. However, it > is difficult to find differential sources. Double balanced mixers and XOR > gates are single-ended. If you are trying to achieve high gain, small > effects can add up.Single-ended XOR gates are single-ended, but DBMs aren't necessarily. The RF and LO ports are both transformer-coupled, so you can drive them differentially with no issues. Even the LO port can be driven differentially for the upconversion. <snip stuff I commented on already> 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 ●August 8, 20222022-08-08
Mike Monett wrote:> Phil Hobbs <pcdhSpamMeSenseless@electrooptical.net> wrote: > >> Mike Monett wrote: > > [...] > >>> I believe it was Bruce Griffiths who championed low gain stages driving >>> back-to-back diodes between stages. This would alleviate the ground >>> bounce problem and allow slew rates down to DC. > >> The wideband noise both adds and intermodulates with the desired signal, >> causing phase noise. In the high-SNR limit, the RMS phase noise >> deviation (rad/sqrt(Hz)) due to additive noise can be found from the >> small-angle approximation: >> >> <delta phi> = 1/sqrt(2 * SNR ). >> >> As long as the intermodulation is small, I agree that the last stage is >> most of what matters, but not 100%. >> >> Noise intermodulation will shift not just the zero crossings, but also >> the times when the amplifier goes in and out of clipping. The next >> filter will turn that into a zero-crossing shift. > > I'm not sure I understand what you mean. The noise is symmetrical. It can > add jitter to the zero crossings, but that's what noise does. You show this > in your equation.It adds jitter to everything, including the time when the amplifier goes in and out of clipping. The filter applies a convolution to the entire waveform, not just the zero-crossings, so that shift is equally important.> > In the Griffiths approach, the limiting is done by back-to-back diodes. > > There is no amplifier going in and out of clipping, so it's not clear how > there can be a shift in the zero crossing.The additive noise does the shifting, even if the rest of the hardware is noiseless. Diodes are not noiseless devices either. 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 ●August 8, 20222022-08-08
Phil Hobbs <pcdhSpamMeSenseless@electrooptical.net> wrote: [...]> Single-ended XOR gates are single-ended, but DBMs aren't necessarily. > The RF and LO ports are both transformer-coupled, so you can drive them > differentially with no issues. Even the LO port can be driven > differentially for the upconversion.Yes, the RF and LO ports are both transformer-coupled. So what difference does it make if these ports are driven single-ended vs differential? How does the transformer know how the input is driven?> Cheers > > Phil Hobbs > >-- MRM
Reply by ●August 8, 20222022-08-08
Mike Monett wrote:> Phil Hobbs <pcdhSpamMeSenseless@electrooptical.net> wrote: ><snippage restored>>> Gerhard Hoffmann wrote: >>> Am 07.08.22 um 22:37 schrieb Gerhard Hoffmann: >>> >>>> The essence of the Collins paper is that it takes several pairs of >>>> (filter + amplifier) in cascade, not a dumb comparator. >>> >>> I forgot: >>> >>> The filters have to be tighter from stage to stage. >>> There is an optimum. >>> In the time nuts archives, there is a spreadsheet >>> that computes the number of stages, gain per stage >>> and bandwidth. >>> >>>> Gerhard >>> >> >> I suspect the minimum will vary depending on the criteria. You don't >> gain much by making the filters so narrow that their parametric drifts >> start going all over the place. Lots of things get worse by factors of >> Q. >> >> Cheers >> >> Phil Hobbs > > I never bought into the Collins theory. A bit of fiddling in LTspice and > simple pen-and-paper work shows the last stage is all that matters. > > Other attempts to improve on Collins fail in the first paragraphs. For > example, Attila Kinali assumes the limiter has hysteresis. As far as I > know, no limiter worth it's salt has hysteresis. See > > http://people.mpi-inf.mpg.de/~adogan/pubs/IFCS2018_comparator_noise.pdf > > It is referenced in > > https://www.mail-archive.com/time-nuts@lists.febo.com/msg08534.html > > One problem with high gain limiters is ground bounce. This can cause > feedback to the input stage that causes effects similar to hysteresis, or > even oscillations. Many limiters restrict the minimum slew rate, or even > do not specify the performance in a band around zero. This means the > circuit cannot be used at low frequencies or even DC. > > I believe it was Bruce Griffiths who championed low gain stages driving > back-to-back diodes between stages. This would alleviate the ground bounce > problem and allow slew rates down to DC.>> Phil Hobbs wrote: > > [...] > >>> Just using fully differential stages (a la ECL) fixes the ground >>> bounce problem pretty well. > >> I should add that it's important that the limiter be fully >> differential, because otherwise you get a bunch of AM-PM >> conversion. > > ECL helps as long as both outputs are equally loaded. For example, > higher capacitance on one output can introduce switching transients. > However, it is difficult to find differential sources. Double > balanced mixers and XOR gates are single-ended. If you are trying to > achieve high gain, small effects can add up. > >> Single-ended XOR gates are single-ended, but DBMs aren't >> necessarily. The RF and LO ports are both transformer-coupled, so >> you can drive them differentially with no issues. Even the LO port >> can be driven differentially for the upconversion. > > Yes, the RF and LO ports are both transformer-coupled. So what > difference does it make if these ports are driven single-ended vs > differential? How does the transformer know how the input is driven?That's the point. You claimed that DBMs were single-ended, and they aren't necessarily. So the fully differential approach is a good solution to the supply/ground coupling problem. 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
