https://www.dropbox.com/s/ryp2m1t9mr9gi17/DDS_Diff_Filter_1.jpg?raw=1 Most DDS chips have complementary DAC current outputs and the appnotes usually waste one and go to some effort to bias up a single-ended comparator. It's not any harder to go full diff on the filter, get twice the swing, and use the zero cross. To make a clock, the filter can have a ghastly Bode plot. It doesn't have to resemble any of the classic mathematical forms. So it can be designed by Spice fiddling with parts in stock. The sketched filter is 5th order, but I might play with 3rd order, maybe with an elliptic notch, and save a couple of inductors. Probably not a good idea, actually. I think we can park the DDS at cos(0) when it's not in use, to bias the comparator way off zero. Some hysteresis would be prudent too. ADCMP562 has that.
DDS differential filter
Started by ●September 1, 2022
Reply by ●September 3, 20222022-09-03
On 2/9/22 01:49, jlarkin@highlandsniptechnology.com wrote:> https://www.dropbox.com/s/ryp2m1t9mr9gi17/DDS_Diff_Filter_1.jpg?raw=1 > > Most DDS chips have complementary DAC current outputs and the appnotes > usually waste one and go to some effort to bias up a single-ended > comparator. It's not any harder to go full diff on the filter, get > twice the swing, and use the zero cross.It's generally a good idea, and I've done it myself, because it ensures that the rejected frequency components cancel out, instead of them appearing at the DDS's DAC power supply pins, where they need to be decoupled into the ground rail. On multi-DDS chips (e.g. AD9959) these signals can cross-talk between the different DDSs before they reach the power pins for decoupling. The better appnotes specify a balun to drive a single-ended filter. Some actual schematics use a fast differential op-amp to subtract the two outputs, but if you do that before the filter the op-amp has to be fast enough to track any harmonics (that the filter will reject) or they can cause pain. Clifford Heath
Reply by ●September 3, 20222022-09-03
On Sat, 3 Sep 2022 20:05:18 +1000, Clifford Heath <no_spam@please.net> wrote:>On 2/9/22 01:49, jlarkin@highlandsniptechnology.com wrote: >> https://www.dropbox.com/s/ryp2m1t9mr9gi17/DDS_Diff_Filter_1.jpg?raw=1 >> >> Most DDS chips have complementary DAC current outputs and the appnotes >> usually waste one and go to some effort to bias up a single-ended >> comparator. It's not any harder to go full diff on the filter, get >> twice the swing, and use the zero cross. > >It's generally a good idea, and I've done it myself, because it ensures >that the rejected frequency components cancel out, instead of them >appearing at the DDS's DAC power supply pins, where they need to be >decoupled into the ground rail. On multi-DDS chips (e.g. AD9959) these >signals can cross-talk between the different DDSs before they reach the >power pins for decoupling.People usually throw away the complementary current output, with maybe a resistor to ground, so the supply currents should cancel. Seems like a waste of signal.> >The better appnotes specify a balun to drive a single-ended filter. Some >actual schematics use a fast differential op-amp to subtract the two >outputs, but if you do that before the filter the op-amp has to be fast >enough to track any harmonics (that the filter will reject) or they can >cause pain. > >Clifford HeathI can see how a balun is useful to output a good sine wave against ground without an amp at the end, but I'll be driving a diff input comparator so the balanced filter looks good. One of my guys ran the NuHertz software and it designed a great balanced filter that uses standard values. Amazing software, crabby guy, ugly screens. https://www.dropbox.com/s/ba8fmhxn9stdlny/NuHertz_LPF_1.jpg?raw=1 It did an elliptic version too, but we'll be tight on board area and the four extra caps aren't worth the space. I'm inclined to split the right end and center-tap to ground, to reduce common-mode junk that might feed from the DDS chip. Gotta keep the unipolar current sources happy too. Worth Spicing.
Reply by ●September 3, 20222022-09-03
jlarkin@highlandsniptechnology.com wrote:> On Sat, 3 Sep 2022 20:05:18 +1000, Clifford Heath <no_spam@please.net> > wrote: > >> On 2/9/22 01:49, jlarkin@highlandsniptechnology.com wrote: >>> https://www.dropbox.com/s/ryp2m1t9mr9gi17/DDS_Diff_Filter_1.jpg?raw=1 >>> >>> Most DDS chips have complementary DAC current outputs and the appnotes >>> usually waste one and go to some effort to bias up a single-ended >>> comparator. It's not any harder to go full diff on the filter, get >>> twice the swing, and use the zero cross. >> >> It's generally a good idea, and I've done it myself, because it ensures >> that the rejected frequency components cancel out, instead of them >> appearing at the DDS's DAC power supply pins, where they need to be >> decoupled into the ground rail. On multi-DDS chips (e.g. AD9959) these >> signals can cross-talk between the different DDSs before they reach the >> power pins for decoupling. > > People usually throw away the complementary current output, with maybe > a resistor to ground, so the supply currents should cancel. Seems like > a waste of signal. > >> >> The better appnotes specify a balun to drive a single-ended filter. Some >> actual schematics use a fast differential op-amp to subtract the two >> outputs, but if you do that before the filter the op-amp has to be fast >> enough to track any harmonics (that the filter will reject) or they can >> cause pain. >> >> Clifford Heath > > I can see how a balun is useful to output a good sine wave against > ground without an amp at the end, but I'll be driving a diff input > comparator so the balanced filter looks good. > > One of my guys ran the NuHertz software and it designed a great > balanced filter that uses standard values. Amazing software, crabby > guy, ugly screens. > > https://www.dropbox.com/s/ba8fmhxn9stdlny/NuHertz_LPF_1.jpg?raw=1That's some gnarly passband ripple, for sure. The comparator doesn't care much, but I bet the phase whoopdedoos are not too pretty 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 ●September 3, 20222022-09-03
On Sat, 3 Sep 2022 14:37:49 -0400, Phil Hobbs <pcdhSpamMeSenseless@electrooptical.net> wrote:>jlarkin@highlandsniptechnology.com wrote: >> On Sat, 3 Sep 2022 20:05:18 +1000, Clifford Heath <no_spam@please.net> >> wrote: >> >>> On 2/9/22 01:49, jlarkin@highlandsniptechnology.com wrote: >>>> https://www.dropbox.com/s/ryp2m1t9mr9gi17/DDS_Diff_Filter_1.jpg?raw=1 >>>> >>>> Most DDS chips have complementary DAC current outputs and the appnotes >>>> usually waste one and go to some effort to bias up a single-ended >>>> comparator. It's not any harder to go full diff on the filter, get >>>> twice the swing, and use the zero cross. >>> >>> It's generally a good idea, and I've done it myself, because it ensures >>> that the rejected frequency components cancel out, instead of them >>> appearing at the DDS's DAC power supply pins, where they need to be >>> decoupled into the ground rail. On multi-DDS chips (e.g. AD9959) these >>> signals can cross-talk between the different DDSs before they reach the >>> power pins for decoupling. >> >> People usually throw away the complementary current output, with maybe >> a resistor to ground, so the supply currents should cancel. Seems like >> a waste of signal. >> >>> >>> The better appnotes specify a balun to drive a single-ended filter. Some >>> actual schematics use a fast differential op-amp to subtract the two >>> outputs, but if you do that before the filter the op-amp has to be fast >>> enough to track any harmonics (that the filter will reject) or they can >>> cause pain. >>> >>> Clifford Heath >> >> I can see how a balun is useful to output a good sine wave against >> ground without an amp at the end, but I'll be driving a diff input >> comparator so the balanced filter looks good. >> >> One of my guys ran the NuHertz software and it designed a great >> balanced filter that uses standard values. Amazing software, crabby >> guy, ugly screens. >> >> https://www.dropbox.com/s/ba8fmhxn9stdlny/NuHertz_LPF_1.jpg?raw=1 > >That's some gnarly passband ripple, for sure. The comparator doesn't >care much, but I bet the phase whoopdedoos are not too pretty either. > >Cheers > >Phil HobbsWe only use it at one frequency at a time! I'm modifying it to account for the DC issues of the DDS diff current sources. I have a sort of hybrid semi-differential filter in mind. https://www.dropbox.com/s/owhvm3dtzjf539u/DDS_LPF_5.jpg?raw=1 This config removes hf common-mode junk too. Filter theory hurts my head so I avoid it.
Reply by ●September 3, 20222022-09-03
On Thursday, September 1, 2022 at 8:49:34 AM UTC-7, jla...@highlandsniptechnology.com wrote:> https://www.dropbox.com/s/ryp2m1t9mr9gi17/DDS_Diff_Filter_1.jpg?raw=1 > > Most DDS chips have complementary DAC current outputs and the appnotes > usually waste one and go to some effort to bias up a single-ended > comparator. It's not any harder to go full diff on the filter, get > twice the swing, and use the zero cross.One of the things I've learned fairly recently is that discrete LC filters are a bag of hurt when it comes to production. I used to use a lot of packaged Mini-Circuits elliptic filters until the last project, when I got tired of paying for them. I can run a filter design program just as well as they can, right? Well, no. Turns out that a lot of parts get swapped around at random, probably during reel changes or 2nd-op hand placement when the PnP machine doesn't quite hold enough reels. Yelling at the factory people doesn't help as much as I thought/hoped it would. This probably happens with ordinary RC components too, but one 0402 bypass or coupling cap is pretty much as good as another, and the same is true for many if not most discrete resistors with values under 10K or so. I'm not going to notice if an I2C pullup is 4.7K instead of 2.2K or whatever. I will if a 22 nH inductor is placed in a spot for an 82 nH inductor, though.... ... or at least, I will notice it if I have the foresight to design a test jig to sweep the filter response and compare it with a limit line, rather than just checking for the expected level at one or two points within the passband. Oops. Where'd *that* spur come from? Anyway, yeah, baluns followed by single-ended filters are a good way to go. For some reason, I get a bit more signal out of certain chips with an SBTCJ--1WX+ 180-degree splitter than I do with a straight transformer or balun. -- john, KE5FX
Reply by ●September 3, 20222022-09-03
On Sat, 3 Sep 2022 12:47:45 -0700 (PDT), "John Miles, KE5FX" <jmiles@gmail.com> wrote:>On Thursday, September 1, 2022 at 8:49:34 AM UTC-7, jla...@highlandsniptechnology.com wrote: >> https://www.dropbox.com/s/ryp2m1t9mr9gi17/DDS_Diff_Filter_1.jpg?raw=1 >> >> Most DDS chips have complementary DAC current outputs and the appnotes >> usually waste one and go to some effort to bias up a single-ended >> comparator. It's not any harder to go full diff on the filter, get >> twice the swing, and use the zero cross. > >One of the things I've learned fairly recently is that discrete LC filters are a >bag of hurt when it comes to production. I used to use a lot of packaged >Mini-Circuits elliptic filters until the last project, when I got tired of paying >for them. I can run a filter design program just as well as they can, right?The mini-ckts mlcc flters are great, but start around 1 GHz or so. I want a 15 MHz filter so I'll have to make it. It's strange that nobody makes a series of lp filters aimed at the DDS market.> >Well, no. Turns out that a lot of parts get swapped around at random, probably >during reel changes or 2nd-op hand placement when the PnP machine doesn't >quite hold enough reels. Yelling at the factory people doesn't help as much >as I thought/hoped it would.Yikes, fire your assemblers. Nothing will work if they mix up parts.> >This probably happens with ordinary RC components too, but one 0402 bypass >or coupling cap is pretty much as good as another, and the same is true for many >if not most discrete resistors with values under 10K or so. I'm not going to notice >if an I2C pullup is 4.7K instead of 2.2K or whatever. I will if a 22 nH inductor is >placed in a spot for an 82 nH inductor, though....What about voltage dividers, gain set resistors, voltage reg programming, all that? Values matter.> >... or at least, I will notice it if I have the foresight to design a test jig to sweep the >filter response and compare it with a limit line, rather than just checking for the >expected level at one or two points within the passband. Oops. Where'd *that* >spur come from?We work in time domain, so some of our testing is oscilloscope mask limits on pulses and such. Our intent is to production test for any possible value or assembly/soldering error.> >Anyway, yeah, baluns followed by single-ended filters are a good way to go.But no passive balun will work from 15 MHz to 1 mHz. You narrowband RF types have it easy.>For some reason, I get a bit more signal out of certain chips with an SBTCJ--1WX+ >180-degree splitter than I do with a straight transformer or balun.My DDS chip has differential current outputs, and my comparator is differential, so a balanced filter makes sense. https://www.dropbox.com/s/owhvm3dtzjf539u/DDS_LPF_5.jpg?raw=1 Still, lots of parts.
Reply by ●September 3, 20222022-09-03
On 2022-09-03 21:47, John Miles, KE5FX wrote: [...]> > Anyway, yeah, baluns followed by single-ended filters are a good way to go. > For some reason, I get a bit more signal out of certain chips with an SBTCJ--1WX+ > 180-degree splitter than I do with a straight transformer or balun. > > -- john, KE5FX >It's likely a splitter with three windings instead of two. That gets you better LF response if the driver is up to the challenge. Jeroen Belleman
Reply by ●September 3, 20222022-09-03
On Saturday, September 3, 2022 at 1:30:05 PM UTC-7, jla...@highlandsniptechnology.com wrote:> It's strange that nobody makes a series of lp filters aimed at the DDS > market.MCL makes tons of HF/VHF filters, although they're all single-ended AFAIK. I used a batch file to scrape all the .S2P files for them a few years back, and wrote a tool to cascade them interactively (http://www.ke5fx.com/s2plan.png). Kinda dangerous, because if they were to ban my IP for abusing their server I'd be hating life.> What about voltage dividers, gain set resistors, voltage reg > programming, all that? Values matter.I don't think they do it all that often with resistors and caps, or we would indeed have had trouble along those lines. My guess is they load those first, then run out of slots somewhere in the middle of the 0603 inductors. They get handed off to a different person who isn't always as careful as they should be. One of those unfortunate cases where we have no choice but to "inspect in quality." I should hang out a shingle building production test jigs, I've done enough of that lately... -- john, KE5FX
Reply by ●September 4, 20222022-09-04
"John Miles, KE5FX" <jmiles@gmail.com> wrote:> One of those unfortunate cases where we have no choice but to "inspect > in quality." I should hang out a shingle building production test jigs, > I've done enough of that lately... > > -- john, KE5FXWhen you sweep a filter, are you looking for ripple or phase anomalies? Do you need a high end HP VNA or can you use a $69 Amazon nanovna? https://www.amazon.ca/s?k=nano+vna -- MRM
