DDS differential filter

On Saturday, September 3, 2022 at 10:54:55 PM UTC-7, Mike Monett VE3BTI wrote:
> "John Miles, KE5FX" <jmi...@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, KE5FX
> When 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 

Or can you use a  noise source and FFT it?

John Miles, KE5FX 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?
> 
> 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
> 
Good gravy, where are you having them assembled? In antarctica by 
penguins?  Seriously? Arbitrarily changing resistor values by more than 
a factor of 2?  I'd fire mine if they used thick film rather than the 
specified thin film.

That sort of nonsense wouldn't fly with me or my consulting/licensing 
customers.

Gotta do more than yell, man.

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

John Miles, KE5FX wrote:
> 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.

You just have to do your abuse in the library or the coffee shop. ;)

> 
>> 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.

Put the inductors on the back of the board. ;)

> 
> 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...

Dunno how you expect to avoid final test.  It's pretty important that 
you know the thing works right before shipping it, yes?

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

On Sat, 3 Sep 2022 15:23:40 -0700 (PDT), "John Miles, KE5FX"
<jmiles@gmail.com> wrote:

>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

We usually buy small Ls that have a color dot or something. The
Coilcrafts do. Our automated VOA machine can check them. Every board
gets inspected with that.

Inspection has a powerful process feedback element. We flog as
required.

On 4/9/22 02:15, jlarkin@highlandsniptechnology.com wrote:
> One of my guys ran the NuHertz software and it designed a great
> balanced filter that uses standard values.

Nice. Does it do worst-case tolerance analysis too?

Clifford Heath

On 4/9/22 06:29, jlarkin@highlandsniptechnology.com wrote:
> On Sat, 3 Sep 2022 12:47:45 -0700 (PDT), "John Miles, KE5FX"
> <jmiles@gmail.com> wrote:
>> 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.

It's difficult to build good LC filters in SMD for HF. The multi-layer 
inductors that work fine at UHF just don't have the required Q at lower 
frequencies.

I designed a nice-looking 7th order bandpass filter for 50MHz, then 
started looking for SMD parts to realise it. Stick the actual Q values 
into LTSpice and weep. 1dB pass-band loss turns into 60dB loss really 
quickly.

Coilcraft make suitable small inductors that are wound, not multi-layer, 
but you pay a lot more for that.

Clifford Heath

On Sunday, September 4, 2022 at 6:40:14 AM UTC-7, Phil Hobbs wrote:
> >> What about voltage dividers, gain set resistors, voltage reg 
> >> programming, all that? Values matter. 

Errors in those areas are rare, and will usually (but obviously not always) 
get flagged in test.  

> Dunno how you expect to avoid final test. It's pretty important that 
> you know the thing works right before shipping it, yes?

Of course there's a test/burn-in cycle, and it's *very* thorough.  Everything in
the front end is actively tested via a custom ATE harness.  However, there are 
four switched LP/BP filters to check, and we were originally testing those by 
simply feeding in a signal near the center frequency and verifying that the 
UUT reported the expected power level +/- the expected tolerance.  That wasn't
sufficient to catch one of the cases where they swapped some 0603 inductors
around.  

Fortunately I caught it myself when diagnosing another problem. The swapped 
inductors were not a major issue in this case -- they wouldn't have caused a
spec violation, and almost certainly would never have been noticed in the 
field -- but that whole episode scared me straight.  We now test at several more
points, and next time around I'll use a full mask test.

I've found enough forehead-slapping production faults in HP and Tek gear 
over the years that I don't harbor any illusions about perfection at the 
assembly or QA stages.  It's an axiom that you're not supposed to rely on testing 
for quality -- I think that goes back to Deming? -- but that's an exercise in
wishful thinking IMHO.  As long as people are involved, you've gotta test 
thoroughly or quality will suffer.

-- john, KE5FX

On Saturday, September 3, 2022 at 10:54:55 PM UTC-7, Mike Monett VE3BTI wrote:
> "John Miles, KE5FX" <jmi...@gmail.com> wrote:
> When 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? 

I wouldn't be surprised to see a lot of those NanoVNAs showing up in
applications like this.  But in our case we use a custom DDS signal generator
(AD9959+some THS3491s) that is driven via SCPI for the stimulus, and 
the UUT itself reports the response.  

We need to be able to test several units at once in a fully-automated fashion, 
given that each unit needs a minimum of 4 hours to pass.  

-- john, KE5FX

On Sunday, September 4, 2022 at 7:40:44 AM UTC-7, jla...@highlandsniptechnology.com wrote:
> We usually buy small Ls that have a color dot or something. The 
> Coilcrafts do. Our automated VOA machine can check them. Every board 
> gets inspected with that. 
> 
> Inspection has a powerful process feedback element. We flog as 
> required.

Good point there, our (Coilcraft) parts also have the dots.  Most of them do, anyway.

-- john, KE5FX

On Sunday, September 4, 2022 at 5:00:24 PM UTC-7, Clifford Heath wrote:
> On 4/9/22 06:29, jla...@highlandsniptechnology.com wrote: 
> > On Sat, 3 Sep 2022 12:47:45 -0700 (PDT), "John Miles, KE5FX" 
> > <jmi...@gmail.com> wrote: 
> >> 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.
> It's difficult to build good LC filters in SMD for HF. The multi-layer 
> inductors that work fine at UHF just don't have the required Q at lower 
> frequencies. 
> 
> I designed a nice-looking 7th order bandpass filter for 50MHz, then 
> started looking for SMD parts to realise it. Stick the actual Q values 
> into LTSpice and weep. 1dB pass-band loss turns into 60dB loss really 
> quickly. 
> 
> Coilcraft make suitable small inductors that are wound, not multi-layer, 
> but you pay a lot more for that. 

By "7th order bandpass" I'll guess you mean 7 resonators. BPF are usually even order. Anyway, you're right that wirewound CCI will be more expensive than multilayer. Multilyers are rarely good enough for bandpass filters.

Just to experiment, I hacked out a 5-coil 10% 50 MHz BPF using 82 nH MIDI (1812SMS-82NGL_) coils, just to experiment.

features
* 50 MHz center freq
* 10% nominal BW (b4 non-idealities)
* 20 dB nominal return loss
* all 5 coils coerced to 82 nH
(coercion "paid for" with additional caps)
* 5 finite transmission zeros
* Coil Q: guess 80 (midi spring)
* Cap Q: guess 250 
* All internal nodes have design capacitance to ground
(enable absorbtion of parasitic capacitance; costs caps)



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