DDS differential filter...

[server]

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.

 

[Clifford Heath]

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

 

[server]

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

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.

 

[Phil Hobbs]

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 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

 

[server]

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 Hobbs

We 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.

 

[John Miles, KE5FX]

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

 

[server]

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.

 

[Jeroen Belleman]

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

 

[John Miles, KE5FX]

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

 

[Mike Monett VE3BTI]

\"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, 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



--
MRM

 

[whit3rd]

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?

 

[Phil Hobbs]

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

 

[Phil Hobbs]

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

 

[server]

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.

 

[Clifford Heath]

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

 

[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

 

[John Miles, KE5FX]

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

 

[John Miles, 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

 

[John Miles, 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

 

[Simon S Aysdie]

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)



Version 4
SHEET 1 6120 3588
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WIRE 624 -736 560 -736
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FLAG -240 -240 0
FLAG 3056 -240 0
SYMBOL res 3040 -528 R0
SYMATTR InstName R_load00
SYMATTR Value 50
SYMBOL voltage -240 -544 R0
WINDOW 0 25 23 Left 2
WINDOW 3 25 93 Left 2
WINDOW 123 36 64 Left 2
WINDOW 39 27 106 Left 2
SYMATTR InstName Vsrc00
SYMATTR Value \"\"
SYMATTR Value2 AC 2
SYMATTR SpiceLine Rser=50
SYMBOL cap 272 -688 R90
WINDOW 0 0 32 VBottom 2
WINDOW 3 32 32 VTop 2
SYMATTR InstName C1
SYMATTR Value 343.705540p
SYMATTR SpiceLine Rser=0.037
SYMBOL cap 384 -512 M0
SYMATTR InstName C2
SYMATTR Value 28.367562p
SYMATTR SpiceLine Rser=0.449
SYMBOL ind 576 -624 R90
WINDOW 0 5 56 VBottom 2
WINDOW 3 32 56 VTop 2
SYMATTR InstName L1
SYMATTR Value 82n
SYMATTR SpiceLine Rser=0.322
SYMBOL cap 560 -752 R90
WINDOW 0 0 32 VBottom 2
WINDOW 3 32 32 VTop 2
SYMATTR InstName C3
SYMATTR Value 75.416515p
SYMATTR SpiceLine Rser=0.169
SYMBOL cap 704 -480 M0
SYMATTR InstName C4
SYMATTR Value 14.427057p
SYMATTR SpiceLine Rser=0.883
SYMBOL cap 848 -688 R90
WINDOW 0 0 32 VBottom 2
WINDOW 3 32 32 VTop 2
SYMATTR InstName C5
SYMATTR Value 63.512945p
SYMATTR SpiceLine Rser=0.200
SYMBOL ind 944 -608 R0
SYMATTR InstName L2
SYMATTR Value 82n
SYMATTR SpiceLine Rser=0.322
SYMBOL cap 976 -416 M0
SYMATTR InstName C6
SYMATTR Value 292.455428p
SYMATTR SpiceLine Rser=0.044
SYMBOL cap 1184 -528 M0
SYMATTR InstName C7
SYMATTR Value 196.936941p
SYMATTR SpiceLine Rser=0.065
SYMBOL cap 1376 -480 M0
SYMATTR InstName C8
SYMATTR Value 6.183996p
SYMATTR SpiceLine Rser=2.059
SYMBOL cap 1296 -688 R90
WINDOW 0 0 32 VBottom 2
WINDOW 3 32 32 VTop 2
SYMATTR InstName C9
SYMATTR Value 29.234988p
SYMATTR SpiceLine Rser=0.436
SYMBOL ind 1568 -624 R90
WINDOW 0 5 56 VBottom 2
WINDOW 3 32 56 VTop 2
SYMATTR InstName L3
SYMATTR Value 82n
SYMATTR SpiceLine Rser=0.322
SYMBOL cap 1552 -752 R90
WINDOW 0 0 32 VBottom 2
WINDOW 3 32 32 VTop 2
SYMATTR InstName C10
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SYMATTR SpiceLine Rser=0.148
SYMBOL ind 1696 -464 R0
SYMATTR InstName L4
SYMATTR Value 82n
SYMATTR SpiceLine Rser=0.322
SYMBOL cap 1696 -608 R0
SYMATTR InstName C11
SYMATTR Value 159.281578p
SYMATTR SpiceLine Rser=0.080
SYMBOL cap 1792 -448 R0
SYMATTR InstName C12
SYMATTR Value 43.812471p
SYMATTR SpiceLine Rser=0.291
SYMBOL cap 2048 -560 M0
SYMATTR InstName C13
SYMATTR Value 111.772393p
SYMATTR SpiceLine Rser=0.114
SYMBOL cap 2240 -480 M0
SYMATTR InstName C14
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SYMATTR SpiceLine Rser=0.113
SYMBOL cap 2160 -688 R90
WINDOW 0 0 32 VBottom 2
WINDOW 3 32 32 VTop 2
SYMATTR InstName C15
SYMATTR Value 170.870341p
SYMATTR SpiceLine Rser=0.075
SYMBOL ind 2432 -624 R90
WINDOW 0 5 56 VBottom 2
WINDOW 3 32 56 VTop 2
SYMATTR InstName L5
SYMATTR Value 82n
SYMATTR SpiceLine Rser=0.322
SYMBOL cap 2416 -752 R90
WINDOW 0 0 32 VBottom 2
WINDOW 3 32 32 VTop 2
SYMATTR InstName C16
SYMATTR Value 45.940816p
SYMATTR SpiceLine Rser=0.277
SYMBOL cap 2560 -480 M0
SYMATTR InstName C17
SYMATTR Value 70.752997p
SYMATTR SpiceLine Rser=0.180
SYMBOL cap 2656 -688 R90
WINDOW 0 0 32 VBottom 2
WINDOW 3 32 32 VTop 2
SYMATTR InstName C18
SYMATTR Value 104.602713p
SYMATTR SpiceLine Rser=0.122
TEXT -1312 -2784 Left 2 ;o
TEXT 5416 1528 Left 2 ;o
TEXT -232 -856 Left 2 !.net I(R_load00) Vsrc00
TEXT -240 -792 Left 2 !.SAVE S11(vsrc00) S21(vsrc00)
TEXT -216 -912 Left 2 !.ac lin 3801 10e6 200e6
TEXT 448 -1024 Left 2 ;\"10% BW\" 50 MHz BPF\\nQL = 80\\nQC = 250