analog mezzanine physical/interface specification

[jordens]

Here is the final summary, based on comments in the thread below. This post will be sticky at the top, containing the most current specification. Items labeled ACTION: are not finalized yet; @dhslichter will edit this comment with the correct information as it is finalized.

Mezzanine geometry/stackup

• mezzanine board 32 x 100 mm, .047" nominal thickness, 4 layer boards
  • top and bottom layers to be Rogers 4350B, .020" thickness (cap construction for PCBs)
  • 50 ohm CPWG on .020" Rogers 4350B is .035" trace with .011" gaps
  • need ground stitch vias every ~.050" or less along both sides of CPWG
• mezzanines are mounted to RTM motherboard using 4x M1.5 screws
• 9 mm stand off between motherboard and mezzanine
• RF shields on both sides, separate cavities for each DAC channel and ADC channel (4 total per mezzanine)
  • interior height under DAC side shield lids needs to be .170" minimum (allowing for fabrication tolerances) to accommodate baluns
  • interior height under ADC side shield lids .120" (3 mm).

Connectors

• power/IO: Samtec hermaphroditic, 40-pins connectors: LSS-120-01-L-DV-A and LSS-120-02-L-DV-A
• RF to/from motherboard
  • DACs: 4 SMP (2 differential pairs) - Molex 85305-0232
  • ADCs: 4 SMP (2 differential pairs) - SMP-MSLD-PCT-10
  • clock: 2 SMP (1 differential pair) generated on sayma_clk mezzanine - either style, depending on side of board where clock circuitry is needed.
• panel RF connection
  • see analog mezzanine front panel connectors selection #7
  • option to have SMA shield isolated from Sayma ground (i.e. not grounded to panel) -- use plastic insulating washers on both sides of panel (e.g. http://www.rf-microwave.com/datasheets/603_Huber-Suhner_77-Z-0-0-8_01.pdf)
  • DAC side of board: 2 SMA (Telegartner J01151A0451)
  • ADC side of board: 2 SMA (Telegartner J01151A0921)
  • ACTION: use multi-coax assembly for front panel?

Power rails

• +12VDC @ 200 mA, max 1 mV p-p noise in 20 Hz-20 MHz bandwidth
• +6VDC @ 1.5 A, max 1 mV p-p noise in 20 Hz-20 MHz bandwidth
• -12VDC @ 50 mA, max 1 mV p-p noise in 20 Hz-20 MHz bandwidth
• -6VDC @ 150 mA, max 1 mV p-p noise in 20 Hz-20 MHz bandwidth
• no switching "spikes" on rails
• pi filters on all power lines
  • 0402 NP0 1 nF cap (16V min rating) parallel 0805 X7R 10 uF cap (16V min rating - Samsung CL21B106KOQNNN or equivalent DC bias performance -- degrades to 7.5 uF @ 3.3V, 5.5 uF @ 6V, 2.7 uF @ 12V) to ground, between mezzanine and choke
  • BLM41PG181SN1L choke
  • 0805 X7R 10 uF cap to ground (16V min rating - Samsung CL21B106KOQNNN or equivalent DC bias performance), between choke and supply
• desired supply voltages to be generated from these rails on mezzanine with dedicated LDOs
• no switching supplies on mezzanines
• +3.3VDC @ 1 A for digital-only devices on mezzanine. Should still be filtered on Sayma to minimize noise. max 10 mV p-p noise in 20 Hz-20 MHz bandwidth acceptable.
• single shared ground for analog and digital, connected to all SMP shields and all ground pins of power/IO connector

Power/IO header routing

• as shown in image below from @gkasprow with exceptions listed below
• power line filters to pi filters as described above.
• digital lines IO0-IO15 3.3V LVCMOS
• I2C SDA/SCL (3.3V), pullup resistors to +3.3V installed on Sayma. I2C fed from I2C mux chips on Sayma card. Easy/compact level shifting on mezzanines (if needed) using discrete MOSFETs: http://www.nxp.com/documents/application_note/AN10441.pdf
• digital IO are general purpose in hardware, but for SPI preferred pinout is:
  • SCLK = IO0
  • MOSI = IO1
  • MISO = IO2
  • CS = IO3
  • additional chip selects starting with IO4 and up
• power rails as shown (7V rail is actually 6V)
• 4x connections to slow ADC (24-bit sigma-delta) for drift/temp/etc monitoring. ADC chip is mounted on See thermal design #11.

[gkasprow]

I did some estimation.
We can easily fit four 32x100mm DAC/ADC mezzanines on RTM. We can even make them longer.
On each of them we will fit four Wurth 30x30mm shields placed on both sides.
In case of RF connectors I consider using:
2 high performance SMA, edge mount for DAC output
2 lower performance, right angled for ADC input.
8 SMP connectors for ADC/DAC. THT version with SMD central pad would give best mechanical properties. The distance between mezzanine and RTM is 9mm which is minimal value for SMP+bullet.
For digital IO and control I'd like to use Samtec hermaphroditic, 40 pins connectors: LSS-120-01-L-DV-A and LSS-120-02-L-DV-A which combined give exactly 9mm. I use them in other projects and they work well with frequent mating.
Mechanical mounting would be ensured by 4 x 2.6mm holes, 4 threaded spacers and 8 M2.5 screws.
There is still place on front panel for SMA clock input
I placed typical bulky SMA connectors to check the clearance between them. It is not that bad. Then are accessible with the wrench when the RTM is outside of the crate.
Attached are PDF3D files with mezzanine and RTM.
mezzanine.pdf
RTM_DAC.pdf
We will leave the space below mezzanines to enable the air flow. Some cut-outs in RTM may be necessary as well.

[gkasprow]

The clock module dimension is 43x80mm
We can make it slightly longer when necessary
here is the RTM with clock module
RTM_DAC_clk.pdf

[jordens]

Thanks! Impressive.

• Is this still with the SMA firmly screwed into the front panel? How would the mezzanines mounted in that case?
• Did we want no common LO clock (for whatever upconversion needs there may be) to the mezzanines?
• I assume that there is still space for the dacs, adcs, power supplies, and clock fanout?

[sbourdeauducq]

I cannot open any of those PDFs, either with mupdf or Evince.

$ mupdf mezzanine.pdf 
error: expected trailer marker
error: cannot parse trailer
error: cannot read xref (ofs=617742)
error: cannot read xref at offset 617742

[whitequark]

@sbourdeauducq Yes. As far as I know PDF3D can be opened only with proprietary Adobe Reader for Linux.

[sbourdeauducq]

Adobe Reader freezes when trying to open those documents.

[sbourdeauducq]

Is there another format we can use for this?

[gkasprow]

we can use step

On 1 October 2016 at 18:10, Sébastien Bourdeauducq <notifications@github.com

wrote:

Is there another format we can use for this?

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[gkasprow]

To view step files I use free Design Spark Mechanic.

[whitequark]

FreeCAD is very bad but it should suffice to view STEP files.

[gkasprow]

Due to 10MB limit I had to zip them
CLK module.zip
mezzanine2.zip
RTM_DAC_clk.zip

[gkasprow]

In case it pdf 3D files you have to use recent Adobe Acrobat Reder

[gkasprow]

Is this still with the SMA firmly screwed into the front panel? How would the mezzanines mounted in >that case?

the mezzanines are mounted to the RTM board using 4 M1.5 screws

Did we want no common LO clock (for whatever upconversion needs there may be) to the >mezzanines?
I'll add another SMP connector
I assume that there is still space for the dacs, adcs, power supplies, and clock fanout?
sure, some of them can be plalced below the mezzanines

[gkasprow]

Is this still with the SMA firmly screwed into the front panel?

There are versions with long thread both right angle and straddle mounted.
But their performance need to be verified since they are low cost.
http://www.amphenolrf.com/132289.html
http://www.rfstreet.com/ch_contentd.asp?id=2043
on the other side the assembly of the modules changed - they will be firmly attached to the rigid RTM board by 4 screws so it may be not necessary to screw them to the front panel

[sbourdeauducq]

Thanks. The STEP files work fine with FreeCAD. FYI, the Acrobat Android app and the Chrome PDF viewer also choke on the PDF3Ds.

[sbourdeauducq]

What is the idea behind having more SMPs than SMAs on the mezzanines? Since this also results in more SMPs than DAC/ADC channels on the RTM carrier, how will they be connected?

[gkasprow]

Did you enable the 3D view in AR?

It needs a while while opening the file

Greg

[gkasprow]

The signals of the ADCs and DACs are differential, that's why we need the SMPs doubled.
The mezzanines inputs and outputs are single ended.
The idea is to place baluns on the mezzanines because the way one feeds the ADC or couples to DAC depends on application.

[sbourdeauducq]

Aren't SMP connectors designed for single-ended unbalanced signals?

[gkasprow]

Well, in this way you can say that traces on PCB are designed for single
ended signals.
If you make differential traces that are loosely coupled, you can treat
them as single ended. When you make traces tightly coupled, then they
interfere with each other and if you split them then their impedance will
increase.
The same applies to SMAs - on every devkit you can connect gigabit
transceivers using single-ended connectors.
To transfer signals using SMPs you have to make impedance conversion from
tightly coupled diff line to loosely coupled diff line and then transfer
via connectors.
So to make 100Ohm diff line you can use 2 traces wit 50Ohm but separated
far away or use two 70Ohm traces coupled in such way that trace field see
each other. In first case you can transfer them wia SMP, SMA or whatever,
in second case you have to transfer the line to the first case.

On 2 October 2016 at 17:08, Sébastien Bourdeauducq <notifications@github.com

wrote:

Aren't SMPs designed for single-ended unbalanced signals?

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[sbourdeauducq]

on every devkit you can connect gigabit transceivers using single-ended connectors.

Yes, but I've been suspicious of this and wondering if it were just a cheap hack. Is it actually working well?

[jordens]

This works perfectly fine. Convince yourself using the method of image charges and symmetries.

[dhslichter]

It's very hard to find true differential signal connectors where the channel-to-channel crosstalk is anywhere near as low as if one uses two coaxial connectors (e.g. SMP) for a differential signal. Sending the ADC/DAC differential signals over pairs of SMP connectors is the best compromise we could come up with given crosstalk/space/price/bandwidth/mating limitations and requirements.

[jbqubit]

Impressive drawings!

Wurth 30x30mm shields placed on both sides

• Should these also cover the back side where the SMP penetrate?
• A single shield on each side would give more freedom for component placement.
• What's the maximum vertical height for components? Is this sufficient for bulky components like baluns, transformers?

Is this still the plan?

• 4-layer PCB, top layer .020" Rogers 4350B
• single ground plane for analog and digital
  • Harty's comment: I tend to agree with the Good Book on EMC(http://www.amazon.com/Electromagnetic-Compatibility-Engineering-Henry-Ott/product-reviews/0470189304/ref=cm_cr_dp_see_all_btm?ie=UTF8&showViewpoints=1&sortBy=bySubmissionDateDescending) on this point that "thou shall have only one ground-plane before thee".
• separate power planes for analog and digital

[dhslichter]

Several points:

• if board will have components on both sides, should use Rogers 4350B on both sides. OK to make it thinner than .020" -- 50 ohm CPWG on .020" Rogers 4350B is 35 mil trace with 11 mil gaps, based on measurements at NIST.
• shields should be multi-cavity (if a single part) to reduce crosstalk between channels on the same daughtercard. Something along the lines of https://leadertechinc.com/product/multi-cavity-slot-lok/ would be good.
• interior height under shield lids needs to be .165" minimum to accommodate baluns
• crosstalk reduction would be improved if shields extend all the way to the end of the board, such that SMP connectors are connected through holes in the shields. Unshielded lengths should be short to the SMA connectors. Notches need to be made in the shields to avoid impedance discontinuities where CPWG traces exit from under the shields to the SMA connectors.

[gkasprow]

The Wurth shields were only placed just to reserve place on PCB. There are
smaller ones like these
http://www.digikey.com/product-search/en/rf-if-and-rfid/rf-shields/3539677?k=&pkeyword=&pv1217=1&FV=fff40036%2Cfff802dd%2C13040001%2C13040009&mnonly=0&newproducts=0&ColumnSort=0&page=1&quantity=0&ptm=0&fid=0&pageSize=25

[dhslichter]

Sounds good to me -- we should probably do some tests (S parameters/TDR) to check whether this creates a problematic impedance discontinuity to have the right-angle connector substantially offset vertically.

[gkasprow]

That's easy to check - one can compare two mezzanines with RA SMAs mounted in standard and elevated way.
By elevating the SMA we increase the centre to centre distance to 10.56mm
Theoretically we can elevate them even more, but I'm afraid of impedance mismatch. Maybe piece of 1mm FR4 with metallized via would help :)

[sbouhabib]

I agree this has to be checked, the impedance mismatch first, then we have an unshielded small antenna (maybe just a minor issue), the idea with some board piece could be a solution, not sure about FR4 in general, I think Rogers class laminate should be for the RF parts. technically it could cause some issues for assembly, and ground continuity/repeatability of connection

[dtcallcock]

Here's a somewhat more radical idea. I think that a dense field of ~200 SMAs (in a full rack) with sub-mm clearances is going to be basically unusable on a day-to-day basis so I can imagine that most people will take everything to patch panels. If that's the case, then why not use a multi-coax connector on the board and supply an SMA breakout with each board?

There are quite a few solutions out there such as the Smith MDHC. Other suppliers include Times, Carlisle and H&S.. I haven't dug into specs too much but it seems you can get beyond-SMA electrical performance.

Whilst I think this will make a much nicer final product, it will presumably add a lot of cost, increases design complexity and is more likely to be obsoleted than SMA (still going strong at 50+).

[jmizrahi]

@dtcallcock in general I think it's a good idea to avoid obscure, expensive connectors and cables if at all possible. I also don't really see what your solution solves -- it requires an SMA breakout panel, just as before. It's definitely more elegant, but I don't think that's a strong enough argument to move away from SMA.

I would say that so long as it is possible to connect the cables with a wrench, even if it has to be done in sequential order, then it's perfectly usable and I would stick with SMA. You wouldn't even necessarily need a patch panel -- a bunch of 100mm SMA cables could just be permanently attached to the front panel, and then all connections and disconnections happen with the 100mm cables.

[ghost]

I also don't really see what your solution solves -- it requires an SMA breakout panel

Probably several panels. These can then have adequate connector spacing, different connectors depending on requirements, and space for sensible cable management. External up/down-conversion or LO distribution could also be mounted here.

avoid obscure, expensive connectors

Probably no more obscure than the other connector on the mezzanine. As for cost, I'll admit I have no idea (though decent SMAs aren't exactly cheap and you have to buy the cable either way). Anyone have experience here @sbouhabib?

even if it has to be done in sequential order, then it's perfectly usable

Sure, for one or two cards this is fine but my worry is how well it scales to ~200 connections. It's going to be a pig to change cards, debug damaged cables/connectors and could easily be a rats' nest that puts a lot of mechanical force on the cards and connectors. Also a bundle of 100mm cables and i-pieces doesn't seem the best solution for crosstalk, ground loops and maintaining the phase and amplitude stability of microwave signals. I'm sure each lab will come up with a reasonable coping strategy but I thought this might be a good juncture to engineer a nice solution.

It seems like the consensus is that I'm overstating the problem though so I'm happy to drop this.

[dhslichter]

Two different github accounts @dtcallcock @allcockd ? Fancy pants!

The other mezzanine connectors are actually standard SMP (~$6 each), plus the power/io pin connector (also cheap). The spec'ed SMA connectors are ~$20-$25 each. My guess about these sorts of integrated connectors, based on what gets charged for standard Micro-D and other fancy space-hardware stuff, is that they'd be several hundred dollars apiece at a minimum. Factor in that you are buying a mating pair as well, with one end integrated into a cable assembly that likely can only be manufactured by the company that makes the fancy connector. $$$$$$$ :D

That said, I don't think we should be ready to drop this right away. We do need to consider that either way (SMA or multi-connector) one is going to want a sturdily mounted SMA breakout panel, and this panel can be used for routing externally generated LO and mounting the required components, among other tasks. What happens to the godawful ratsnest of cables that would emerge from a fully populated crate is going to have to get figured out by each lab independently -- connector changes have no effect there one way or the other.

[dhslichter]

Also would need the shields of the coax cables not to be grounded together or to the panel if we care about ground loops a la @hartytp

[dtcallcock]

plus the power/io pin connector (also cheap)

I mean isn't this connector an 'obscure' Samtec interface (not that there's anything wrong with that)?

$$$$$$$

But I'd be interested to know actual cost/feasibility from someone who's done something like this.

connector changes have no effect there one way or the other.

The ratsnest can be a lot tidier if it's a smaller number of blind mate connectors and wiring harnesses and not a bunch of cables you have to be able to delve into with your hand and an SMA wrench.

What happens to the godawful ratsnest of cables that would emerge from a fully populated crate is going to have to get figured out by each lab independently

Even if we stick with SMAs it'd be good to share ideas on how to do this. Perhaps something to figure out once we've got prototypes and tried a few things out.

I have no idea who @allcockd is.

[jmizrahi]

@dtcallcock my thinking was along the lines of what @dhslichter said, namely that these connectors would be rather custom and quite expensive, and likely unique to whatever company they came from (that's what I meant by "obscure"). But, you make a strong argument that it would make the front panel quite a bit more manageable. Would you be willing to get some quotes so that we have concrete numbers to discuss? I also hadn't realized just how expensive those SMA connectors are -- at $25 a piece, then a $200 8x connector would be a wash, at least as far as direct connector cost is concerned.

[sbourdeauducq]

I also like Smith MDHC. Quote requested.

[gkasprow]

Samer, you shown us during last meeting at WUT some DSUB-like multipin RF connectors.

Do you remember their PN?

[gkasprow]

These are the fbm connectors.but after 2ghz they become quite "hard" to
match properly.theoretically doable but hard and in general narrowbanded

Samer Bou Habib

20.10.2016 10:56 "Grzegorz Kasprowicz" G.Kasprowicz@elka.pw.edu.pl
napisał(a):

Samer, you shown us during last meeting at WUT some DSUB-like multipin RF
connectors.

Do you remember their PN?

[dhslichter]

It looks like a 4x connector (single-width mezzanine) is 35.6 mm wide, so I don't know if we can really put them all next to each other (mezzanines are 32 mm wide now, not sure what the gap is).

Thus if we go Smith MDHC, we should use double-width mezzanine cards with 8x connectors (53.4 mm width, easily fits). Thus there would be 2 connectors per RTM, covering all 16 DAC/ADC lines.

[dhslichter]

MDHC has -85 to -95 dB crosstalk between connectors, return loss better than -20 dB to 13 GHz (more like -30dB at 3GHz and below)....I'd say performance-wise it's definitely suitable.

[dtcallcock]

I've also asked for quotes on:

Carlisle Core HC
Carlisle Core GD
Rosenberger Multiport Minicoax
The latter system has a few parts on Mouser to give you an idea: link

These might be a bit too fine pitch and fragile but see what you think.

[dhslichter]

Any love on these quotes @dtcallcock @sbourdeauducq ? What pricing/lead times are being given?

[sbourdeauducq]

No reply. BTW we should break down this issue into smaller ones.

[dtcallcock]

I got the Rosenberger quote. Still working on Carlisle.

Rosenberger 23C21E-40ML5
4ch solder pin right angle
Qty 100-250 $56.82ea, 5-7wk lead time
Datasheet

Rosenberger L99-816-30E
Cable set 4ch, 30cm, SMA M termination
Qty 100-250 $157.01, 13-15wk lead time
Datasheet

As I said, these things might be too compact.

[jbqubit]

@dtcallcock Thank you for getting these quotes. Price is good economy vs individual SMA.
@sbourdeauducq Please break this issue into smaller ones.

[dhslichter]

@jbqubit @sbourdeauducq I will break this down into smaller issues.

[gkasprow]

http://uk.rs-online.com/web/p/sma-connectors/7123264/
http://uk.rs-online.com/web/p/sma-connectors/7123254/