Single-Board Universal Module

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27 Feb 2015 21:00 #29178 by midelic
Replied by midelic on topic Single-Board Universal Module
I'm not expert either...but read a lot...In fact I'm interested you succeed and make this work.

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27 Feb 2015 21:01 - 27 Feb 2015 21:18 #29180 by moss
Replied by moss on topic Single-Board Universal Module
The sequencing requirement seems odd PITA , can you try this on a EVAL board to see if this is critical or else?

SKY says: "The CMOS decoder should have VDD applied before a logic high is
applied to one of the control lines."

...besides the sequencing requirement. Dump that part.

The PE part states:

"Latch-Up Avoidance
Unlike conventional CMOS devices, UltraCMOS® devices are immune to latch-up."
Last edit: 27 Feb 2015 21:18 by moss.

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27 Feb 2015 21:22 #29182 by PhracturedBlue
Replied by PhracturedBlue on topic Single-Board Universal Module
moss, you said you have used the PE 4:1 in a project before. I assume you've used them in a bi-directional situation? If I switch to another 4:1 mux without understanding the issue with this one, I'd like to have some confidence it will actually work.

The datasheet shows a 1M-ohm resistor to VCC and a bypass cap (I think 100pf would be fine here). I think the resistor is just to ensure that the control signals don't float (which they never will in our design), so I should be able to get away without the resistors, right? I really need to keep the component count down as space is really at a premium.

I would definitely prefer to use a 4:1 mux, and the layout changes look minimal for the PE 4:1. Also I can source them from China and Digikey which is nice (I like having multiple suppliers).

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27 Feb 2015 21:24 #29183 by hexfet
Replied by hexfet on topic Single-Board Universal Module
This paper indicates the switches are bidirectional. Though it doesn't mention the 13384 specifically it cites other members of the family and the appendix describes the same technology (pHEMT) used in the 13384.

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27 Feb 2015 21:42 #29184 by moss
Replied by moss on topic Single-Board Universal Module
I am using these PE switches bi-directionally, for a receiver.
I'm not using pull-ups at all. I think this is just for the demo board to not malfunction with CMOS inputs.
You can get them from DigiKey or RFMW in the US.

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27 Feb 2015 22:19 #29185 by PhracturedBlue
Replied by PhracturedBlue on topic Single-Board Universal Module
Yes, I am quite confident that the sky13384 is bi-directional, but it also obviously doesn't work, and we've done the basic debug to indicate that it should. I'd rather take a chance on an alternate design than bang my head against this wall any further. I'll likely put together the updated board design this weekend and place an order for new parts. I may just go ahead and order both a 2:1 switch version and a 4:1 switch version, since the lead time is several weeks, and it is only ~$25 for the boards

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27 Feb 2015 23:29 #29186 by octagon
Replied by octagon on topic Single-Board Universal Module
for $30 you get 10 ea. 5cmx5cm boards with ENIG from seeed. Shipping?
Do you need a foot print for the PE part?

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28 Feb 2015 00:12 - 28 Feb 2015 00:13 #29188 by PhracturedBlue
Replied by PhracturedBlue on topic Single-Board Universal Module
I think SEEED is $40 + shipping for 10 4-layer 5x5 boards (we can only fit one universaltx per board)
OshPark is ~$25 for 3 boards including shipping
More importantly ,we designed to the OshPark design rules, which means the boards are likely to actually work :)

And the footprint for the PE part is the same as the Sky13384 though the pin-out is different
Last edit: 28 Feb 2015 00:13 by PhracturedBlue.

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28 Feb 2015 00:24 - 28 Feb 2015 00:24 #29189 by octagon
Replied by octagon on topic Single-Board Universal Module
The pin out is different, not much, but enough to not make the old one work. The code is also different, for 2 and 3 ports.
OshPark seems good. I ordered boards from seeed that actually worked. The RF explorer is made by seeed, plus many other well known parts, so I'm not worried about them. They also have some eagle rules that you can use if you want. I'm not a fan of tented vias so I did not.
Last edit: 28 Feb 2015 00:24 by octagon.

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28 Feb 2015 01:26 - 28 Feb 2015 01:37 #29195 by mikemacwillie
Replied by mikemacwillie on topic Single-Board Universal Module
The oshpark 4 layer stackup is a bit different than most others. If the microstrip impedance was calculated with the OSHPark layup, that's what you should stick with. OSHPark is one of the only that I'm aware of cheap PCB manufacturers that offers a controlled impedance laminate.

I did a little more digging with the current switch.. I too was concerned that it could be a latch-up problem due to the lack of pull-down resistors on the control lines and the required power-up sequence, but there aren't any measurable power-up transients on the control lines, and adding the pull-down resistors for good measure had no effect.
Last edit: 28 Feb 2015 01:37 by mikemacwillie.

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28 Feb 2015 02:06 #29196 by PhracturedBlue
Replied by PhracturedBlue on topic Single-Board Universal Module
have you tried removing the blocking cap on the output (leaving it open, not connected to the amp), and measuring the signal there? I'm wondering if we should be using something like 10pF instead. Looking around 8-10pF seems to be more typical for 2.4GHz DC blocking.

Also the PE42641 docs are unclear.
Initially it says: 'No DC blocking capacitors are required'
Then later it says 'Blocking capacitors needed only when non-zero DC voltage present' (which seems to indicate DC blocking caps ARE needed)

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28 Feb 2015 02:18 - 28 Feb 2015 02:27 #29197 by octagon
Replied by octagon on topic Single-Board Universal Module
There is no contradiction in that statement.
Blocking caps are built in, but have low breakdown voltage, so if you plan to apply DC to a port, add a cap. Adding a resistive load is OK for this part, but harmful for the SKY part, as it would load down the internal circuit.

An 8pF (murata GRM1555C1H8R0DZ01) would have a loss of 0.04dB at 2.44GHz. The same part would have a self-resonance at 2.56GHz thus be good at killing the signal as a bypass cap.
Last edit: 28 Feb 2015 02:27 by octagon.

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28 Feb 2015 02:28 #29198 by mikemacwillie
Replied by mikemacwillie on topic Single-Board Universal Module
I'm actually using an 8pF cap already. I hadn't realised you were using 100pF. It's not going to make a 20dB difference though.

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28 Feb 2015 02:35 - 28 Feb 2015 02:44 #29199 by octagon
Replied by octagon on topic Single-Board Universal Module
Finding the correct 50 Ohm line width is easy with TX-line and other free utilities. Controlled impedance is usually an extra cost.
The effect of some difference in impedance may not be that much, as these lines are not used for tuning, and are very short.
www.awrcorp.com/products/optional-produc...sion-line-calculator

100pF or 8pF should make no difference here, 100pF would be better at the low frequencies you don't need.
Last edit: 28 Feb 2015 02:44 by octagon.

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28 Feb 2015 05:45 - 01 Mar 2015 00:52 #29202 by PhracturedBlue
Replied by PhracturedBlue on topic Single-Board Universal Module
Here is my attempt at a reworked board with the PE42641:
EDIT: replaced with newer version, see below

The schematics/board can be found here:
bitbucket.org/deviationtx/universaltx

It is basically identical to the 0.9 board, but I fixed the LDO, added some probe points, added caps to the muxsel control lines, and swapped the 4:1 switch. I also removed the blocking cap between the switch and the amp, but I'm not sure that is a good idea.
Last edit: 01 Mar 2015 00:52 by PhracturedBlue.

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28 Feb 2015 16:04 - 28 Feb 2015 19:39 #29207 by octagon
Replied by octagon on topic Single-Board Universal Module
Nice!

Adding a coupling cap wont hurt, but as I remember the RFX2401 does not produce DC on pin 4 (?).

Why not pour ground over the whole board? Using OshPark's stack-up you can calculate line width and isolation with TX-line with CPW-ground. (9 mil isolation layer 1 -2?). Use the "isolate" number in the polygon setting to adjust for the CPW-ground result.
Like with a 9 mil dielectric e=4.4, 10mil gaps, and 15 mil line-width. Pins on IC's can start at 9 mil and change to 15 a bit away from the pins.

Use layer 41 "tRestrict" polygons to keep ground from pouring where you don't want it.

Keep a complete unmasked area under the chip slug, control the amount of solder paste by disallowing "cream" on SMDs, and draw you own "cream" layer on pads.
(default cream is typically too much)/

Use layer 41 "tRestrict" polygons to keep ground from pouring where you don't want it.

More ground vias can't hurt.

The two big mounting holes can be vias and assigned to VSS for making large low inductance ground vias. Some vendors charge more for un-plated holes are this is an extra process step.

The USB pads should be grounded and given vias to strengthen them. I've pulled off a few of these. For occasional programming use it may be OK.
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Last edit: 28 Feb 2015 19:39 by octagon.

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28 Feb 2015 19:45 - 28 Feb 2015 19:54 #29212 by PhracturedBlue
Replied by PhracturedBlue on topic Single-Board Universal Module
Thanks for the feedback.
I've connected the USB pads to ground, that should help some. adding vias to the pads isn't feasible due to the LDO on the bottom (which I have no place to move to)

I don't pour ground because I don't like the way it fills on the top of the board. Maybe if I play with it more, I can get something I like.

I thought 'd calculated 9mil as ok for the width, but using txline with Oshpark's stackup, I get about 13mil:
FR408 = 3.6 dielectric constant
1.4mil copper
6.7mil prepreg
support.oshpark.com/support/articles/122...p-and-specifications

Using vias rather than mounting holes is a good idea, I'll give that a shot too.

Adding more vias overall is always a good idea, there isn't too much room unless I fill the RF section though.
Last edit: 28 Feb 2015 19:54 by PhracturedBlue.

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01 Mar 2015 00:24 #29218 by octagon
Replied by octagon on topic Single-Board Universal Module
Parasitic capacitance from ground pour can be fixed with the application of "tRestrict" polygons where needed.

Via inductance can be a problem, like in series with bypass caps where it would make bypassing less effective.
Using larger drills if possible is preferable. Minimize the use of 13 mil drills to where it only possible.
I'd rather have those inductances in parallel on poured ground than isolated.

www.saturnpcb.com/pcb_toolkit.htm

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01 Mar 2015 00:32 #29219 by PhracturedBlue
Replied by PhracturedBlue on topic Single-Board Universal Module
I spent a couple hours pouring the top plane and after I had it looking the way I wanted, I remembered another reason why I didn't do it previously. My concern was that the 3.3V layer had become less robust and may be a risk to delivering stable power to the RF chips. So I spent some more time moving vias around and I think it looks ok now.

Here is the updated design:
oshpark.com/shared_projects/H6IjHydR
I've also checked the updates into bitbucket:
bitbucket.org/deviationtx/universaltx


I ended up not replacing the mounting holes with vias. I don't need to with OshPark and would rather not change anything I don't need to.

We've already created custom cream layers and I had no issue applying it with a stencil, so I'm not going to worry about that either.

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01 Mar 2015 01:27 #29222 by octagon
Replied by octagon on topic Single-Board Universal Module
The largest current draw on the board is the RFX chip at 100mA, which is not a heavy load. Bypassing the DC feed point to that chip with- and giving good ground should be a priority. I would not be too worried about LF aspects of this. Good ground should be a higher priority. RFaxis may have some eval board layouts for this.

Cutting out ground from around the PE switch serves no purpose, and is not done in their model layout. Keep the ground solid around it.

Adding line and via inductance to the output highpass filters capacitors ground leads alters the filters response, did you run an RF simulation with these added?
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