Single-Board Universal Module

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29 Sep 2014 21:11 #25882 by PhracturedBlue
Replied by PhracturedBlue on topic Single-Board Universal Module

moss wrote: scraping off the solder mask under microscope with an X-acto blade is not that hard, easy no, but not that bad. Once the mask is well scraped off, add some solder to the copper.
Then you just need to cut a gap in the scraped trace, and reflow on the cap over the gap. Adding "pads"? Not necessary.

Look at eBay for "10X-20X Boom Arm Widefield Stereo Microscope" and you can find a decent solder scope for under $200.

Heh...as if it were actually so easy. Let me tell you how it goes at my house....scrape resist off...looks good...add solder to exposed lines...oh it won't stick...add more heat, crap I jut bumped it and got solder on the neighboring cap....carefuly try to remove it, now the cap came off too...and brought the pad with it...damn, now try to put the cap back, looks ok, but there isn't a connection..add more solder...go back up a couple steps and repeat...house is now on fire and everyone is running for their lives...the board is a smoking ruin of course. Well, maybe not quite, but you should have seen the massacre that resulted from my trying to clean up a few solder bridges on the STM32 processor.

In all seriousness, I have decent equipment (including a temperature controlled iron, hot-air rework, and binocular scope). What I don't have is a steady hand or much experience working with these tiny components. I am amazed I was able to assemble one of these boards on my own and actually get it to work. I am not going to push my luck and damage one further unless absolutely necessary :)

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29 Sep 2014 21:55 #25883 by webbbn
Replied by webbbn on topic Single-Board Universal Module
Congrats on the progress of the universal module. I'm looking forward to seeing how the RF testing goes.

A couple of comments:

Have you seen the nRF51822? I'ts on-air compatible with the nRF24L01, and it includes a pretty decent Cortex-M0 MCU. It might help simplify the design a bit by combining the nRF24L01 and STM32F072.

Also, you may need an attenuator doing the RF testing. I believe the RFExplorer (at least the one that I have) only accepts up to 4dBm.

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30 Sep 2014 01:49 - 30 Sep 2014 22:06 #25885 by octagon
Replied by octagon on topic Single-Board Universal Module
solder mask scraping:
It is easy to get solderability when the mask is completely cleared off. Even a thin layer of mask will still stop the solder, and more heat will not help.
Bright red, raw, shiny, copper is needed. It takes solder like a wick, if not, scrape some more! Solder mask may be 2 mils thick wile the copper is ~ 1.3 mil, so go easy, keeping the x-acto blade parallel with the board.

Moving the light source to a low angle can reveal new stuff.

Have fun!
Last edit: 30 Sep 2014 22:06 by octagon.

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02 Oct 2014 22:20 #25930 by mikemacwillie
Replied by mikemacwillie on topic Single-Board Universal Module
DC Blocking caps are in. :) The OSHPark solder mask is really soft and scrapes quite easily. I use a #15 scalpel under the microscope and it didn't give me any trouble. The pic isn't the greatest (Only have my phone with me currently), but it gets the point across.

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02 Oct 2014 22:22 - 02 Oct 2014 22:23 #25931 by octagon
Replied by octagon on topic Single-Board Universal Module
Perfect!
An iPhone cam in the microscope eye-let works pretty good, I've found. Some vignetting but usable.
Last edit: 02 Oct 2014 22:23 by octagon.

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03 Oct 2014 22:07 #25936 by octagon
Replied by octagon on topic Single-Board Universal Module
stupid question:

Could not a single RF chip handle multiple protocols? Or are the RF-encodings too different, differences in PN-numbers,PLL-locking abilities, whatnot?

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04 Oct 2014 02:24 #25940 by PhracturedBlue
Replied by PhracturedBlue on topic Single-Board Universal Module
We've been looking for such a chip for years, and never found one with sufficient overlap in capability. There appear to be a couple of possibilities ofr the CYRF to communicate with a CC2500, but not for the common protocols. The nrf and a7105 also have some overlapping modes, but not at the same data rate.

Sorry I haven't made much progress lately. there were some issues at home that distracted me this week. I have finished some software for testing the RF performance though. I expect I'll get to it this weekend.

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04 Oct 2014 18:55 #25945 by octagon
Replied by octagon on topic Single-Board Universal Module
sorry to bother you PB, with simple questions;

some modes, data rates, and protocols differ?

The overlap is so weak as to make interoperability a non-starter?

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05 Oct 2014 02:15 #25951 by PhracturedBlue
Replied by PhracturedBlue on topic Single-Board Universal Module
I was able to add the 100pF cap to the board as well. It really wasn't too difficult.

I have checked in my latest code which adds a test-mode for the RF modules.

There is good and bad news.
The good news is that I seem to be able to send a signal out of the module:



The down-side is that the signal strength is very weak. it is about 13dBm down from the BUYCHINA cyrf module I have at the same settings. Note that you should ignore the absolute number, I don't have the calibration right for the attenuators I have installed.

I haven't done any debug yet other than to verify the same code works on the Discovery board. It is possible I have some settings wrong. I also haven't gone through and tested all 4 modules yet. But getting any signal at all out is a good start.
Attachments:

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05 Oct 2014 20:06 - 05 Oct 2014 20:10 #25954 by mikemacwillie
Replied by mikemacwillie on topic Single-Board Universal Module
That's definitely better than no output at all! It'll be interesting to see what you get with the other ICs; should help narrow down the area of degradation.
Last edit: 05 Oct 2014 20:10 by mikemacwillie.

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06 Oct 2014 00:09 - 07 Oct 2014 02:46 #25956 by PhracturedBlue
Replied by PhracturedBlue on topic Single-Board Universal Module
Well, I cycled through the 4 transceivers.
CYRF: -41dBm @ 54mA
A7105: -31.5dBm @ 54mA
CC2500: -48dBm @ 73mA
NRF24L01+: no signal @ 33mA

At full power, I would expect the current to be ~400mA, so I think we're nowhere near max power. I need to check that the stm32 I/O is working as expected. Something definitely looks fishy here.
Last edit: 07 Oct 2014 02:46 by PhracturedBlue.

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06 Oct 2014 00:21 #25957 by mikemacwillie
Replied by mikemacwillie on topic Single-Board Universal Module
That does seem quite a bit low. Unfortunately the RFAxis datasheet is pretty light on test data. I wonder if it's getting lost at the switch?


I placed an order for an RF explorer today (Been on the fence for quite a while), so I'll be able to duplicate some testing in a couple weeks.

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06 Oct 2014 01:58 #25958 by PhracturedBlue
Replied by PhracturedBlue on topic Single-Board Universal Module
Note that you'll need the following:
* U.FL to SMA connector
* DC-Block SMA connector (I use this as it also has a 6dB attenuator and power limiter: 'Power Limiter for RF Explorer', but you can find similar items on Ebay)
* 30dB attenuator (I found one on Ebay searching for 'DC-3 Ghz 30dB SMA attenuator')
* sma cables and adapters to connect the RFExplorer to the board.

You can also use antenna on both sides, but this can be tricky if there is any wifi or bluetooth in the area since you'll get a lot of interference.

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06 Oct 2014 02:22 #25959 by mikemacwillie
Replied by mikemacwillie on topic Single-Board Universal Module
Thanks! I already have most of that, but one thing I don't have is a DC block. I'll make sure I grab one. I went for the ISM combo, seems to be the best compromise of features and cost for the bands I'm likely to use it in. The 2.4ghz input is only good up to - 15dBm though. It's amazing how inexpensive they are.

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06 Oct 2014 03:33 #25961 by octagon
Replied by octagon on topic Single-Board Universal Module
In this circuit you could chuck the coil in the output, and replace it with a 1nF cap, or any large size, and remove the two pi-filter caps if you have to.
The right pi-filter can be added later.
No good having DC in RF circuits unless you really need it.

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06 Oct 2014 15:48 #25970 by victzh
Replied by victzh on topic Single-Board Universal Module
@octagon - basically everything is different for the main 4 chips and protocols used. CRC, packet handling (length of address), possible bit rates - all are specific to each of the chips used. Some chips has the direct mode - raw set of bits at given bit rate, frequency, and modulation. Some have even better - I and Q output at given frequency. They require all the packet handling in software which is a bit too much ;-).

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06 Oct 2014 21:22 #25980 by moss
Replied by moss on topic Single-Board Universal Module
@victzh,
Thanks for your patience. I'm sure this is old hat. The different chip mfgs are not really interested in making commodity parts, and by adding unique features this can be avoided. Good for them.
I'm really excited about this project.

There are low cost RMS reading RF-power meters out there, the 5.8GHz video guys sell them, this is a simpler and more direct way of measuring RF power, if you are pretty sure you don't have very strong harmonic output (which is unlikely). Spectrum analyzers are good for analyzing spectral content, while they are OK for measuring RF power, nothing beats an RF-power meter.
Hell, even a calibrated detector diode is good, the output can be fed into a scope input to look for power waveforms.

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06 Oct 2014 21:26 #25982 by mikemacwillie
Replied by mikemacwillie on topic Single-Board Universal Module
Yeah, good point. I do have a diode based detector. If we're only concerned about relative measurements, it could be workable.

I had tossed around the idea of buying the Immersion RC power meter, but it is pretty much the same price as the RF explorer, and I'll get much more use out of the RF explorer than I would the power meter.

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07 Oct 2014 03:59 #25985 by octagon
Replied by octagon on topic Single-Board Universal Module
You could calibrate a detector diode, with a sig gen, not perfect over temperature, but if you don't deviate too much from room temp it can be OK. That Immersion deal probably have one of the numerous great RF detector chips now available.
Some detector diodes can be quite expensive too.

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07 Oct 2014 12:54 #25988 by PhracturedBlue
Replied by PhracturedBlue on topic Single-Board Universal Module
After further experimentation, I don't think the switch is working.
The original code wasn't ever changing the switch controls. However even after fixing the code and verifying the pins are switching, I got the exact same result.
I will next check everything at the switch and make sure I have proper connections there.

It is also possible that the PA isn't working, and what I'm reading is some sort of RF coupling from the board. But I've verified that the PA is enabled as expected.

Also, the CC2500 signal IS there. It is very weak (just above the noise threshold) but I can see it is definitely there.

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