Author Topic: Designing a better RFM69 radio module  (Read 10524 times)

Felix

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Designing a better RFM69 radio module
« on: May 26, 2016, 09:44:24 PM »
What is this about?
I see a lot of interest in this forum from folks who experiment with all kinds of RF techniques, antennas and matching etc.

I bragged around that I made a RFM69 clone, which is true but it was just an experiment, which I am sure could be much improved, so this is an effort to reach out to the RF experts and ask them to voice their RF wisdom and share advice. I don't have the expensive RF tools to do tests or measure loads matching etc (VNAs and whatnot).

Here's my version of the RFM69 next to Hope's:


Some more background
We've seen threads like this one where discussion starts to go into the details of the RFM69/semtech design of the module.
HopeRF has a sx1231h board that has followed the Semtech design almost to the letter but they cut a few corners to allow the same PCB to assemble into a RFM69W or RFM69HW. They add a few components like a transistor to invert the RF switch control.
They use a specific GaAs RF switch (the G4C marked SC70 package chip) which is discontinued but they seem to have vast amounts of this or source it from asia somehow since all the radios I see many of their radios contain this RF switch (for instance the LoRa radios, HW radios, RFM22b etc). The W radios (13dbm like RFM69W) do not contain this switch or the transistor that drives it and the reason is obvious from the transceiver DS, and has a slightly different output path but uses the same PCB.
One thing that confuses lots of people is the main chip used on the RFM69 which is marked as "RF69". This is a custom QFN28 packaged sx1231h, instead of the original QFN24 "SX1231h" which you can buy from digikey, see photo above for comparison and package pin count difference.
This blue board I made can be used to make Ws or HWs. I used a RF switch that had the logic table inverted so I didn't need the extra transistor+resistor driving it.

SX1231h Datasheet
RF Switch on the RFM69 Datasheet

What I'd like to focus in this discussion:

- use the sx1231h chip to design a board that is 100mW capable, similar to the blue one above
- discuss whether it's worth using a common PCB that can morph into a W or HW or whether optimizing the design for a single variant can reduce any losses and make a better module
- discuss the possibility of a 4 layer and what the real gains are - middle layers would have GND and VCC to follow RF design patterns and minimize capacitive coupling etc
- discuss the necessity of a metal can cover
- discuss the possibility of replacing the matching network with a single component - i've seen these on some RF boards but I don't know what/how/if this can replace the many 0402 components. If it's possible this would reduce complexity, perhaps reduce losses, maybe even cost
- RF chips usually are very dense and use 0402 components as industry standard, but we've seen the RFM12b using 0603 which is MUCH less challenging to assemble on a pick and place
- pinout should be unchanged, with the exception that the NC (not connected pin) could be used for something useful like telling the difference between a W and HW, which could then be used in software (a logical HIGH on that pin could mean HW for instance)
- substrate PCB should be no thicker than 1mm
- of course, made in the USA :)
- price will be more for many reasons: a basic BOM is easily a few dollars (sx1231h, good quality crystal, rf switch). The quest here is to see if a better design is possible and if the tradeoff with cost makes it justifiable to produce.

I will add to this list as ideas come up.

Please share your advice, RF experts!
Let's watch this space produce something great, but keeping it focused, with information backed by experience and data.

WhiteHare

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Re: Designing a better RFM69 radio module
« Reply #1 on: May 27, 2016, 01:02:58 PM »
It's not an RF issue per se, but since you're wanting to sell these, one obvious "improvement" would be to move from a 2.0 pitch to a 2.1 pitch.  That would obsolete the need for breakout boards, making it an easily obtained win-win differentiator.  Also, maybe doing that would give you enough extra space that you could use the slightly larger components that you've said would be easier for you to pick-and-place.   :)
« Last Edit: May 27, 2016, 02:14:14 PM by WhiteHare »

TomWS

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Re: Designing a better RFM69 radio module
« Reply #2 on: May 27, 2016, 05:24:51 PM »
It's not an RF issue per se, but since you're wanting to sell these, one obvious "improvement" would be to move from a 2.0 pitch to a 2.1 pitch.
Who uses 2.1mm pitch?  2.0 pitch IS a standard, it's used on quite a few castellated pin modules, eg ES-12E, RTX4140, as well as all the 'Bees (Zig, X, etc)...

Tom
Updated to correct phrasing on 'Bee modules.
« Last Edit: May 27, 2016, 06:16:54 PM by TomWS »

WhiteHare

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Re: Designing a better RFM69 radio module
« Reply #3 on: May 27, 2016, 06:36:14 PM »
Yes, I obviously meant a 2.54mm pitch, and should have said so instead of 2.1mm. I guess I somehow conflated it with 0.1" to yield the erroneous figure.  Anyhow, 2.54mm would translate to a lower cost for some users (those buying and using breakout boards because of the wonderful 2.0mm "standard"), and that's all I was trying to say.  If you prefer to hand that market to adafruit and sparkfun rather than harvest an easy target, then by all means stick with 2.0mm.  Maybe they'll send you a thank-you card.
« Last Edit: May 27, 2016, 08:14:49 PM by WhiteHare »

captcha

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Re: Designing a better RFM69 radio module
« Reply #4 on: May 27, 2016, 08:24:02 PM »
0.1" pitch would indeed be a welcome improvement for beginners as I still remember soldering little bits of wire to each leg to make my own breakout board. Sure, for further stages many would prefer to design their own pcb and would care less but for those starting with these modules I think it would help them to get on board (pun intended) quicker.

Is there a need to have all pins lined up in two rows? Once you change the footprint you may as well have pins on all four sides.

One other suggestion I can make is to see if a bigger decoupling cap is still required. In some posts I've read that the 100mW RFM6HW is pushing the envelope a bit and would welcome some extra caps close to the pa.

I like the idea of an rf shield over the relevant components. Maybe just have pads for mounting pins at first.

Not sure whether 4-layer is required. If you can get a nice GND/shield layer underneath with a 2-layer board then go for it.

TomWS

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Re: Designing a better RFM69 radio module
« Reply #5 on: May 27, 2016, 08:31:53 PM »
Yes, I obviously meant a 2.54mm pitch, and should have said so instead of 2.1mm. I guess I somehow conflated it with 0.1" to yield the erroneous figure.  Anyhow, 2.54mm would translate to a lower cost for some users (those buying and using breakout boards because of the wonderful 2.0mm "standard"), and that's all I was trying to say.  If you prefer to hand that market to adafruit and sparkfun rather than harvest an easy target, then by all means stick with 2.0mm.  Maybe they'll send you a thank-you card.
Well the marked up market might exist in the breakout board domain,  but the volume market is clearly on the side of 'smaller is better'.  I'd buy one breakout board to 100 PCB mounted radios in the smallest form factor possible.  And WRT the RFM69, you don't even need a breakout board to fan out the 2mm pitch pad/holes to a 0.1" breadboard strip.  You just need about 8 - 10 22ga wires cut to 1/2".  Works like a champ at very low cost.  Did this with both RFM69 & XBee footprinted WiFi radios.

Tom


john4444

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Re: Designing a better RFM69 radio module
« Reply #6 on: May 27, 2016, 10:40:33 PM »
Quote
What I'd like to focus in this discussion:

- use the sx1231h chip to design a board that is 100mW capable, similar to the blue one above
Wow. These chips are really getting good. The spec sheet shows some extremely good TX/RX capabilities.
Quote
- discuss whether it's worth using a common PCB that can morph into a W or HW or whether optimizing the design for a single variant can reduce any losses and make a better module
While it appears that you need only a coupling cap for 11dBm output, It may be tricky to utilize the "PA_Boost" output because of the antenna matching network needed at the higher power. Also, those components will be slightly different for the 433 vs 915-Mhz bands.
Quote
- discuss the possibility of a 4 layer and what the real gains are - middle layers would have GND and VCC to follow RF design patterns and minimize capacitive coupling etc
The schematic does not look to me as if more than 4 layers would be needed. The only "critical" part will be the antenna matching components & layout. The others are digital control lines, bypass caps and the crystal. Those items should not be much of an issue.
Quote
- discuss the necessity of a metal can cover
Even at 915-Mhz you are unlikely to need a shield over the chip or antenna matching network. Besides adding to the cost and complexity, it will increase the operating temp of the chip. I would avoid RF shielding if possible. However, just as the data-sheet says over and over, you will need lots of bypass caps.
John AE5HQ

Felix

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Re: Designing a better RFM69 radio module
« Reply #7 on: June 01, 2016, 07:58:27 AM »
Now if only Felix could make the RFM69CW not only better, but also smaller!
Joe
The CW only has passives and a crystal. So if all that passive network could be replaced by a single matching network component this could be much smaller yes, and perhaps better tuned. I just don't know if that's possible, that's why I started that other thread to see if others can steer me in the right direction.

joelucid

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Re: Designing a better RFM69 radio module
« Reply #8 on: June 01, 2016, 08:51:01 AM »
Quote
I just don't know if that's possible, that's why I started that other thread to see if others can steer me in the right direction.

I had looked at that some time back. What I took away is that these components are custom Integrated Passive Devices (IPD). Usually the radio chip provider partners with an IPD company to create a custom chip for a given radio. See https://www.silabs.com/Support%20Documents/TechnicalDocs/AN904.pdf for example.

I didn't find any for the SX1231(H).

joelucid

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Re: Designing a better RFM69 radio module
« Reply #9 on: June 01, 2016, 08:53:10 AM »
BTW the performance of the IPD in the doc I linked is really great compared to discrete matching. It would be more than a size advantage if we could find one that fits for the SX1231H.

john4444

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Re: Designing a better RFM69 radio module
« Reply #10 on: June 01, 2016, 12:52:06 PM »
Regarding the antenna matching network - It is usually very desirable to carefully match all the impedances throughout the system in order to achieve minimum losses.
However, there are also a couple of options that could be looked into.

1     The antenna can be tuned to match the output impedance of the final drivers
            (without a matching network).
2     A small balun can be purchased or wound to match the type of antenna connected.
            (the balun could be different for loops, 1/4-wave, 5/8-wave, dipole, etc.)
3     Within wide limits, it is not necessary to have a matching network between the antennas / drivers.
            (You can still receive "good" signal levels even though there may be significant mismatches.
             You have to decide if it is "good" enough to accomplish what you want to do.)

John
AE5HQ
« Last Edit: June 01, 2016, 01:52:05 PM by Felix »
John AE5HQ

perky

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Re: Designing a better RFM69 radio module
« Reply #11 on: June 01, 2016, 04:46:20 PM »
You could also use the spare IO to manually over-drive the mux input of the RF switch (series resistor between the TXRX of the si1231h and the RF mux, brought out to the spare IO). You could then have full power range available by driving the IO pin to permanently select RFIO.

Felix

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Re: Designing a better RFM69 radio module
« Reply #12 on: June 02, 2016, 02:16:37 PM »
This is the difference between all passives vs some passives and IPD:


I didn't see a partnership mention in the case of this example, i know there are vendors that make these types of matching integrated components, like the one in this example.
BTW in the linked example the RF driver chip has a differential output, which is different than the Si1231h.
« Last Edit: June 02, 2016, 08:53:29 PM by Felix »

Felix

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Re: Designing a better RFM69 radio module
« Reply #13 on: June 02, 2016, 03:27:11 PM »
You could also use the spare IO to manually over-drive the mux input of the RF switch (series resistor between the TXRX of the si1231h and the RF mux, brought out to the spare IO). You could then have full power range available by driving the IO pin to permanently select RFIO.

I understand the wiring of that, but I guess I'm not following how that would give access to the full power range? Which range are we talking about here, the TX power range?
Looking at the 20dBm schematic, my best guess is you mean that in TX we can override the RXTX pin to tun off the RFIO part maybe? That could be a very erroneous guess :P
But could you expand on that a little please? This could be an interesting development.
« Last Edit: June 02, 2016, 08:54:02 PM by Felix »

TomWS

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Re: Designing a better RFM69 radio module
« Reply #14 on: June 08, 2016, 03:57:25 PM »
I've got one for ya, Felix.  How about you find a way to unequivocally encode whether the radio is HW or W!!!!   No more "I didn't know that I was supposed to uncomment that line..."

Tom
PS: I know this isn't an RF feature but it sure beats trying to get people to RTFM!