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Author Topic: reliable datagram RF22 library question  (Read 2802 times)
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The LP2985-3.3 is rated @ 150 mA. Perhaps that's OK for 2 wire pull-ups and some of the smaller sensors But IMO 100 mA is too low for a general purpose 3V3 supply But neither did the designers as the 'manual' for the Mega (I own 2 of them) Clearly states that there is a 50 mA limit on the current that can be drawn from that source. The Nordic and the others in that family don't draw a great deal of quiescent current but on transmit there is a lot going on very quickly or what would be called a high DI/DT or the change in current over the change in tine, current from 2 places one is the SPI bus in the transceiver and the other is the PA after the synthesizer...
The problem and why the 4700 uF cap makes the radio's work is that it stores energy from the 3V3 source to 'help' out when there are any demands on the 3V3 source. The issue that makes this necessary is that the copper to the 3V3 pin is likely not very wide making a resistive connection to the 3V3 source pin and the by-pass in the 3V3 source on the Mega board is C13, A 1uF MLM ceramic cap this is the minimum value for stability. 2200 to 4700 uF at the end of 3 - 4 inches of breadboard is enough to make the issue go away.
For higher power devices in the 40 to 80 mA class I would NOT recommend using the 3V3 source from the Mega Or for that matter at least the R3 Uno and I would expect the previous versions are the same or similar.. as is the Leonardo C14, 1uF MLM Ceramic).
The implications are worse If you plan on using the RFM22-3B radios as they draw 80+ mA on transmit and more if the antenna isn't right.
I use at least 3300uF and as much as 4700 uF when using a Mega board and either the Nordic or Hope RF RFM12-22/3 radios for test and a separate 3V3 regulator for the finished project.
My one recommendation would be that If there is any doubt... Use a separate 3V3 regulator in your project. I do.

Bob
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I am only using the 4700mFd while I am checking it out on the Arduino board, as the onboard 3v3 supply cant handle the rush, but I scoped the cap and it hardly dips at all.

That changes things a lot. I don't think you mentioned which Arduino bd is being used,
some of them only provide 50 mA or so at 3.3V. I'm almost to the point of soldering up
my RFM22, and will see how a regular v.reg handles it. I'm using this,
http://www.digikey.com/product-detail/en/NCP1117DT33RKG/NCP1117DT33RKGOSCT-ND/1967215

I've been using the same layout and lashup with XBee Pro modules which draw 250+ mA
on transmit, and not seen any problems.
« Last Edit: March 22, 2013, 08:54:47 pm by oric_dan » Logged

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I have given up on these RFM22 modules, and dug out the SIM20 modules, which I am trying to use. I have just done a range check and ran out of road, with one unit on my bench ( with I have just noticed, only a 120mm antenna ! ) and me going walkabout.
The furthest I could get was 170m away from my desk, through 4 houses and 3 walls, and it was still solid.

Tommorow I will fit correct antennas and do the open space test   As this post no longer refers to the title, I have started a new post at http://arduino.cc/forum/index.php/topic,155930.0.html
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Ok, I don't understand why you gave up on the RFM22s. You couldn't get enough range?
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I thought the range would be better, but I am sure its that I dont now how to set them up, and there doesnt seem to be anyone who has tried ?  They should have good performance with 18dbm Tx  and 118dbm Rx sensitivity .....  but I was doing something wrong.
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@ Boffin1 +18 DBM @ 50 ohms is nearly a quarter watt of power. 0 DBM = 1 mW, +10 DBM = 10 mW, +18 DBM is about 30 mW because the power doubles every +3 DB. This 'should' be good for .5 KM direct line of sight and no interference.    - 118 DBM is about .2 uV RX sensy {Edit RKJ}
I suspect two things, First as you mentioned the 'base' antenna leaves something to be desired. Second that the match which becomes most important at higher power is very poor and this has the impact of negating the output power simply because your antenna leaves something to be desired. A 1/4 wave antenna at 433 MHz is a wire about 17 cm in length... with 4 equidistant 'Radials" or elements (wires of the same type as the center radiator) drooping down at a 30 to 45 deg angle. If you need to 'visualize' the process it's that the antenna needs something to push against, a 'counterpoise'.
There is one last consideration and that is that both antenna's should be clear of other obstructions and that they both be in the same plane or pointing the same way or there can be as much as 6 DB of additional path loss due to cross polarization. If there is any kind of diversity control available.. I'd certainly try it. I've seen it offered before with some of those receivers...
Diversity in it's simplest is two receive antenna's and a means of electrically switching between them. The receiver switches back and forth between two antenna's looking for the strongest signal...

Bob
« Last Edit: March 23, 2013, 04:18:24 pm by Docedison » Logged

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For testing I am just using the quarter wave vertical, without any form of groundplane for now.

The actual project will have dipoles at least, but at the moment, I am just comparing apples with apples.

The same quarter wave wires on the SIM20 modules ( in fact I noticed after the test that one was only 120mm long instead of 170mm ) at the same power give me 500m range, but as I said I dont trust my setup, and cant use the spreadsheet on my PC.

I will have another look at these modules one day when I have time, but this is a rush job now.

( I worked with diversity combiners in the 60s, and wondered what happened to them since,  the first time I noticed them on commercial equipment was on base stations for wireless mics 40 years later )

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From what I can tell, a major factor in regards usable range is the RF transmit
data rate. The longer range modules, everything else being the same, have low
data rates like 9600 bps, whereas those XBee modules and RFM22s are more like
120-240 Kbps.
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Yes, when I slowed the data rate with the setupcode from my previous ( simplex ) project, it took the range from 7 to 150 meters.

I think the RFM22 will run at much slower baud rate, I think they run it with pulsed output like morse code for the really long range tests, but I have run out of time.

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Boffin, I assume you've moved on and dealt with your problem by now, but FYI, I just
ran across this long-range XBee socket compatible transceiver, albeit costly, also maybe
not licensed for SA.

https://www.sparkfun.com/products/11634
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That does look good, I will see how the SIM20 pans out, its quite handy only using 3 pins.
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