Author Topic: RFM69HW power amplifier configuration  (Read 5295 times)

mav

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RFM69HW power amplifier configuration
« on: January 22, 2014, 08:22:07 AM »
Hi all,

I am new to this forum, so before I start I would like to thank Felix for providing the library.

I have been playing around with a RFM69HW and the RFM69 library and I think the power amplifier configuration should be changed in the library. For a RFM69W the configuration is fine: It is configured to use PA0 with power level 31 (+13 dBm). But for a RFM69HW it is configured to use PA1 + PA2 at power level 31 with the high power settings enabled (+20 dBm).

+20 dBm equals 100 milliwatt and I think that is way to high to start off with as default value.

As a default for a RFM69HW it makes much more sense to start with only PA1 enabled, power level 31 and high power settings off. And provide methods to switch to higher power amplifications modes. This would mean that for both the RFM69W and the RFM69HW the default would be to start with +13 dBm (about 20 milliwat). This also significantly reduces power consumption!

Clarification of the power amplifier modes:

I think people misunderstand the amplifier modes: both the RFM69W and RFM69HW modules use the same chip (RF69) and therefore they both have exactly the same registers. The difference between the modules is in how the antenna is connect to the chip. The RFM69W and RFM69HW do not show schematics for the modules, but the RF69 datasheet has one (figure 42) which gives some clues.

The RFM69W module has the antenna connected to a impedance matching circuit which in turn is connected to RFIO. RFIO is used for RX and at the same time the output for PA0. Trying to use PA1 and PA2 with a RFM69W is pointless because the PA_BOOST pin is unused.

The RFM69HW module has the antenna connected to a RX/TX switch which is controlled by the RF69 chip. The RX side of the switch is connected to RFIO. The TX side of the switch is connected to an impedance matching circuit which is connected to PA_BOOST. Trying to use PA0 with a RFM69HW is pointless because of this switch.

Marco

Felix

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Re: RFM69HW power amplifier configuration
« Reply #1 on: January 22, 2014, 08:32:29 AM »
Marco,
Thanks for shedding some light about the amplifiers. Yes, they are often misunderstood.
Most people just want to plug and play, which is fair, since that's in line with the Arduino philosophy. So HW makes sense when someone just wants that extra 7dBm. For 13dBm people can just get the RFM69W and do nothing else. The HW can be scaled down in terms of RF output power which has the same effect as turning off PA2.

mav

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Re: RFM69HW power amplifier configuration
« Reply #2 on: January 22, 2014, 08:55:14 AM »
Fair enough... its your party.

Word of warning: The +20dBm setting caused problems for me. I have the RFM69HW connected to a Arduino Micro and the +20dBM setting overloads the 3.3V regulator when transmitting. The result is that the RFM69HW locks up. The RFM69HW keeps transmitting an 433Mhz carrier wave (with pretty good power completely drowning out everything else at this frequency). And the arduino is caught in a never-ending wait for-mode-ready-loop.

+17dBm or lower is not a problem for the little regulator.

Marco

Felix

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Re: RFM69HW power amplifier configuration
« Reply #3 on: January 22, 2014, 06:11:54 PM »
Haven't really seen that type of behavior. There's also a output power limitation in the RFM radios, it's 95mA by default from what I recall, could be wrong on that though. I haven't had to do anything to have them work on the Moteino (with the mcp1703 regulator).

Quote
The RFM69HW keeps transmitting an 433Mhz carrier wave (with pretty good power completely drowning out everything else at this frequency)

Not sure I'm following on this, could you go into more detail?

mav

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Re: RFM69HW power amplifier configuration
« Reply #4 on: January 23, 2014, 07:45:54 AM »
Yes, its 95mA but it has to be deactivated for the highest power mode (OCP_OFF in setHighPower). So the default for your lib for an RFM69HW is PA1 + PA2 + high power with OCP off.

Your Moteino probably won't have problems with the regulator but the Arduino Micro has an 3.3V regulator on board which I think is not up for the task (I believe its 150mA max).  What happens I think is that transmission at +20dBm overloads the regulator which causes the voltage to drop too low and leaves the RFM69 in a weird state. Maybe an extra capacitor of lets say 100uF would help since 150mA should be enough.

I only noticed it when I decided to go to bed and tried to turn off my lights with a KAKU switch. But that did nothing because the RFM69 was still transmitting :) Before that I was trying to debug the code in order to find out why it was locking up. When I switch to lower output (PA1 only) or (PA1 + PA2 at 17dBm) then the code works fine with no lockup.

I'm not worried about it. I think its nice to have the extra power available if needed but I don't want it transmitting at 100mW default. The +13 dBm (about 20mW) is more than enough around the house. In fact just PA1 with power level 0 (-2dBm about 0.6mW) seems to be just about enough to carry the signal around the house.

I'm thinking extending the library to include the RSSI of the received signal when sending out an ACK. And writing some form of power-level-auto-adjust scheme to tune the whole network of nodes.

pko

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Re: RFM69HW power amplifier configuration
« Reply #5 on: January 23, 2014, 10:28:05 AM »
although the 3.3V regulator on the Arduino Micro can support 150mA output, the 3.3V rail on the Micro is only designed to do 50mA

DC Current for 3.3V Pin   50 mA (as per http://arduino.cc/en/Main/arduinoBoardMicro).

This may have something to do with it sharing the power output from the 5V regulator with the rest of the components on the Micro.

As the RFM69HW typically uses 130mA on transmit, there is not enough current from the 3.3V rail to power the RFM69HW,

hdphilip

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Re: RFM69HW power amplifier configuration
« Reply #6 on: April 19, 2014, 05:18:19 PM »

what would be the best way to disable PA2?

philip