It's alive!

RFM12 Wireless module discussion.

It's alive!

Postby chrisw957 » Thu Apr 24, 2008 4:15 am

I was able to transmit and receive my first packets today.

I used a modified version of the PIC example that HOPE provides. I'm using nIRQ to know when a byte has been received, and that seems to work fine.

I'm confused about the antenna connections though. I'm using the RFM12B modules, and they have two solder pads labeled ANT. I think 1/4 wavelength of 434 mhz is around 6.8 inches. Should I just solder a 6.8" wire to one of the pads, or should it be a loop between the two pads?

It looks like the outside pad is connected to ground.

thanks.
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Re: It's alive!

Postby seaton » Thu Apr 24, 2008 8:29 am

Well done!!!!

As to your antenna, just solder your piece of wire to the pad that is not connected to ground, do not loop.

the length, as measured from ones sent from factory is

915MHz ~ 87mm
433MHz ~ 173mm

Hope that helps.

chrisw957 wrote:I was able to transmit and receive my first packets today.

I used a modified version of the PIC example that HOPE provides. I'm using nIRQ to know when a byte has been received, and that seems to work fine.

I'm confused about the antenna connections though. I'm using the RFM12B modules, and they have two solder pads labeled ANT. I think 1/4 wavelength of 434 mhz is around 6.8 inches. Should I just solder a 6.8" wire to one of the pads, or should it be a loop between the two pads?

It looks like the outside pad is connected to ground.

thanks.
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Re: It's alive!

Postby chrisw957 » Thu Apr 24, 2008 10:14 pm

Changed out my whip antennas for ones of the correct length, but I'm still disappointed by the range.

I'm using the rfm12b at 434 mhz, 4800 baud, 90 khz of deviation at full output power. Sending 16 byte packets, I can only get a few meters before I start getting bad CRCs...

I see you mentioned getting good distance from the 900 mhz version, but what are others seeing with 434 mhz?

Also, I believe here in the USA, the RFM12B at max power setting emits too much power at 434 mhz to fit under that part 15 rules.
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Re: It's alive!

Postby seaton » Sun Apr 27, 2008 1:59 am

chrisw957 wrote:Changed out my whip antennas for ones of the correct length, but I'm still disappointed by the range.

I'm using the rfm12b at 434 mhz, 4800 baud, 90 khz of deviation at full output power. Sending 16 byte packets, I can only get a few meters before I start getting bad CRCs...

I see you mentioned getting good distance from the 900 mhz version, but what are others seeing with 434 mhz?

Also, I believe here in the USA, the RFM12B at max power setting emits too much power at 434 mhz to fit under that part 15 rules.


Some suggestions.

Could be interference, I can imagine that 433 spectrum would be well used. Try moving the Frequency around a bit.

What receiver bandwidth are you using? Try 200KHz as a starting point and work in. 67KHz bandwidth on the receiver and the transmitters deviation of 45Khz should optimal for the datarate your using. too wide a bandwidth will be prone to more interference. The transmitter deviation should be smaller than receiver bandwidth.

While not normally needed you can check the tuning of all transceivers, to do this, enable and measure the CLK output with either a frequency counter or Oscilloscope, change the frequency to 10mhz, and then change the capacitance by the config command, 0x8000, until it reads exactly 10MHZ, I think around 12pf works for me. This helps to make sure that the transmitter-receiver frequency offset coming from crystal inaccuracy is minimal.

Also as a starting point for the receiver set your AFC set to auto offset independent of VDI, +3/-4 max dev, fine mode, enable offset register, enable calculation of offset.
and set your datafilter to auto mode, clock recover set, digital filter, DQD threshold is 4.

if you are not using your CLK then turn it off

Hope this helps.

Stephen...
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Re: It's alive!

Postby chrisw957 » Tue May 06, 2008 1:07 am

Thanks for the suggestions. The first thing I checked was the TX and RX bandwidth settings.

RX was set with 0x94C0, which should mean i = 110 binary, or a receiver baseband BW of 67 khz.

TX was set with 0x9850, which I believe means M = 5, and deviation of +/- 90 khz, for a total BW of 180 khz. Based on your comment, I changed to M=2, for a deviation of +/- 45 khz. I set this by writing 0x9820.

What a difference. I can now reach all thru the house. I think the farthest I tested was about 30 meters thru a three walls, but I wasn't seeing any CRC errors. I expect I will be able to turn down the transmit power and still get the range I need.

It's funny because my original settings were based on the PIC example provided by hope. I guess they didn't check their work too well?

chris
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Re: It's alive!

Postby seaton » Tue May 06, 2008 10:11 am

chrisw957 wrote:Thanks for the suggestions. The first thing I checked was the TX and RX bandwidth settings.

RX was set with 0x94C0, which should mean i = 110 binary, or a receiver baseband BW of 67 khz.

TX was set with 0x9850, which I believe means M = 5, and deviation of +/- 90 khz, for a total BW of 180 khz. Based on your comment, I changed to M=2, for a deviation of +/- 45 khz. I set this by writing 0x9820.

What a difference. I can now reach all thru the house. I think the farthest I tested was about 30 meters thru a three walls, but I wasn't seeing any CRC errors. I expect I will be able to turn down the transmit power and still get the range I need.

It's funny because my original settings were based on the PIC example provided by hope. I guess they didn't check their work too well?

chris


Well done!!! These are great little beasts once you get the settings right....the problem is getting the settings right ;) Unfortunately there is not much info on them and it took me a while to find the right settings as the datasheets are very vague, and yes the Hope examples do not work lol. I'm working on a FAQ with all the settings etc so it makes life easier.

Stephen...
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Re: It's alive!

Postby ayesha » Wed Jun 04, 2008 5:03 am

It's great ur really a gernious and a lucky guy can u plz send me the sechamaitcs and code on this adress engr.aahmed@gmail.com. I have spents months working on RFM12BP but no sucess. an u plz help me out. :?:
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