Author Topic: Might the RFM69W be (effectively) FCC compliant, but not an unimpaired RFM69HW?  (Read 7914 times)

Felix

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Adding to this interesting discussion is a report of FCC ETSI compliance testing for a RFM69CW-433 module: view it from here.
« Last Edit: April 18, 2016, 09:16:23 AM by Felix »

WhiteHare

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3.  "Everything else" (as you call it).  Governed by Part 15.249.  The *average* Tx power is limited to no more than -1.12Dbm (roughly 0.77mw) and can have a *peak* of no more than 18.9dbm (about 78mw).  Here's the rub: to calculate the average power using the rules of Part 15.249, you must determine what the worst possible duty cycle will ever be over a 100ms window.  Applying a duty cycle correction factor based on that worst case duty cycle to the Peak Power yields a number which must be no more than -1.12dbm.  So, the way I see it, you must either guarantee a very short worst-case duty cycle, or else your peak Tx power must be low, or some combination of both.  Maybe there exist applications where the RFM69x can easily operate within those constraints, but if so, it's not intuitively obvious to me what they all might be.  Speaking generally, the Part 15.249 rules don't appear to leave much wiggle room.  That said, I suppose there's a chance you might get lucky when it comes to fitting your application within those constraints.  For instance, maybe operating the RFM69 at a high bitrate while transmitting only a single very short packet (with, say, a 100ms rest interval between packets) might get you in or near the ballpark while still allowing a relatively high Tx power.  Returning to my OP (above) and perhaps answering my own question, maybe the RFM69HW might outshine the RFM69W in that scenario and prove its worth all while maintaining compliance with the relevant FCC rules.  :)

Closing the loop on the OP and putting numbers on this:  assuming my math is right and assuming I haven't overlooked some other restriction, then wrt 15.249 it appears that as long as an RFM69HW does not exceed a peak transmit power of 18.9dbm, it could have a duty cycle as high as 10% at 18.9dbm, provided it does not Tx more than 10ms during any 100ms time interval.  That's good news, as that should be easily manageable at higher bitrates.   :)  Unless I'm mistaken,  15.249 does not imposes any minimum bandwidth requirements.  Therefore, to pin down the other end of the range, the final step would be to compute minimum bitrate based on minimum packet size, and that would dictate the minimum bandwidth from the picklist.   Obviously, lower peak Tx power would allow longer duty cycles.

It would be great if someone besides just me would read 15.249 and run the numbers independently so as to either confirm or refute.

Wrt to 433Mhz, I get the impression that it is legal to use the band, provided the applicable Tx power and duty cycle requirements are met for the equipment's applicable category of intended use.  IIRC, in addition there's a minimum wait of either 10 or 20 seconds (?) between transmissions in the 433Mhz band for non-privledged uses, but for a lot of applications (e.g. temperature sensors) that would also be easily manageable.  I do think it's worth looking into.


« Last Edit: December 31, 2015, 03:59:51 PM by WhiteHare »

TomWS

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Adding to this interesting discussion is a report of FCC compliance testing for a RFM69CW-433 module: view it from here.
You should sticky this!

emjay

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Curious that the "Test Certificate" explored supply voltages well above the radio chip "Absolute Maximums".  Anyone recognise the testbed board - perhaps an LDO on there feeding the RFM?

WhiteHare

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Adding to this interesting discussion is a report of FCC compliance testing for a RFM69CW-433 module: view it from here.

@Felix,

The report I get when I click on the link in your post is Report No: DDT-RE140701.  As near as I can tell it documents compliance testing only wrt to European standards, not FCC standards.  Was there a different document you meant to link instead?

WhiteHare

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It looks as though Part 15.231 is what would govern a 433Mhz RFM69 (cf. https://www.maximintegrated.com/en/app-notes/index.mvp/id/3815).  So, looking at Table 1 of that document, the maximum average Tx power at that frequency would be approx -14.4dBm.  So, peak Tx power would be limited to 20 - 14.4 = 5.6dBm. 

So, wrt to power, how would that compare to what might be allowed in the 902-928Mhz range?  I'd be curious to know.  Back of the envelope:  If propagation at 433Mhz is the equivalent of roughly 6dB (?) better than at 900Mhz, then comparing peak Tx powers in some sort of apples-to-apples manner might be roughly 5.6dBm + 6 = 11.6dBm versus 18.9dBm.  It would seem that 900+Mhz spectrum would win the bake-off, hands down.

I suppose what might modify that conclusion is the strength of local interference and how congested the spectrum is.  Maybe if 900Mhz range is jammed packed full of powerful interference, then maybe the 400+Mhz spectrum might offer refuge.  Likewise, maybe there is legally accessible spectrum at the lower frequencies where you're much more unlikely to cause interference, and I suppose that might tilt the balance also.

Am I wrong?  Comments?
« Last Edit: January 01, 2016, 12:17:21 PM by WhiteHare »

WhiteHare

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So, anyway, at least for letter-of-the-law FCC compliance at 433Mhz, the RFM69HW would be a bad choice, as it can't transmit low enough  Definitely RFM69W if it's a forced choice between the two.

Moving on: It seems that most, if not all, of the non-Hope SX1231/H modules seem to have a metal can on them. I presume that's primarily to mitigate against RF pollution from harmonics or other undesired frequencies (?).  What's the best way to add de facto shielding to the RFM69 to mitigate against the same?  Put it in a metal box?  Wrap in insulation and then wrap  it in aluminum foil with the insulated antenna sticking through?

Suggestions?

john4444

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I realize that this topic has been quiet for several weeks now.
However, while searching for something else, I ran across this: https://transition.fcc.gov/Bureaus/Engineering_Technology/Documents/bulletins/oet63/oet63rev.pdf.
This gov doc purports to answer questions about non-licensed transmitters.

The information may be there.
I can see that the max signal strength for 433-MHz is at the bottom of p17 and top of p18
while 915-MHz radios are at the bottom of p18 & top of p19.
They even provide the formula for calculating power on p29.

I'm not smart enough to figure out what is “legal”.
Possibly someone with more knowledge and experience can provide that definitive answer.

John AE5HQ
John AE5HQ

WhiteHare

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Like you, I also did a quick read to better understand Part 15, so let's compare notes.

Ignoring things like cordless phones, which have their own special carve-out allowing them high powered transmissions in the 902-928Mhz ISM band, I read the FCC rules governing the 902-928Mhz band as comprising 3 buckets rather than just the two you mentioned:

1.  Spread spectrum and frequency hopping are allowed up to 1 watt. Governed by Part 15.247. However, RFM69x doesn't do spread spectrum, so it doesn't match that bucket.  I'm not sure what potential, if any, it has for doing frequency hopping (though there is a section of the datasheet, Section 4.2.5, entitled " Optimized Frequency Hopping Sequences" which I haven't yet read).  I need to look to into that more, but for now, until proven otherwise, it would appear the RFM69 doesn't fit this bucket.

2.  Digital Transmission System (DTS).  Also governed by Part 15.247.  These devices are limited to a peak of 8dbm across any 3khz band.  Also, the 6db bandwidth must be at least 500Khz.  The Semtech paper I cited above is clearly trying to identify modes of operation (Tables 1 and 2) for the RFM69 to make the case that an RFM69 could qualify as a DTS if operated within those parameters.  Why?  Because the alternative is the third bucket (below), which appears to be generally more restrictive.

3.  "Everything else" (as you call it).  Governed by Part 15.249.  The *average* Tx power is limited to no more than -1.12Dbm (roughly 0.77mw) and can have a *peak* of no more than 18.9dbm (about 78mw).  Here's the rub: to calculate the average power using the rules of Part 15.249, you must determine what the worst possible duty cycle will ever be over a 100ms window.  Applying a duty cycle correction factor based on that worst case duty cycle to the Peak Power yields a number which must be no more than -1.12dbm.  So, the way I see it, you must either guarantee a very short worst-case duty cycle, or else your peak Tx power must be low, or some combination of both.  Maybe there exist applications where the RFM69x can easily operate within those constraints, but if so, it's not intuitively obvious to me what they all might be.  Speaking generally, the Part 15.249 rules don't appear to leave much wiggle room.  That said, I suppose there's a chance you might get lucky when it comes to fitting your application within those constraints.  For instance, maybe operating the RFM69 at a high bitrate while transmitting only a single very short packet (with, say, a 100ms rest interval between packets) might get you in or near the ballpark while still allowing a relatively high Tx power.  Returning to my OP (above) and perhaps answering my own question, maybe the RFM69HW might outshine the RFM69W in that scenario and prove its worth all while maintaining compliance with the relevant FCC rules.  :)

I was going to edit this earlier post, but it seems to have been made unmodifiable.  The addendum I was going to add is that after a second reading of the rules, it looks like frequency hopping might possibly be a viable option after all:  https://lowpowerlab.com/forum/index.php/topic,1819.msg13036.html#msg13036