Author Topic: Might the RFM69W be (effectively) FCC compliant, but not an unimpaired RFM69HW?  (Read 8809 times)

WhiteHare

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Looking through the examples of allowable FSK configurations in Tables 1 and 2 here:  http://www.semtech.com/images/datasheet/fcc_digital_modulation_systems_semtech.pdf

The tables relate to the SX1231 (whereas the RFM69x is technically the SX1231H).  Nonetheless, on first look it would seem that (perhaps) all of the example configurations use a Tx power that's below the RFM69HW's datasheet minimum.  Is that correct?
It's not stated that the tables are complete,  so maybe there are examples where the Tx power of the RFM69HW would be allowed (?).

I really do hope examples exist where the RFM69HW might be compliant, but if not, then the RFM69W would seem to be the preferred module if you live in the US or otherwise care about FCC compliance.

Anyone know?
« Last Edit: December 27, 2015, 04:54:06 PM by WhiteHare »

WhiteHare

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I'm going to take a stab at answering this.  I hypothesize the RFM69HW would be FCC compliant if running FSK at 300kbps with the parameters set as in Table 2, because some measurements (documented in the RadioHead library, see excerpt below) suggest that the actual power output by the RFM69HW are about 3db less than what one might expect based on the SX1231H datasheet:

/// We have also made some actual power measurements against
/// programmed power for Anarduino MiniWireless with RFM69-HW
/// Anarduino MiniWireless (with RFM69-HW), USB power
/// - 10cm RG58C/U soldered direct to RFM69 module ANT and GND
/// - bnc connecteor
/// - 2x12dB attenuators
/// - BNC-SMA adapter
/// - MiniKits AD8307 HF/VHF Power Head (calibrated against Rohde&Schwartz 806.2020 test set)
/// - Tektronix TDS220 scope to measure the Vout from power head
/// \code
/// Program power           Measured Power
///    dBm                         dBm
///    -18                         no measurable output
///    0                           no measurable output
///    13                          no measurable output
///    14                          11
///    15                          12
///    16                          12.4
///    17                          14
///    18                          15
///    19                          15.8
///    20                          17

So, if those measurements are accurate, then if set to the 11dbm output level (corresponding to programmatic power level 14), it would appear the RFM69HW could fit within the criteria for the last entry (300kbps) in Table 2 of the document in the post linked above.  If that's the case, then there exists at least one setting where the RFM69HW could in theory be FCC compliant.  That said, I don't see that the RFM69HW confers any advantages over the RFM69W for running an FCC compliant FSK: the linked Semtech document is suggestive that most if not all of the RFM69HW's higher Tx power maybe can't be utilized and still be FCC compliant.

Granted, the cited measurements were taken on an Anarduino, not a Moteino, so I can't know if they would be the same or not on a Moteino. Maybe the Anarduino has RF impairments which aren't normally experienced on a Moteino.   On its face, ~3db of impairment certainly seems like a lot of impairment.  Without actual measurements, this becomes a rather shakey theory to pin hopes on. 

Hopefully someone more knowledgable or with access to more on-point facts can comment.  If there are other ways to make the RFM69HW be FCC compliant without losing the advantages of its higher transmit power (e.g. frequency hopping?), I'd be very interested, but I'm not aware of it being baked into any of the demo code, and at the moment this is as far I've gotten.

« Last Edit: December 27, 2015, 05:19:19 PM by WhiteHare »

john4444

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Hi WhiteHare

In the US, the FCC allows unlicensed radio transmissions - at low power on the ISM bands. The ISM bands are for equipment that producese radio emissions as part of its operation such as, microwave ovens, certain welders and types of industrial heaters. In 1985, when the FCC made the allowance for unlicensed radios, the argument was that low-power transmissions would not adversely affect ISM approved equipment and any interferrence by ISM equipment would be tolerated.

From the Googling I did, there are two different maximum power levels allowed: 1-W for spread-spectrum transmissions and -mW (-W?) for everything else. You should note, this is simply power (Volts times Amps) to the final stage, not the transmitted power or power to the antenna.
(At 3.3-V, 500uW is only 151-uA while -W is 150-mA.) http://urgentcomm.com/techspeak/radio_truly_watershed_event

So... in regards to your question, spread-spectrum radio transmitters can legally operate without a license at higher power than other unlicensed radios in the 902 to 926-MHz ISM band.

Maybe someone can find the actual provisions in 47 CFR Part 18 and provide more details.
John AE5HQ
John AE5HQ

WhiteHat

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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.  :)
« Last Edit: December 28, 2015, 02:30:45 AM by WhiteHat »

perky

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I'm a little confused by your option 3. As far as I can see the power should be measured with a quasi-peak power device below 1GHz, which would preclude using duty cycle to calculate 'average' power in the 915MHz band. So it appears we're stuck with 0.77mW unless we use frequency hopping.

Right now I'm looking for an equivalent band in the USA to the 868.0MHz-868.6MHz band, which allows 25mW erp and with < 1% duty cycle. It's looking like I have to use frequency hopping and >= 50 channels if my hopping channel bandwidths are < 250kHz to do it legally :(.

WhiteHare

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I've tried searching this forum, and I don't have confidence that any of the posts so far have truly gotten to the bottom of it.  The topic appears to be long overdue for some on-point guidance.
« Last Edit: December 29, 2015, 12:07:10 PM by WhiteHare »

perky

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I agree. Some simple basic rules from an authorative source would help considerably. It really annoys me that the official documentation is so tied up in complexity and doesn't get the basic information across properly.

WhiteHare

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I just did a brief survey of some of the newer FCC certified FSK modules that are listed on Mouser, and I didn't find any that allowed for greater than 12db of Tx power.  That doesn't prove anything per se, but they are datapoints.

In the absence of any meaningful on-point guidance, it might be prudent to operate within the DTS modes of Table 1 and Table 2 that I referenced in the OP.  Semtech seems to be saying that those could (in principal) be compliant.

The FCC rules do seem to allow for up to 6db of antenna gain before needing to reduce the conductance power.  So, there's that if you care to fiddle with it.

Because of the way the rules are written, LoRa strikes me as the no-brainer choice.  Those who proclaim disinterest...well, I don't understand why, unless they live outside the US.

john4444

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It seems that I was not very clear. My understanding based on what I have read in 47CFR 18:

The LoRa/RFM95, which is a spread-spectrum radio, would be legal in the US at frequiencies in the ISM band (902 to 926-MHz), power less than 1-W.

Other radios (RFM12, RFM69 etc.), which are not spread-spectrum radios, would be legal in the US, again in the ISM band, at Input Power less than 100-mW.

Sections of 47CFR Part 15 apply to the ISM band as well as the 434-MHz band radios.
But, man are they hard to read.
John AE5HQ

TomWS

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<snip>
But, man are they hard to read.
No kidding!  And talking to the 'guys' is even worse.  However, unless you are planning a product which, of course, needs FCC certification or you're really swamping the daylights out of your neighborhood which would bring in the 'Van', who is gonna care that you are violating the letter of the 'regulation' with your occasional moisture/TH/barometric reading?

On the matter of this thread, I do agree with others that 'knowledgeable' source's input would be valuable to this discussion, however the fact of the matter is, unless you are planning to make a product, FCC 'compliance' is less important than making sure you don't annoy your neighbor with your 'loud music'.

And that, believe it or not, can be resolved simply by talking to your neighbor...

Tom

john4444

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Here are two more small contributions to Tom's point from 47CFR 15.5.

1) No one has any vested right to continued use of any given frequency.

2) Operation of a radio is subject to the conditions that no harmful interference is caused and that interference must be accepted.

[I interpret these to mean: If your neighbor complains that your Moteinos cause a problem... do something like change bands and fix it quick.
If your neighbor's stuff mess up your Moteino reception... it is still your problem.
Luckly, You can probably fix it by changing bands.]

My experience with the Moteino RFM69 radios has been - because of the low-power and short transmit-time, you won't be causing your neighbors much interferience. For example, I can hear 433-MHz transmissions from my outdoor weather station on my 915-MHZ wireless headphones inside the house, but I hear nothing on the headphones from the 915-MHz Moteinos around the house.

Back to the subject of the thread,
I think I identified the relevent regulations for the 915-MHz RFM12/69/95 transcievers.
I have no reason to question un-licensed use of 433-MHz units. I just cannot find it in the CFR.

John AE5HQ
John AE5HQ

WhiteHare

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You guys are persuasive, I'll grant you that.

As an aside, my main reservation about LoRa is that it's a slower bitrate.  37500 appears to be the RFM9x's maximum LoRa bitrate, so with the shortest possible packet, the Semtech's calculator says the shortest "time on air" is 1.82ms.  So, that suggests a listen-Mode Rx window of ~4ms, which is considerably more than what's possible with the faster FSK bitrates.

Also as an aside, for those who like the RFM69 FSK,  you'd probably like the RFM9x's FSK even more.  As near as I can tell, the RFM9x's also offer the same FSK capabilities as the RFM69, but with an improved receive sensitivity.   

Happy (soon to be) New Year!

perky

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@WhiteHare: Unfortunately the Semtech app note doesn't help the range problem. It's good for high bit rates that already use wideband (with their corresponding reduction in range), but not for lower bit rates that could use narrow band and low receiver bandwidths specicially to increase range. You're stuck with a receiver noise floor set by your (large) receiver bandwidth.

WhiteHare

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@perky So, are you saying the RFM9x's FSK is actually worse than the RFM69's FSK at lower narrowband bitrates, or simply that it's not any better?  I'd be curious to know, as I haven't done an in-depth comparison.  If the latter, then you could switch to LoRa mode and get the extra range you might need, as you'd already be at low bitrates anyway.  Likewise, the extra range at the higher FSK bitrates from improved receive sensitivity would be nice, because you can maybe operate at faster bitrates so as to save energy at the receiver (the preferred strategy being used by those using Listen-Mode).

The wonderful thing about this topic is (for me at least) it seems there's always more to learn!

perky

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I'm sure the RFM9x's FSK at narrow band is actually better than RFM69's due to it's improved sensitivity for a given power level.

Edit: I removed the rest of this paragraph, it wasn't very well thought out ;-)

LoRa modulation may well come to the rescue here as it allows spreading of the signal energy even at low bit rates, and appears to do a much better job of spreading over a much wider bandwidth too. I need to look into this further.

« Last Edit: December 30, 2015, 05:43:15 PM by perky »