Author Topic: Channel width  (Read 952 times)

brolly759

  • Jr. Member
  • **
  • Posts: 64
  • Country: us
Channel width
« on: September 25, 2017, 04:53:43 PM »
I have been trying to figure out the width of the default channel and some places say 125Khz on either side so 250Khz in total. I have a network scanner and I see close to 500Khz in transmit. What am I missing?

brolly759

  • Jr. Member
  • **
  • Posts: 64
  • Country: us
Re: Channel width
« Reply #1 on: September 25, 2017, 05:19:40 PM »
I am attaching what I am seeing, the larger band is mine and I am not connected to a hub currently so you should see 3 spikes for the auto retry of 3 times. Everything should be default settings.

captcha

  • Full Member
  • ***
  • Posts: 108
  • Country: au
Re: Channel width
« Reply #2 on: September 25, 2017, 06:35:39 PM »
How close is this to the transmitter? I mean, if unshielded you could see noise spillover from the rf generator on the chip, rather than just what's coming out of the antenna. Does it look the same 50m away?

brolly759

  • Jr. Member
  • **
  • Posts: 64
  • Country: us
Re: Channel width
« Reply #3 on: September 25, 2017, 07:34:12 PM »
It was very close, about 3-4 feet away. When I put the sensor about 50 feet away it looks to be about +- 100KHz. Does this mean that I need a metal can to be able to FCC certify this device? :(

captcha

  • Full Member
  • ***
  • Posts: 108
  • Country: au
Re: Channel width
« Reply #4 on: September 26, 2017, 06:23:16 PM »
Maybe.

I've never gone through the list of requirements for approving something under the FCC guidelines, but I wouldn't be surprised if there's something in there that says that at distance x the generated RF from the device (e.g. through the psu, chips on the board, out of the antenna, etc..) can't be more than y in any direction.

There's also the guidelines specific to ISM bands.

Google EMC (electromagnetic compatibility) for getting an understanding that your device may not be 'harmful' to other devices, but also that your device can tolerate a certain rf noise environment from other devices. You may even *want* to put your project in a metal can if your device is highly susceptible to outside noise.

If you're using a plastic enclosure, investigate 'metallic paint' (similar to what you sometimes see inside laptops, not sure of the proper name of the substance) to provide an rf shield on the inside of your enclosure. That could be all you need.

brolly759

  • Jr. Member
  • **
  • Posts: 64
  • Country: us
Re: Channel width
« Reply #5 on: September 26, 2017, 09:13:26 PM »
What is the default width of transmit, both sides total 125KHz or is each side 125KHz for a total of 250KHz?

Thanks

brolly759

  • Jr. Member
  • **
  • Posts: 64
  • Country: us
Re: Channel width
« Reply #6 on: September 26, 2017, 09:23:26 PM »
Another question as well, if its 125Khz width, do I need a guard band so instead of 200 channels, I would have 100 channels?

captcha

  • Full Member
  • ***
  • Posts: 108
  • Country: au
Re: Channel width
« Reply #7 on: September 26, 2017, 09:57:04 PM »
This is quickly going out of my area of expertise  ;) ..but I would like to link to this thread which discusses bandwidth in (much) greater detail.

source: https://lowpowerlab.com/forum/rf-range-antennas-rfm69-library/definition-of-rxbw-with-rfm69/

brolly759

  • Jr. Member
  • **
  • Posts: 64
  • Country: us
Re: Channel width
« Reply #8 on: September 27, 2017, 10:10:30 AM »
I read that link yesterday and that is out of my area expertise ;)