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I've been looking at building a ~900MHz frequency hopping transmitter that conforms to FCC Section 15.247.

See section 3.2.1 Frequency Hopping Spread Spectrum Requirements in the Semtech document below.

Semtech's write up on FCC Section 15.247

There are all kinds of channel separation measurements, and whatnot in the regulation, but I am wondering how do you ACTUALLY measure the peak conducted output power that goes up to 1 watt (+30 dBm) according to 15.247?

I've got an EMI testing antenna with known antenna factors, and conduct 3 meter readings in my office for field strength measurements; however, I've never encountered this kind of measurement? For whatever regulatory reason, the FCC doesn't specify a field strength measurement for these systems.

Is measuring the conducted output power, simply just hooking up a coax pigtail to your TX IC output line (or using an SMA / UMCC connector, etc.) and running the TX IC output into your spectrum analyzer?

If it is -- one word of caution is that +30 dBm would blow-up the front end of my signal analyzer which is rated for up to +20 dBm; so you might want to look at an attenuator.

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  • \$\begingroup\$ It does appear to tell you in the linked document and yes, using an attenuator may be sensible. \$\endgroup\$
    – Andy aka
    Aug 24, 2017 at 16:45
  • \$\begingroup\$ Andy -- is that a valid measurement for measuring output power? Is there any kind of gotchas in using a coax pigtail for this measurement beyond just cable & connector loss? This is more of a physical implementation question... \$\endgroup\$
    – Leroy105
    Aug 24, 2017 at 16:49

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Antenna port conducted measurement is usually the preferred method for measuring power. If the measurement is not possible due to device having PCB or SMD antenna, radiated measurement is also allowed. If your device have antenna port, you can use connect coax cable to attenuator then to spectrum analyser.

You might want to see ANSI C63.10 for the measurement method. There is also a KDB related to 15.247 measurement that might be helpful for you.

Mind that if your device employ less than 50 hopping channels the limit is 24 dBm instead of 30 dBm.

If you need more reference, go to FCC website and look for test report that do 15.247. They usually have pictures of test setup.

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