SX1280 : RF power less than expected - Page 1
Forum section: electronics / rf-microwave

author: joseph1235
author_rank: Newbie
timestamp: June 10, 2025, 08:51:31 am
content: Dear community,I'm working on sx1280 with home made board ( STM32 + SX1280 over SPI).The first basic uses case was to make a ping-pong over two board.It's work, but the range is not conforme as expected ( RF communication fault over 45m).In spide of the RF setting was set to +13db Tx power, the effective RF power measured by spectrum analyser directly on component RF pin ( without Z adaptation), give me +2db.this following picture show my measure in function of RF settings.my question, is anyone here have already use this IC ? measure is RF Power ? a mistake on my reasoning ?thanks in advance.

author: radiolistener
author_rank: Super Contributor
timestamp: June 10, 2025, 11:06:46 am
content: A few points need clarification to better understand the issue:1) Power Measurement Units: You mentioned a discrepancy between the configured +13 dB Tx power and the measured +2 dB. However, "dB" alone is a relative unit (a ratio) and does not specify absolute power. Typically, RF power is measured in dBm (relative to 1 mW) or dBW (relative to 1 W). Talking about power in "dB" with no mentioning reference level just doesn't have sense. Could you confirm which reference level you are using?2) Possible Power Limitation: The lower-than-expected output power could be related to regional regulatory settings (e.g., country-specific WiFi channel restrictions), which may enforce power limits on certain frequencies if you didn't set WiFi country code or set country code for which power limit is applied.If you use WiFi module manufactured for specific market (for example for USA), the power may be hardware limited.3) Measurement Methodology: The WiFi chip does not emit a continuous carrier, so measuring its output power requires careful consideration. If you used a spectrum analyzer in average power mode (rather than peak or with proper triggering), the reading might be underestimated.

author: joseph1235
author_rank: Newbie
timestamp: June 10, 2025, 11:34:36 am
content: First thanks for your reply.A few points need clarification to better understand the issue:1) Power Measurement Units: You mentioned a discrepancy between the configured +13 dB Tx power and the measured +2 dB. However, "dB" alone is a relative unit (a ratio) and does not specify absolute power. Typically, RF power is measured in dBm (relative to 1 mW) or dBW (relative to 1 W). Talking about power in "dB" with no mentioning reference level just doesn't have sense. Could you confirm which reference level you are using?You have right, all "dB" must be change in "dBm"2) Possible Power Limitation: The lower-than-expected output power could be related to regional regulatory settings (e.g., country-specific WiFi channel restrictions), which may enforce power limits on certain frequencies if you didn't set WiFi country code or set country code for which power limit is applied.If you use WiFi module manufactured for specific market (for example for USA), the power may be hardware limited.very interresting ! i test it now !3) Measurement Methodology: The WiFi chip does not emit a continuous carrier, so measuring its output power requires careful consideration. If you used a spectrum analyzer in average power mode (rather than peak or with proper triggering), the reading might be underestimated.The SX1280 have a Continous transmittion wave option. This mode have been set on for the measure

author: ftg
author_rank: Frequent Contributor
timestamp: June 10, 2025, 03:10:19 pm
content: It is hard to make guesses about the reasons without seeing the schematic and RF layout of the design.What instrument are you using to measure the output power?

author: joseph1235
author_rank: Newbie
timestamp: June 11, 2025, 07:00:53 am
content: the measure have been done with a "Rohde & Schwartz FSL6".Custom board + custom firmware -> RF power problemCustom board without RF IC + official SX1280 devBoard + custom firmware -> RF power problemArduino board + official SX1280 devBoard + custom firmware from Lib Arduino from web -> RF power problemi think that the problem is electronic but firmware. BUT i'm not able to verify my reflexion with commun arduino lib.i'm tottaly lost.i'm looking for someone who already use the SX1280/81 AND get RF measure

author: uer166
author_rank: Super Contributor
timestamp: June 11, 2025, 07:54:34 am
content: I've used sx1276 and had some wrinkles but eventually figured out that it was a measurement error.Post your full schematics and transceiver configuration.

author: joseph1235
author_rank: Newbie
timestamp: June 11, 2025, 02:45:44 pm
content: see attachement

author: uer166
author_rank: Super Contributor
timestamp: June 11, 2025, 08:44:44 pm
content: Quote from: joseph1235 on June 11, 2025, 02:45:44 pmThat's some decently heavy impedance matching. FYI there is no real point in making those intermediate traces 50R, since, well, it's not 50R impedance.Did you try measuring the power at the SMA output connector? What is it and how does it compare to your original measurements.

author: RFDx
author_rank: Regular Contributor
timestamp: June 12, 2025, 06:49:34 am
content: Quote from: joseph1235 on June 11, 2025, 02:45:44 pmUnless your layout has some hidden features not reflected in the schematic, the matching/filtering network, as shown, does not provide a 40 Ohm load to the transmitter. Still, even with the mismatch, you would see at most a 3dB lower output power and not the 11dB you are experiencing.At the lowest output power (-18 dBm), you are already 7 dB short. Can you confirm that the R&S FSL is OK? Check the supply current of the transmitter, at full power (~13 dBm) it should be around 24mA and at 2 dBm around 12mA.

author: joseph1235
author_rank: Newbie
timestamp: June 30, 2025, 12:02:02 pm
content: Dear all,the problem is solved !The cause : RF cable quality AND a RF attenuator (value not correct) .I have discover that by replacing my device with a signal generator with exactl the same path.Now i'have the correct rf power.thanks for your help, yours idea.SOLVED