Author Topic: Detect antenna transmission SX1276  (Read 848 times)

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Offline littleboot

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Detect antenna transmission SX1276
« on: January 05, 2017, 02:34:07 am »
Hello,

I'm currently working on a LoRa module that uses the sx1276 LoRa transceiver from Semtech based on the sx1276MB1MASreference design. (https://developer.mbed.org/components/SX1276MB1xAS/
In this design there are RF-switches that are used to switch the transmission circuit  or receiver circuit to the antanna. The are 2 options to control the RF-switches 1: software control from the MCU (default) 2: controlled by the transceiver RXTX/RF_MOD pin.
The datasheet pin description states: "pin 20 = RXTX/RF_MOD = Rx/Tx switch control: high in Tx" but there isn't any information about the time it takes for this pin to go High and the actual start of the transmission. Because I use different RF switches and 2 inverters in serie (buffer and inverting the signal) to control these switches it takes some time to switch from RX to TX. If the transmission starts before the switch is complete a small part of this transmission may be lost so I would like to measure this delay with my logic analyser.
Can I connect my logic analyser directly to the antenna and RF_MOD pin?(I think not) (Also I can't find anything about the voltages on transmission line when transmitting.)
How do I measure this delay correctly? (maybe connect a 50ohm resistor and measure the voltage across it?)

I already measured the delay between the software and the hardware  RF_MOD pin, and the the software pin is activated 100uS earlier then the hardware pin (in de demo project).

 

Offline mjs

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Re: Detect antenna transmission SX1276
« Reply #1 on: January 10, 2017, 07:16:41 am »
You can probably see the transmission by putting a small diode (even 1n4148 should be ok) to the output and rectifying it to a small (22-100pF) cap paralleled with 1k resistor. Use oscilloscope to measure output and RXTX pin relative timing. Try with high transmit power and the PA_BOOST output to get enough signal for simple detector.
 
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