NRF24L01+ Constant Carrier

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TelephoneBill

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I'm playing with NRF24L01+PA/LNA modules and Teensy 3.1. Having good success so far.

I wondered if anyone has managed to switched on "Constant Carrier" via Teensy. I'd like to have a look at the carrier on my SDR radio using HDSDR.

My basic SDR hardware only reaches 2.0 GHz, but I bought a Frequency Mixer that can reach from 2 GHz up to 12 GHz, so will employ this to bring the signal into visible range. Had great success with this mixer looking at 10 GHz radar module signals.
 
Hi Bill,
Although not having actual experience with "Constant Carrier", the scanner example has code to switch CCW ON & OFF.
But perhaps you already tried this...

Paul
 
That scanner example you gave me is brilliant. Its worth sharing the results to show not only how to transmit constant carrier, but also how to use the receiver to monitor activity in the WiFi spectrum. I'm very impressed.

The nRF24L01+ device has a built-in low noise amplifier. So with just a short quarter wave stub antenna, it can pickup any local WiFi activity that may be taking place. The sketch first prints details of the device itself, but then goes on to display a complete scan of the 2.400 to 2.525 GHz spectrum.

You have to be aware that this Tx/Rx module is not directly WiFi compatible. So it won't talk to a WiFi router, or decode WiFi packets. The radio channel numbers mentioned differ from WiFi channel numbers, and the modulation used is also different. The module uses GFSK modulation and has channel numbers that are 1 MHz wide, so there are a total of 126 channels going from 2.400 GHz to 2.525 GHz.

The example sketch will auto scan every channel in turn (0 - 125) and then display the received channel signal strength as a hex digit (0 - F). Here is a sample scan that I made this evening...

Scan2d4GHzChannels 01.jpg

Where I have annotated the picture with an arrow, you can see the point where I switched on the constant carrier on channel 40 from a second nRF24L01 module, placed only a few feet away. But noticed that there is radio signal activity received before it was switched on.

Notice too that there is no radio signals appearing above Channel 100. This is equivalent to 2.500 GHz, which is beyond the WiFi channels (WiFi CH14 = 2.484 GHz).

My construction for this experiment is shown below. I built the plywood bases originally to use for the ADF4351 Signal Generator modules, but I decided to use the same SPI bus for the nRF24L01 modules. I use different pins for Chip Select etc, and switched the SPI clock from pin 13 to pin 14, but it worked a treat on first switch on.

NRF24L01 Board 01.jpg NRF24L01 PointToPoint 01.jpg
 
Thanks for sharing your test results and explanation.
[never seen such neat breadboard wiring before]

Paul
 
nRF24L01+ Radio Spectrum

I have now managed to investigate the radio spectrum output of the nRF24L01+PA/LNA module as shown...

nRF24L01 2420 MHz.jpg

The constant carrier is shown to be present at 1420068 KHz = 1420.068 MHz = 1.420 GHz approximately on the SDR frequency scale. This is the IF (Intermediate Frequency) output from the RF Mixer when fed with a 1.000 GHz LO (Local Oscillator). So the true RF input frequency is 1.420 + 1.000 = 2.420 GHz. Channel 0 starts at 2.400 GHz and each channel is 1 MHz wide (0.001 GHz), so this corresponds to Channel 20, which is indeed what the constant carrier was set to in my code.

I did confirm that changing the channel to other numbers also saw the signal move accordingly to the expected new frequencies. The signal does drift slightly as the on-board 16 MHz crystal warms up, but this is only a few KHz.

The power output is variable, but the maximum is only 0 dBm, which equates to 1 mW. So the signal in the picture is only small. As to be expected, there appears a bit of phase noise either side of the signal peak.
 
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