User Manual

The higher the dBuV noise value the stronger the signal and better chance there's a real station broadcasting there.

The exact values will differ based on your area and the nearby stations but in general a small value under 32 is

probably unused, and a large value above 40 is a strong radio signal. Notice in the screen shot above 107.7mhz has a

strong signal with noise value of 45 dBuV, this makes sense because a nearby FM station, Seattle's 107.7mhz FM The

End (https://adafru.it/Bq9), is broadcasting at that frequency. However at 107.35mhz the signal has a much smaller

noise value of 29 and likely indicates an unused frequency.

Transmitting

Once you find a frequency that's unused you can configure the transmitter to broadcast its audio input on it with the

tx_frequency_khz property. Set this to a value in kilohertz, again only within the range 87.5mhz - 108mhz and at

50khz steps, to change the transmitter frequency. For example to use 107.35mhz:

You can also set the transmitter power with the tx_power property. Set this to a dBuV value from 88 - 115, or set 0 to

turn off the transmitter entirely. You typically want the maximum power because these low power FM transmitters

don't have much range by design. For example to set the maximum 115 dBuV transmit power:

At this point the Si4713 should be transmitting anything sent to the audio jack or LIN & RIN inputs over the configured

FM frequency. Plug in an audio source and try tuning an FM radio nearby to hear the signal! Remember the range of

these low power FM transmitters is limited and you might need the radio in the very same room or close by to pick up

the signal.

You might need to increase or decrease the volume of your audio source to ensure it's at a high enough level for the

transmitter to pick up and send (or you might need to turn it down if it's too high and 'overmodulating' the FM signal).

You can actually check with the chip to see if it's getting a good audio signal and potentially overmodulating or running

into other transmission issues. Simply read the input_level property to see the audio level (in dB) and the

audio_signal_status property to check if there are problems with the signal:

si4713.tx_frequency_khz = 107350

si4713.tx_power = 115