Question about TX Carrier Frequency Range

Hello. I have a quick question about the TX carrier frequency range of the LimeSDR-USB. I understand that the minimum LO frequency that one can set is 30 MHz (according to the function LMS_GetLOFrequencyRange). That means that the minimum carrier frequency that can be transmitted is 30 MHz.

However, If that is correct, then why does these plots of the LimeSDR HF perfomance for the TX paths ( have data points below 30 MHz? Can someone clarify me, please? Is it possible to transmit below 30 MHz?

Thanks in advance.

Because you can introduce frequency shift in digital domain using NCO.


Thanks for the quick reply. However I still can’t transmit at frequencies below 30 MHz right? Or is there a way to bypass the analog mixer and somehow produce a real signal from employing the digital mixer only?

What I thought is that I could select, for instance, a LO for the TX path of, say, 30 MHz. Then if I produce a vector with complex digital IQ samples at a frequency of 5MHz (somehow with the DACs), the output of the Mixer would generate a signal with -25MHz which is a signal with 25MHz with some phase. So I would be transmitting something with 25 MHz. Is this correct?

If not, can you clarify me how to transmit below 30 MHz?


Imagine the LO is tuned to 30MHz and the device is actually transmitting 60MHz of bandwidth.centered around that 30MHz. So 55MHz of that has no signal (or is zero padded), and the 5Mhz that is your desired signal for transmission was mixed in the digital domain. That is my understanding of it, but I could be totally wrong.