I have just started to learn LimeSDR, and currently, I have just compiled BasicTX through Visual Studio and created its exe file. Above 500 MHz, the transmitted signal is sinusoidal. However, below 100 MHz, especially at the frequency I am interested in, 64 MHz, is really messy. I have read the other posts and was unable to find an answer.
Do you have any advice or idea why this might be happening?
The reason is possibly this: S11 Measurement Results
For TX signal analysis and purity use spectrum analyzer, not oscilloscope. You can’t expect signal output to look like Si5351 or similar devices.
please take a look at this post: Trouble using NCO to transmit at lower frequencies
I have the same problem. I’m trying to use LimeSDR Mini to transmit a sinusoidal signal in the range (21 MHz - 100 MHz) i.e. a simple carrier without any modulation / tone.
When I get to frequencies below 60 MHz, the signal gets really degraded.
Around 30 MHz I’m not able to calibrate Tx and Rx anymore.
I’m not sure if it’s a problem of the LimeSDR Mini Hardware or a problem in the API that could be avoided by programming the board in a “lower” level.
What is for sure is that the sinusoidal output signal has not the expected quality I’d like. As @yt7pwr said, it might be that the board transmit at the intended frequency and that you might be able to read this frequency with a spectrum analyzer, but the signal in this form has not the quality I’d need for my application. I’d like to know however if it’s the board Hardware or the API the source of the problem.
We have looked with spectrum analyzer before, and I forgot to take pictures of that. But what we saw was several peaks in different frequencies.
So, from what I understood, is it impossible to have a good sinusoidal below 100 MHz without any modifications? @yt7pwr
This is received spectrum BasicTX example on 21.768KHz using Mini sa TX and Airspy as RX. As you can see all spurs are at least 60dB below transmited signal. Strongest unwanted signal is SXT on 30MHz (not visible on picture). For HF operation 30MHz LO can be adjusted using DC corrector to fine null LO leak plus additional attenuation using final PA LPF. Using LimeSDR or any other SDR just for generation sine wave is waste of SDR technology potential. LimeSDR can transmit whatever waveform you create and send to it using I/Q data stream.
p.s. take a look to your first image (900MHz). Sine line is thick and if your scope is faster resulting image would be almost identical as second one (64MHz).