Shape of signal from generator

[PittG]

Hello,

I spotted that in Signal Generator mode of LibreVNA from frequency exactly 25MHz and above shape of output waveform from signal generator is changing from sinusoidal to trapezoid. Is it correct ? Is there a way to increase this cut-off frequency to above 30MHz ?

Best regards,
Pit.

[jankae]

Hi Pit,
you can sort of do that. I think it helps to understand why the signal changes its form at that frequency.

For frequencies below 25 MHz, the signal is generated by the Si5351C which has a 40 MHz lowpass in its path:

For frequencies above 25 MHz, the signal is generated by the MAX2871 PLL which only has a 900 MHz lowpass so the amount of harmonics for e.g. 26 MHz are a lot higher:

This transition is hardcoded in the firmware to happen at 25 MHz. If you want to use the Si5351C all the way up to 30 MHz, you have two options:

1. Build your own custom LibreVNA firmware with a different band switch frequency. As far as I know, this should be as simple as changing a single line but I have not tested this:
   

   LibreVNA/Software/VNA_embedded/Application/Hardware.hpp

   Line 44 in 8a5d56e

   static constexpr uint32_t BandSwitchFrequency = 25000000;

2. If you just want to quickly generate a 30 MHz sine wave and do not care about configuring a few more things in the GUI, you could look into the manual control dialog. This is more complicated to set up than using the signal generator mode but you have full flexibility and can set the Si5351C to 30 MHz without modifying the firmware. This could look like this:
   
   Use the marked sections to control the frequency, amplitude and the port. For a simple signal generation you can ignore everything else.

[PittG]

Thank you very much for detailed explanation.
I'll try to do your recomendation.

All the best for you
Pit