This article documents the Port 1 waveform of my NanoVNA-h4 v4.3 which uses a MS5351M (Chinese competitor to the Si5351A) clock generator.
There are many variants of the NanoVNA ‘v1’, and most use a Si5351A or more recently a loose clone, and they will probably have similar output.
Above is the Port 1 waveform with 50+j0Ω load (power setting 0). By eye, it is about 3.3 divisions in the middle of each ‘pulse’. At 50mv/div, that is 165mVpp, 0.136W, -8.7dBm. The fundamental component (which is what is used for measurement below ~300MHz) is 3dB less, -11.7dBm.
Importantly, is is not a sine wave, it is a complex wave shape that is rich in harmonics. See square wave for more discussion.
Above is the Port 1 waveform with no load. By eye, it is about 6.6 divisions in the middle of each ‘pulse’. At 50mv/div, that is 330mVpp.
The voltage measurements are low precision, but suggest that if it behaves as a Thevenin source, since loaded voltage is half unloaded voltage, assuming that then Zsource=Zload=50+j0Ω.
How can you make good measurements with a square wave?
The instrument is mostly used to measure devices with frequency variable response, so how does that work (well)?
The NanoVNA-H is essentially a superheterodyne receiver that mixes the wave (send and received) with a local oscillator (LO) to obtain an intermediate frequency in the low kHz range, and digital filtering implements a bandpass response to reject the image response and reduce noise. So, whilst the transmitted wave is not a pure sine wave, indeed is rich in harmonics, the receiver is narrowband and selects the fundamental or harmonic of interest. The fundamental is used below about 300MHz, then the third harmonic is selected by the choice of the LO frequency above that, and still higher the fifth harmonic is selected.
Of course the power in the harmonics falls with increasing n which reduces the dynamic range of the instrument in harmonic modes. Though the NanoVNA adjusts power for the harmonic bands, one can observed a reduction in dynamic range.
How did you get those waveforms when it sweeps?
There is a stimulus mode labelled CW which causes output to be on a single frequency. In the sense that CW means continuous wave, and unmodulated sine wave, this is not CW but simply a single frequency approximately square wave. If you dial up 600MHz, the output is the fundamental 200MHz approximately square wave.
That is of little consequence if you use the NanoVNA’s receiver as a selective receiver, but if you use a broadband detector or power meter (which responds to the fundamental plus harmonic content), you may get quite misleading results.
The NanoVNA is not a good substitute for a traditional sweep generator that produced levelled sine waves of calibrated amplitude and had a source impedance close to 50+j0Ω