Hi,
I have two ways of getting the fundamental frequency if I am not wrong.
1. I use the provided plot fundamental frequency in the RF manual and get a equation. Something like this: (L0 is the inductor connected to the output pin).
harmonic(xval(getData("/L0/PLUS" ?result "pss_fd")) '1)
The first-order Harmonic would be the fundamental wave.
2. I use a DFT transfer to transfer the transient output waveform to a discrete waveform, bar plot. By observing it, you could notice a high magnitude value at (or around) a frequency because the property of waveform being discrete. (X-axis is frequencies and Y-axis is Magnitude). At that frequency, I think it is the fundamental target frequency.
With one pss simulation result, I could compare the two values. There are some mismatch of the two frequencies which I can understand. DFT only gives values at fixed frequency dependent on user specific settings. For instance, I applied 'ymax' which will return the x values corresponding to the maximum Y values in the DFT resultant waveform. I got 4.6G (method 2.) , but the method 1. gave me 4.70118G.
(a) I am wondering which way is the correct way to get the target fundamental frequency of a VCO design? It looks the method 1. gives more accurate result. (1. is better)
(b) I also noticed there are some energy (Y-axis values in the DFT waveform) at the Zero Frequency. Sometimes if not properly designed, the target frequency's energy (the magnitude of the complex number representing the DFT resultant waveform) is smaller than the energy at the Zero Frequency. Like Mag@4.6G = 1 < 1.3 = Mag@0. I understand according to the definition of the Discrete Fourier Transformation, there are always some energy at the Zero Frequency. If the Magnitude at the target frequency is smaller than that at 0, it means the energy of the oscillation at the target Feq. is not strong enough. Therefore if using the method 2. I may get the target fundamental frequency with low energy which is smaller than the energy at the Zero frequency. On the contrary, it looks method 2. is better and secure.
Does anyone have a clear understanding on this?
Thank you,
Alex