How to measure output power in dBm of an LC tank VCO?

Dear Yasir,

Yasir Shafi said:

I would like to know how to measure the power of an LC tank VCO? I have seen the spectre RF workshop document, The PSS simulation is performed they have kept the R value to 10 kΩ. I don't understand the logic behind it. We shall measure with respect to default value i.e. 50 Ω load. Anyone who has performed this kind of simulation? The snapshot below is attached

In most high-frequency resonant based VCO, you are correct, the output power is typically measured when the output of the VCO is driving a 50 ohm impedance. Just to verify, are you trying to measure the power dissipation of the VCO or its output power. Both serve as VCO requirements.

I do not believe the Direct Plot "power" radio button was intended to measure VCO output power and VCO total power. I believe it is intended for IP1 or IP3 type measurements of an amplifier (these are typically a function of output power) - at least I have never seen it used in the context of VCO design.

The output waveform of a resonant (LC) VCO based on an on-chip or off-chip relatively low Q resonator such as an integrated inductor or external non-quartz based resonator will generally have significant harmonic content and hence, you need to establish if you are interested in the output power at only one of the harmonics or a power measurement that includes other harmonic terms. From that information, you may measure the output power using a DFT of the output waveform to include those harmonics defined in your VCO specification. If the output power has is no specific frequency at which it measured, then there is no need to perform a DFT of the output waveform as the power measurement will include all harmonics in the simulated output waveform.

The output power will vary with the tuning voltage - so it is also important to examine it over the required tuning range. There is some information on this in the forum post at URL:

community.cadence.com/.../vco-output-power

Does this help at all Yasir?

Shawn

Hi Shawn,

Thanks for the help, I have been measuring the output power. yes you are right I was looking to find the output power of the fundamental harmonic frequency. I attached a load of 10k at the output and in the PSS window I described the load of 10k as the value of R and then measured the power across the load attached. I was measuring at a fixed Vtune voltage as well.

Dear Yadira,

if you open the Results Browser for your pss analysis, you will find the frequency domain data resulting from the pss analysis. I believe or is named pss-fd. You may plot or export this to view the frequency content of the steady-state output signal you specified. The power at the frequency your specificstion is given may be ascertained from this plot knowing the load resistance you used in your simulation.

Shawn