VCO output power

Dear Dhara,

I would not characterize the change in power with output frequency a “drawback” as much as an expected behavior.

There are a multiple of reasons and this is not an extensive response, but two are:

1. If it is a CMOS based ring VCO, its rms current will vary as the frequency changes in a fashion analogous to a CMOS logic gate. Further, during start-up, its devices may be biased close to mid rail and hence will draw far more current than a logic gate in a static state.

2. If it is a resonant tank based VCO, the losses of the LC tank change with frequency. Specifically, the Q of the LC network changes with frequency. As a result, the sustaining amplifier must have more gain to overcome the higher loss of the tank. Typically, the losses will be greatest at the low end of its frequency range. Hence, the overall VCO current will typically increase at lower frequencies or at the frequency where the losses of the LC tank are greatest. The “LC” tank I am referring to includes the loss of the varactor capacitances.

Shawn

Over the whole tuning range if loop gain remains constant then we can have constant output power. please correct me. 

Any technique to stable the output power of VCO?  

What is the relation between quality factor of tank, harmonic distortion and constant output power??

Dear Dhara,

Let me make an attempt to answer your additional question. Others may have more insightful answers!

Assuming you are most interested in a resonant tank based VCO, the real part of its impedance is not constant over its tuning range. It might be instructive for you to simulate the impedance of a resonant tank with a finite inductance series resistance and some finite capacitive resistance over its resonant frequency and measure its real part. Since the real part varies with frequency, the loss changes over the tuning range and the oscillator sustaining amplifier must make up for this loss to continue to oscillate, Please recall that in the steady-state, the gain of the oscillator must be close to unity to sustain oscillations. Hence, its current (and hence power) will vary over the tuning range. If you can minimize the loss of the tank, the variation in oscillator power over the tuning range will be less.

The amount of harmonic distortion is highly dependent on the limiting characteristic of the sustaining amplifier about its operating point where its negative resistance is equal and opposite to that of the real impedance of the resonator. As the Q of the tank changes, the operating point of the sustaining changes which will impact its steady-state waveform and the resulting harmonic distortion.

Does this help!

Shawn

Thank you shawnlogan...

I am intensively studying this topic.