circuit design - 250-350 MHz voltage controlled oscillator

I need to build a 250-350 MHz voltage controlled oscillator.

I have been checking out ring oscillators but they cannot reach these frequencies (max ~20 MHz).

Thus, I would like to know which are the usual designs for the range between 250-350 MHz. I suppose the best solution is a Colpitts oscillator, nevertheless I am not pretty sure if they are the only solution.

Some IC oscillator also would do the trick.

Answer

Sinewave oscillators at this sort of frequency tend to be the colpitts type oscillator because it is easily tuned with a vactor diode. This is the general idea: -

The varactor can change capacitance maybe by a factor of 6:1 and this results in a change of frequency that is \$\sqrt6 :1\$ (2.45:1). This then becomes a voltage controlled oscillator (VCO).

You can hang a PLL around it like the ADF4111 like this: -

Notice the VCO shown as a block top right. I've used this sort of design quite successfully at frequencies between 100 MHz and 600 MHz but the tuning range is only a bit over 2:1.

If you want wider than 2:1 there is a design on the net that uses a varactor and also tunes the inductor by varying the permeability of the inductor core by using DC current. You can get maybe 7:1 (frequency range) from this and I have been considering it: -

Or if you want a bigger range, go for a dual VCO method and a mixer. Set one VCO to tune from (say) 600 MHz to 800 MHz and another VCO to fall from 600 MHz to 400 MHz - mix the two outputs (balanced modulator) and after filtering you can get a frequency from the low MHz up to maybe 400 MHz. You need to have quite stable VCOs or use a couple of VCO control loops to keep basic frequency stability reasonably good.

You don't need two VCOs because you can mix a fixed frequency with a variable frequency to get a decent range but filtering is harder if you want sinewave purity. With two VCOs spanning in opposite directions the sum frequency is always constant (1200 MHz in the above example) and a notch filter can pretty much eradicate this unwanted product in the mix.

One word: it's never as easy as it sounds! A lot of care and attention to detail is required to get good performance.