VCO uses no varicaps. How does it work?

I do not remember where I have found this circuit but I was wondering how is that possible to provide a variable frequency?
I have never seen this capacitor configuration in the LC, usually a varactor is used in place not just fixed capacitors.
Maybe he exploits a property of these Y5V capacitors I am unaware of?
Or maybe it relates somehow to my older post https://www.edaboard.com/showthread...riable-capacitor&highlight=variable+capacitor

Hi,

Y5V ceramics capacitors have a relatively large change in capacitance when there is DC voltage involved.

Every Y5V ceramics capacitor datasheet should show the behaviour.

Klaus

KlausST said:

Hi,

Y5V ceramics capacitors have a relatively large change in capacitance when there is DC voltage involved.

Every Y5V ceramics capacitor datasheet should show the behaviour.

Klaus

Click to expand...

How interesting is that!
https://www.maximintegrated.com/en/app-notes/index.mvp/id/5527

I expect Y5Y to be terrible in temperature variations, so their use in HF VCOs LC circuits may not be suitable. However I have not tested it and maybe it is not much worse than a varicap...

I expect Y5Y to be terrible in temperature variations, so their use in HF VCOs LC circuits may not be suitable. However I have not tested it and maybe it is not much worse than a varicap...

Click to expand...

Expect magnitudes worse! They are not particularly temperature or mechanically stable either. Note the oscillator in the schematic runs at maximum 3KHZ, scaled to HF the environmental effects would be disastrous.

I recently had to investigate a digital office telephone design that intermittently lost connection. The problem was a ceramic capacitor on the PCB that was part of a PLL loop filter. It worked perfectly for days on end when not making calls but would sometimes 'crash' when a number was being eyed in. The obvious culprit was a software bug in the dialing routine but it turned out to be the pressure of the keys being pressed was flexing the PCB and that ceramic capacitor was modulating the PLL and taking it out of lock. It was only running at 44KHz so you can imagine how fragile they would be at MHz frequencies.

Brian.