You get a much, much wider range of frequencies with multiple capacitors in any oscillator that uses capacitors as part of the frequency tuning circuit.Please check this
Yes thanks it is less than the actual multiples of C it has to also be divided by 2.It is even more subtle than that because identical capacitors in series ha a value of C/2. So C || C = C*C/(C+C) = C/2
Capacitors in parallel add so:
C/2 + C = 3C/2 and so forth. So there a 7 switch combinations that represent valid tuning ranges. The fine tuning is done with an (old school) varactor, nowadays we use varactor diodes in a very small package.
The use of the term "capacitor bank" had me confused, thanks for the schematic.
That looks interesting.This is very common in modern PLL/VCO chips, this is from the ADF4351 datasheet that sums it up nicely:
View attachment 244173
3 VCOs, with 16 sub-bands each - achieved with switching in capacitors, covering 2200MHz - 4400MHz, along with respectable phase noise performance and all in a 5mm x 5mm package.
The ADF4351 is quite old, newer devices have capacitor banks giving hundred(s) of sub-bands. The ADF5610 datasheet shows a bit more more detail, and there are many others from ADI and other manufacturers.
Sounds like a DDS, nothing to do with VCOs.That looks interesting.
There is another one i got to use. It only went up to 40MHz but it was digital 32 bit word controllable. It came on a complete PC board with filters and everything and it was under 20 dollars USD. Dont remember the part number offhand. It produces a fairly clean sine wave and because of the 32 bit word the frequency increments were very small so you could effectively dial in just about anything you needed.
Havent used it a while now though.
I can find the part number if anyone is interested, but again it only goes up to 40MHz but it is crystal controlled so very stable.
Yes we know they (may) have different names, but i figure that someone interested in a VCO may be interested in the digital version. As i am sure you know, it's just a matter of how they are tuned (digital word or voltage).I guess that's why they have different names: VCO and NCO
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