I am thinking about starting a project to demonstrate simple RC combinations as filters.
I have digital Resistor ICs and am trying to find a digital Capacitor IC but with no luck, either I am using the wrong search terms or they don't exist.
I've never seen a digitally-adjustable capacitor but you could make something like a digital [u]capacitor decade box[/u] with several capacitors and analog switches.
There is something called a switched capacitor filter, but that's a different concept.
Variable capacitors in silicon are restricted to very low capacitance and variation. AFAIR about 10pF, radio amateurs may know better.
There exist rotary capacitors (500pF) as used in ancient radios, which can be made rotate by a motor.
With some chemical knowledge a variable electrolytic capacitor can be built, with extractable plates, but this is only a DIY approach, nothing commercially available.
There may exist capacitor emulation circuits which I'm not aware of.
For demonstration purposees I'd use a capacitor switch box and a variable resistor.
Variable capacitors in silicon are restricted to very low capacitance and variation. AFAIR about 10pF, radio amateurs may know better.
They are known as Varactor diodes and their capacitance changes with the applied reverse bias D.C. voltage. In fact any diode will exhibit the same effect but these are used for oscillators about 100MHz or above. They are often used as a VCO in a PLL.
If this purely for simulation, you can mimic the whole circuit in the micro, simply by adjusting the input values.
That could be keypad, serial - or using simple pots - to adjust the R C values going in to the algorithm.
The output numbers will be the same, and if you need an electrical output signal, then a D-A stage will be needed.
The original idea was to use a signal generator as input and variable resistor and capacitor plotting the output and outputs using arduino oscilliscope code and a speaker to illustrate the changes.
I will revisit the project idea using a small number of fixed capacitors which should still get the ideas across.
Yes, the practical way is using a multi-way analog switch IC and fixed capacitors.
In CMOS logic processes deliberate capacitances are measured in pF to nF, (stray capacitances in fF),
ie about 4 to 6 orders of magnitude too small for your requirements...
Large capacitances need large areas, typical film capacitors unrolled have areas measured in 10's and 100's
of square cm. IC integrated capacitors are measured in square microns, and some are highly non-linear
too (standard MOS capacitors are actually modified FETs).
srik:
There exists a digital capacitor. search term would "digitally tunable capacitor" Psemi is the industry.
Read reply #6
The op is not wanting to operate at 100MHz, which is the minimum frequency those things work at. They seem to be in the range 1 to 5 pF, which puts it in the VHF region of the spectrum.