""The smaller the tank capacitance, the more sensitive the theremin" is not true for the parallel tank EWS which has a relatively huge tank capacitor and relatively tiny tank inductor (compared to the EQ coil) but still manages a quite respectable -4.5 %F/pF. However I believe that quote does apply to parallel tanks without EQ, and to series tanks with or without EQ. " - Dewster
Absolutely! (or perhaps not.. Just seen what I highlighted above.. Let me think about that! ;-) - The sensitivity derived from Fred Nachbaur's calculations are only extremely crude "ball park" calculations and do not apply at all for topologies which include any equalizing method.. the "The smaller the tank capacitance, the more sensitive the theremin" applies only to non-compensated "direct antenna capacitance" designs.. Hz/pF is an utterly useless 'parameter' as both variables have a non-linear relationships - for music, Hz needs to be exponential, and for distance pF is inverse square law.. Put these together and one gets the (musically) non-linear response of distance <-> difference frequency.
This is why I have split theremin front-end designs into two groups -
(1) those where the antenna capacitance is "part" of the tank circuit directly - as in, the antenna capacitance is in parrallel with the tanks fixed capacitance - in these designs, the tanks fixed capacitance must be small (<500pF, but usually in the order of 220pF or even less) and the tank inductance needs to be large, and the operating frequency is usually high so that the tank inductance can be kept managable and to reduce the troublesome effect of inductance variation due to temperature (the bigger the inductance, the more thermal effects on this inductance will influence the oscillator frequency I think)..
The size of the fixed tank capacitor will directly influence the sensitivity - it is easy to change the sensitivity simply by increasing or decreasing the size of this capacitor.. but low frequency operation will require a bigger inductance, as one cannot reduce the operating frequency by increasing the tank capacitance, as this will impact sensitivity.. The SC front end has typical (and well selected) values of 680uH and 180pF for the tank, giving an operating frequency of 455kHz - and the change in antenna capacitance is "seen" as a change in total tank capacitance, and due to the small size (180pF) of the tanks fixed capacitance, is significant enough to give reasonable sensitivity.
(2) those where the antenna capacitance is not directly "seen" by the tank circuit - with these, changes in the antenna capacitance are converted to larger changes in the "virtual inductance" imposed across the tank inductor by the antenna equalizing circuit.
With these designs, even for low frequency oscillation, the tank inductance is small.. Sensitivity is determined by the ratio of the change in the antenna circuits inductive reactance || the tanks inductive reactance, so the antenna circuit can be seen as a "virtual variable inductor" in || with the tanks fixed inductor. The tank capacitance can be extremely large and the combination of tank L and C forms an oscillator which is, effectively, inductively tuned.
The two different topologies are mirror images of each other, and completely incompatible.. The "direct antenna capacitance" topology is better suited to higher frequencies, has small capacitance and large inductance, and has no equalization mechanism of any kind.
The "antenna with seperate resonator" topology requires small tank inductance and large tank capacitance, is well suited to lower frequency operation, and the conversion process from capacitance change to imposed inductive reactance change confers some linearization.
Sensitivity is easier to adjust / control in the "direct antenna capacitance" topology because this topology is far simpler - there are no complex interactions and only a single network defining the operating frequency.