Over on the "scratchpad" thread, two seperate lines of investigation are sort of happening.. One being the design of a RCA "Clone" using semiconductors, the other being exploration of the theory on how the original RCA functions.
Both activities are needed - without understanding, one cannot replicate - or one may simply try to 'replicate' but miss vital aspects and have a "clone" which is clearly not a "clone" (my first ignorant design made assumptions which were sensible, but completely wrong)
I want to move the mainly theoretical discussion to this thread, and leave the other thread for the 'practical' aspects - there will be overlap -
Data on RCA components :
To fully verify the understanding, anyone having unpublished data or measurements of actual RCA theremin component values - Please, lets have these! - In particular, the inductance values are needed for confirmation.
I believe I have a copy of everything published to date (RCA Manuals, Art Harrisons analysis, Construction notes etc).. but am puzzled by some inconsistancies on these.
In particular, I think there is a tendency for those analysing the RCA to take "shortcuts" in the same way that I originally did - For example, you know the tank resonant frequency (172 kHz or therabouts) you know the tank capacitance (say 1000pF) so it is easy to calculate the tank inductance as 598uH.. This is close to the value I calculated, and is the value given for each T8 winding by Art Harrison in his documentation..
But it is wrong! - This value worked with my simulation when I built the simulation circuit the way I thought it should be.. An Armstrong oscillator with normal tank and tickler coil.
But the Lev oscillator isnt a standard Armstrong oscillator - When I "built" an oscillator exactly like the RCA, the simulation returned completely "wrong" results - the frequency dropped way below 100kHz for starters.
At this point I was puzzled (to put it mildly) and asked for data on what the real inductance values for T8 were.. Rob (w0ttm) calculated the inductance based on the physical (winding and diameter) data for the coils, and gave a value of 165uH - Plugging these values into the simulator, the frequency became same as that of the RCA.. But completely wrong for a standard Armstrong oscillator.
Understanding the oscillator:
Thr reason that the inductance calculated from circuit theory (known C, known F, unknown L - simple to calculate L..) is that the circuit is a series resonant, not parrallel resonant one - The two inductors are also coupled, and this coupling increases the combined inductance - it is a circuit like this:
----T8 Winding A ------ 100pf fixed capacitor --- 100pf Variable capacitor --- T8 Winding B --
All in series, with winding A and B coupled, quite tightly (using coupling of .95 in the simulation, with value of 165uH gave close to the right frequency - we know that the theoretical inductance required is is about 600uH, and the additive and coupling (effectively increasing the number of turns) result of coils wired as above gives this kind of value.
Then there is the real simulation challenge - the antenna circuit 'connected' across the lower (B) winding.. Having an antenna circuit across a parrallel tank is usual territory, having it across one half of a series resonant tank, well, thatsv new (to me, anyway!)
I think wrong (but understandable) assumptions have been made when the RCA circuit has been looked at in the past.
The first "mistake" is probably calling the oscillator/s an "Armstrong" oscillator - It is certainly derived from an armstrong type design, but I believe it is probably a different type of oscillator - probably a Lev Termen invention.