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9.26.05
Tuning
for Perfect Pitch Linearity
by Christopher
The
ideal pitch response from a theremin antenna occurs when the pitch
oscillator is tuned slightly "above" the LC
resonance of a coiled antenna. This is tuning to the sweet spot for the maximum
response of the pitch field. The antenna LC resonance is slightly below the
oscillator frequency to avoid frequency cross-over as your hand
approaches.
Modern
day pitch field thinking is fixated on passive hand capacitance
alone; my research
points to the added influence of current flow into the theremin antenna.
Lev Sergeyevich Termen referred to the pitch field as electro-magnetic
not just capacitive, the antenna was also referred to as an electrode not a
capacitor plate.
The Lev antenna coil spring has parasitic capacitance between the
coil turns and the environment. The overall spring stretch should be a maximum of
1/2" or less.
Stretching semi tunes the coil. IMHO the closely gathered spring coil turns
resonate
as part of the tank circuit around 922 kHz.
The pitch oscillator should have
the highest Q while the antenna
response I believe is broad band
behavior.
Moving your hand closer to the
Lev Antenna, hand capacitance effect-1
will lower
the variable oscillators frequency but it also begins
to de-tune the antenna from the balanced oscillator circuit. This detuning attenuates the transfer of
energy from the pitch oscillator
into the antenna. Less
current into the antenna increases the variable oscillator
Xc capacitance
reactance which decreases the tank capacitance effect-2
and this drives the variable oscillators frequency higher.
Amazingly these
two
capacitance effects together
counter balance and generate a perfectly linear
pitch field response relative to the musical scale!
The highest notes would
have the most attenuated current flow. This is why with proper "Lev" LC tuning,
compression of the higher notes cannot occur next to the antenna. It is
at this point next to the antenna that the pitch field energy has been
reduced to a minimum by the off tuning of the antenna LC.
This effect occurred
using either tube/valves or BJT transistors. Also no inline chokes are
used as the coil antenna is a direct connection to the variable
oscillator tank circuit. I did not experience non-linearity until I
purchased an EtherWave Standard years later.
I originally
saw this balancing effect while studying the LC
character of a
coiled spring
antenna back in 2004. My tests made it obvious that
something unusual was happening.
"The true
phenomenon found in Lev
Sergeyvich's heterodyne theremin antenna design is that the musical notes
developed from the pitch field
are spread out to match a piano keyboard perfectly."
This happens all the way up next to the antenna!
Christopher
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