Aetherwellen-Musik.de

Home-brewed Theremins ... and other strange devices

Theremin Measurements

Introduction

Measurements have been performed on certain Theremin circuity.
The presented diagrams are associated to the descriptions of: 'General Functionality of the Rockmore- and Rosen- Tube-Theremin Instruments according to Leon Theremin’s Design'

Audio Pulse-Width / Duty-Cycle and Sound-Spectrum of EW-REB 12-2020

Circuit: EW MOD 12-2020 of EW Standard Theremin Modifications

The sound spectrum of the modified EW/EW+ Theremin instrument consists of the lower-, mid- and high-pitch segment which slightly changes with volume. The spectrum is dependent on the pulse width (duty cycle). The diagrams below show the pitch dependent pulse-width and duty-cycle characteristics of the EW(+) with added  EW-REB 12-2020 extension board.

f [Hz]

Pitch

PD (T) [ms]

NW [ms]

Duty NW [%]

PW [ms]

Duty PW [%]

PD / NW

NW / PW

27.5

A0

36.364

6.2

17.1

30.164

83.0

5.9

0.2

55

A1

18.182

4.1

22.6

14.082

77.5

4.4

0.3

110

A2

9.091

2.24

24.6

6.851

75.4

4.1

0.3

220

A3

4.545

1.12

24.6

3.425

75.4

4.1

0.3

440

A4

2.273

0.55

24.2

1.723

75.8

4.1

0.3

880

A5

1.136

0.26

22.9

0.876

77.1

4.4

0.3

1760

A6

0.568

0.098

17.2

0.470

82.8

5.8

0.2

As can be seen, the duty-cycle (Duty NW) of the mid pitch-range is typically 25%, corresponding to a pulse-ratio (PD/NW) of 1:4. The lower-register shows a decline of the duty-cycle below 110Hz (A2), corresponding to a pulse-ratio up to 1:6. The upper-register shows a sharp decline of the duty-cycle above 880Hz (A5), corresponding to a pulse-ratio up to 1:6.

The following pictures show selected audio pulses and the resulting spectra. For signal comparison, the EW audio-output signal (upper trace, yellow) and the speaker output as received by a microphone (lower trace, blue) are displayed at the oscilloscope screen-picture at the right.

EW-REB 12-2020 Theremin Pitch A0 27.5Hz



EW-REB 12-2020 Theremin Pitch A1 55Hz


EW-REB 12-2020 Theremin Pitch A2 110Hz


EW-REB 12-2020 Theremin Pitch A3 220Hz


EW-REB 12-2020 Theremin Pitch A4 440Hz


EW-REB 12-2020 Theremin Pitch A5 880Hz


EW-REB 12-2020 Theremin Pitch A6 1760Hz


The sound file of the recorded pitch-sweep used for pulse-width measurement, which holds the signal of the EW audio output on the left and the speaker signal, captured by a microphone on the right channel, can be downloaded below:

AWM_RM-12-2020_SWEEP.mp3
Sound file of EW-Rockmorizer Modification 12-2020
AWM_RM-12-2020_SWEEP.mp3 (3.98MB)
AWM_RM-12-2020_SWEEP.mp3
Sound file of EW-Rockmorizer Modification 12-2020
AWM_RM-12-2020_SWEEP.mp3 (3.98MB)

Pitch RF-Oscillators

Under construction!

Pitch Antenna and Extension Coil

Circuit: RT-6SH2P-Schematic of the  RT-6SH2P Tube Theremin

Notes: The circuit applies the general tube circuitry (alike those of the Rockmore / Rosen Theremin) of the pitch-oscillators, the phase-detector and the limiter amplifier. Russian 6SH2P/6J2P (6AS6) have been used. Anode (plate) Supply voltage was +40V.

Description: The  oscilloscope screen views below of the 6SH2P tube Theremin measurement show the input voltage, current and their phase-relation of the antenna extension coil at various distances of the hand to the pitch antenna. Approaching the antenna reduces the inductive current, touching leads to capacitive current. Zero current appears a few millimeters from the antenna, not touching it.

Ch.1 (yellow): 2mA/DIV antenna extension coil input current of the 6SH2P tube Theremin measurement
Ch.2 (blue): 1V/DIV antenna extension coil input voltage of the 6SH2P tube Theremin measurement


Ch.1 (yellow): 2mA/DIV antenna extension coil input current of the 6SH2P tube Theremin measurement
Ch.2 (blue): 1V/DIV antenna extension coil input voltage of the 6SH2P tube Theremin measurement

Pitch Phase Detector

Circuit: RT-6SH2P-Schematic of the  RT-6SH2P Tube Theremin

Notes: The circuit applies the general tube circuitry (alike those of the Rockmore / Rosen Theremin) of the pitch-oscillators, the phase-detector and the limiter amplifier. Russian 6SH2P/6J2P (6AS6) have been used. Anode (plate) Supply voltage was +40V.

Description: The oscilloscope screen views below of the 6SH2P tube Theremin show the phase-detector RF input voltages at the detector-tube control- and screen grid (detail of the amount of amplitude variation - unsymmetrical upper and lower RF envelope) and the output voltage (single-sine pulse) at the secondary of the first interstage-transformer. The control-grid of the tube shows the largest AM like RF amplitude variation (grid current).



Ch.1 (yellow): 5V/DIV AF output voltage of phase-detector stage of the 6SH2P tube Theremin measurement
Ch.2 (blue):
1V..2V/DIV RF input voltage @ detector-tube control- and screen grid of the 6SH2P tube Theremin measurement

Description: The oscilloscope screen views below of the of the 6SH2P tube Theremin measurement show the anode (plate) current of the phase-detector tube (RF current pulses) through the first interstage-transformer and the resulting output voltage (single-sine pulse) at the secondary.



Ch.1 (yellow): 500uA/DIV anode (plate) current of phase-detector tube of the 6SH2P tube Theremin measurement
Ch.2 (blue): 2V/DIV AF output voltage of phase-detector stage of the 6SH2P tube Theremin measurement

Pitch AF Limiter Amplifier

Circuit: RT-6SH2P-Schematic of the  RT-6SH2P Tube Theremin

Notes: The circuit applies the general tube circuitry (alike those of the Rockmore / Rosen Theremin) of the pitch-oscillators, the phase-detector and the limiter amplifier. Russian 6SH2P/6J2P (6AS6) have been used. Anode (plate) Supply voltage was +40V.

Description: The diagrams below of the 6SH2P tube Theremin measurement show the AF output voltage of the phase-detector stage (single-sine pulses) at the secondary of the first interstage-transformer and the limited resulting AC-coupled output voltage (limited single-sine pulse) at the anode (plate) of the voltage controlled limiter-amplifier tube.

Ch.1: 5V/DIV single-sine audio signal @ secondary of first interstage-transformer (phase-detector output) of the 6SH2P tube Theremin measurement
Ch.2: 2V/DIV limited single-sine audio signal @ output of limiter-amplifier of the 6SH2P tube Theremin measurement

Volume Detector

Under construction!

Volume Oscillator

Under construction!

Volume Antenna and Extension Coil

Under construction!

AF Power Amplifier (PA)

Under construction!

Speaker

Under construction!

Pitch Indicator

Under construction!

Power Supply

Under construction!

Tubes

Under construction!

Coils

Under construction!

Audio Transformers

Under construction!

Sound Analysis

Under construction!