FREQUENCY SHIFT OF A RELAXATION OSCILLATOR DUE TO
AN EXTERNAL ELECTRODE*
O.S. STOICAN
INFLPR, P.O. Box MG-36, RO-077125, Bucharest-Magurele, Romania
E-mail: stoican@infim.ro
Received December 15, 2007
Experimental observation on the frequency variation of a relaxation oscillator
due to an external electrode is reported. The experimental setup consists of a gas filled
voltage regulator tube inserted into a relaxation oscillator circuit. The oscillations
frequency for various operating conditions has been measured. The results are discussed.
Key words: relaxation oscillator, gas filled voltage regulator, glow discharge.
1. INTRODUCTION
The use of the properties of the plasma generated inside the glow lamps or
gas filled electron tube as an unconventional detection method for various physical
effects was reported in several papers. For example in [1], [2] the experimental
setups containing discharge tubes used to detect the external microwave field are
presented. In this paper the effect of an external electrode on the electrical
characteristics of a gas filled voltage regulator tube is described. The change of the
discharge properties is observed by measuring the oscillations frequency of a
relaxation oscillator circuit. A similar method has been described in order to detect
the optogalvanic effect [3–5].
2. OPERATING PRINCIPLE OF THE RELAXATION OSCILLATOR
Generally, a relaxation oscillator consists of an appropriate non-linear switch
K, a capacitor C, a resistor R and a direct-current power supply E. The electrical
diagram of a typical relaxation oscillator is shown in Fig. 1 (Top). The Ri simulates
the equivalent resistance of the switch K when it is closed (state “on”). The
*
Paper presented at the 8th International Balkan Workshop on Applied Physics, 5–7 July,
2007, Constanţa, Romania.
Rom. Journ. Phys., Vol. 54, Nos. 3–4, P. 385–390, Bucharest, 2009
386
O.S. Stoican
2
equivalent resistance of the open switch K (state “off”) is supposed to be infinite.
The basic requirements for a non-linear switch that may be used as a relaxation
oscillator are as follows. The state of the switch depends on the voltage Uc across
its terminals. If the voltage Uc>U2 then the switch state is on, while Uc<U1, with
U1<U2, the switch state is off. The threshold voltages U2 and U1, respectively, are
intrinsic parameters of the non-linear switch. The state of the switch when U1
<Uc<U2 can be either on or off depending on the previous state (hysteretic
characteristic). The capacitor C is connected parallel to the non-linear switch and is
charged through the series resistor R. When voltage Uc reaches value U2, the
switch K is closed and the capacitor C is discharged suddenly through the switch
K. When voltage Uc becomes equal to U1, the switch K is open, the discharge of
the capacitor C is interrupted and the cycle restarts. The operating frequency f of
the relaxation oscillations can be approximated by the relation:
 E − U1 
1
≅ RC ln 

f
 E − U2 
(1)
The waveform of the voltage Uc for an ideal relaxation oscillator is shown in the
Fig. 1 (Bottom). By analyzing the time variation of the voltage Uc, the parameters U2 and
Fig. 1. – Electrical diagram of a relaxation oscillator (Top). Waveform of the voltage Uc across the
terminals of the non linear switch K (Bottom).
3
Frequency shift of a relaxation oscillator
387
U1 can be estimated. Any change of these parameters determines a variation of the
oscillations period which can be easily measured. Usually, these parameters are
related to some properties of the device used as a non-linear switch. As a
consequence the relaxation oscillator principle provides a sensitive tool to study
such kinds of physical systems.
3. EXPERIMENTAL SETUP
The experimental setup consists of a gas filled voltage regulator tube inserted
into a relaxation oscillator circuit (Fig. 2). A gas filled voltage regulator tube is an
electron tube electrically equivalent to a Zener diode. Basically, this component
represents a dc electrical discharge tube operating in the glow discharge regime. For
a dc electrical discharge tube the breakdown voltage is larger than the minimum
voltage necessary to maintain the discharge. This feature allows the use of a dc
discharge tube as a non-linear switch for a relaxation oscillator. In this case U1 and
U2 represent minimum voltage necessary to maintain the discharge and discharge
breakdown voltage, respectively. The voltage regulator tube RFT Str 90/40 (identical
to Philips 90C1) was used in our experiments. Its electrodes are enclosed in a
cylindrical glass envelope filled with a gas mixture. This kind of electron tube does
not use a heater electrode. Accordingly to the datasheets [6] the nominal regulating
voltage is 86–94V for discharge current varying in the range 1–40mA.
Fig. 2. – The schematic diagram of the experimental setup.
The electron tube has been placed inside a copper cylinder. The copper
cylinder is made of 0.1 mm thick copper sheet, is 18.3 mm in diameter and 55 mm
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O.S. Stoican
4
in length. Its inner diameter is equal to the electron tube outer diameter. The copper
cylinder is sliding so that electronic tube can be partially covered. A supplementary
resistor Ra is connected in series with the electron tube allowing the precise
measurement of the oscillations period. A short voltage peak occurs across the
terminals of the resistor Ra when the capacitor C is discharged through the electron
tube. In this way the oscillations period may be accurate measured as the time interval
between two consecutive voltage peaks. The values of the components R, C and Ra are
90.5kΩ, 470nF and 110Ω, respectively. The voltage E provided by a regulated
power supply can be varied in the range 0–170V. The copper cylinder is connected
or disconnected to the negative pole of the power supply using the switch S.
4. EXPERIMENTAL RESULTS
The oscillation frequency of the relaxation oscillator has been measured in
two cases. In the first one the copper cylinder is removed. In the second one the
copper cylinder is connected to the ground (negative pole of the power supply).
Fig. 3. – Waveform of the voltage Uc across the terminals of the voltage regulator tube when the
cylinder is removed.
In Fig. 3 and Fig. 4 are shown the waveforms of the voltage Uc in the two
cases. In both cases the supply voltage E=125V. The waveforms have been
recorded using a digital oscilloscope. As seen in Fig. 3 and Fig. 4, the oscillations
frequency increases from 25.07 Hz to 27.79 Hz. This variation appears due to the
decrease of threshold voltage U2. The frequency measurements have been verified
using a digital frequency counter. The same results are obtained if the copper
5
389
Frequency shift of a relaxation oscillator
cylinder is disconnected from the negative pole of the power supply without the
need to remove it. If the copper cylinder is connected to negative pole of the power
supply but it is partially removed (~ 50%), the measured oscillations frequency was
26.63 Hz. The measurements on the frequency shift of the relaxation oscillator as a
function of the supply voltage E are summarized in Table 1. The result shows that
the frequency shift ∆f increases with operating frequency.
Fig. 4. – Waveform of the voltage Uc across the terminals of the voltage regulator electron tube when
the copper cylinder covers the whole glass envelope of the electron tube and it is connected to the
negative pole of the power supply.
Table 1
The operating frequency of the relaxation oscillator for various values of the supply voltage E
Supply voltage
Operating frequency
Operating frequency
when copper cylinder is when copper cylinder is
connected to the
removed
negative pole of the
power supply.
Frequency shift
E [V]
f1[Hz]
f2[Hz]
∆f=f2-f1 [Hz]
164.3
160
155
145
140
135
130
126
62.1
58.2
53.6
44.6
40
35.4
30.5
26.4
67.8
63.7
58.76
49
44.1
39.2
34.1
30
5.7
5.5
5.16
4.4
4.1
3.8
3.6
3.6
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O.S. Stoican
6
5. CONCLUSION
The experimental results suggest that the copper sheet acts as an additionally
external electrode. When this electrode is proper biased the glow discharge
breakdown voltage U2 in the gas filled electron tube decreases. In the usually
conditions the relative variation ∆U2/U2 due to this electrode was found to be
around 1.3%. This value is small and it is very difficult to be evidenced by
measuring the voltage levels of the waveforms. Using a relaxation oscillator circuit
the corresponding relative variation ∆f/f of the oscillation frequency is around 10%.
This frequency shift can be easily measured using a digital frequency counter. The
magnitude of the effect depends on the relative position electron tube-copper sheet
and the relaxation oscillator operating frequency, respectively. These characteristics
show that the external copper sheet is capacitively coupled with the internal
electrodes of the electron tube. Further work will be devoted to evaluate more
accurate the influence of the operating frequency on the effect.
This work is done in the framework of the project PRET (2-Cex 06-11-7) and is supported by
the National Authority for Scientific Research (ANCS)-Romania.
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