Sinusoidal and Relaxation Wide Range Voltage Controlled Oscillators

A voltage controlled oscillator (VCO) is one of the important basic building blocks in analog and digital circuits. For example, a VCO is the main building block in phase locked loop (PLL) and clock generator circuits [1]. This paper presents the design of 2 types of oscillators that are continuously voltage tunable. The frequency changing for all oscillators is based on optically coupled photoresistor (Vactrol) with 100 dB dynamic range [2]. The first is Wien-bridge oscillator with frequency range from approx. 0.1 Hz to 250 kHz with sinusoidal output. The Vactrol is used also for amplitude stabilization. The second oscillator is relaxation oscillator based on digital circuit with frequency range from 10 Hz to 9 MHz with square wave output. It is important to note that the control voltage and oscillator part are optically coupled.


Introduction
A voltage controlled oscillator (VCO) is one of the important basic building blocks in analog and digital circuits. For example, a VCO is the main building block in phase locked loop (PLL) and clock generator circuits [1]. This paper presents the design of 2 types of oscillators that are continuously voltage tunable. The frequency changing for all oscillators is based on optically coupled photoresistor (Vactrol) with 100 dB dynamic range [2]. The first is Wien-bridge oscillator with frequency range from approx. 0.1 Hz to 250 kHz with sinusoidal output. The Vactrol is used also for amplitude stabilization. The second oscillator is relaxation oscillator based on digital circuit with frequency range from 10 Hz to 9 MHz with square wave output. It is important to note that the control voltage and oscillator part are optically coupled. The Wien-bridge oscillator consists of OA 2 where C; R A ; R B are in positive feedback. The buffer OA 3 and diode bridge D, R 3 , R 4 , R 5 and OR 3 are used for automatic gain control. The output frequency is given as

D R
Resistance of OR is approximately where k OR is OR constant and i d is current through OR LED diode. Suppose that the R A =R B =R in this case the output frequency is given by Measured output frequency vs. input voltage is shown in Fig. 4. The frequency spectrum is shown in Fig. 5 (spectral quality is better than 50 dB).

Optically coupled photoresistor characteristic
Vactrol consists of a LED diode and photoresistor (Fig. 1). A photoresistor or light-dependent resistor (LDR) or photocell is a resistor whose resistance decreases with increasing light intensity. Optically coupled photoresistor (OR), also called photoresistive opto-isolator or Vactrol (after a trademark introduced by Vactec, Inc.) offers 100dB dynamic range, fast response time, and very high dark resistance. Some technical parameters for VTL5C1 [2] are: Min. isolation Voltage @ 70% Rel. humidity: 2500 VRMS; Max. resistor power: 175 mW; Max. resistor voltage: 100 V; Max LED current: 40 mA; Response time to 63% final R ON 2.5 ms. The measured output resistance vs. input current is shown in Fig. 2 (logarithmic scales are used for both the X and Y axes).

Wide range voltage controlled Wien-Bridge oscillator
The voltage controlled Wien-bridge oscillator [3 -5] is shown in Fig. 3. The operational amplifier OA 1 is used to form voltage controlled current source. The current i D , flows through diodes D 1 and D 2 of the OR 1 and OR 2 . The output current is i R where V i is input voltage and R 1 is resistor connected to inverting input.
and block k B (in Fig. 6) is used for the balancing of different values of a 1 and a 3 . In Fig. 7 the block diagram of compensated integrator is shown (top) and compensated integrator with OA (bottom). The compensated integrator with OA can be described by eq. (7) where R 4 (i d ) is photoresistor controlled by current i d .
For ideal compensation of R D

Amplitude control in quadrature sinusoidal oscillator
In this part another principle of the amplitude control of the quadrature sinusoidal oscillator is used. The block diagram of the quadrature oscillator with amplitude (energy) stabilization is presented in Fig. 6. The method is based on compensation of the parasitic dissipation parameters -a 1 and -a 3 by the multipliers connected in parallel along the dissipation blocks [6 and 7]. Desired value of the amplitude A of the oscillator signals is fed into the amplitude control block. The quadrature outputs of the oscillator (x 1 and x 2 ) are also connected to the amplitude control block. Amplitude control is based on eq. (5) where ideal steady state is sin c os sin c os

Wide range voltage controlled relaxation oscillator
The relaxation oscillator uses one Schmitt trigger capacitor and resistor [8 -11]. The circuit diagram of voltage controlled relaxation oscillator with output buffer is shown in Fig. 9.
The output frequency vs. supply voltage (with fixed V i =13 [V]) is shown in Fig. 10. The output frequency vs. input voltage (with fixed V CC =15 [V]) is displayed in Fig. 11. The frequency spectrum of quadrature oscillator with dissipative parameters compensation is shown in Fig. 8. Output frequency vs. input voltage of the linearized oscillator is displayed in Fig. 13. Measured values are in Table 1.

Conclusion
In this paper the 2 wide range simple V-f oscillators were described. For frequency control, the optically coupled photoresistor was used. The first is Wien-bridge oscillator with sinusoidal output with spectral quality greater than 50 dB. The second is linearized relaxation oscillator. All oscillators were constructed and measured. It is important to note that these oscillators can be used in different applications including PLL, frequency locked loop and low cost frequency synthesizers. The new method for amplitude control of sinusoidal quadrature oscillators with high spectral quality based also on optically coupled photoresistors and PI controller was also described. The circuit diagram of linearized relaxation oscillator with output buffer is shown in Fig. 13. Linearization is based on supply voltage increasing together with input voltage. The OA 2 is used as power source for V i ≥ 2.3 V. The supply voltage is given by eq. (10): . .
Measured values of output frequency versus input voltage for linearized relaxation oscillator