Triangular oscillator - Frequency - II part

This article is dedicated to the frequency modulation of the VF driver. The article presents different ways to change frequency in run-time. Also, we will see the difference between ideal and real frequency equation as well as the difference between ideal and real components.

Frequency modulation

VF driver stands for variable frequency in terms that signal frequency can be changed in runtime. It is a signal modulation since frequency is changed during runtime mode, but common term is variable frequency driver. It is very important to have a square wave generator with possibility to modulate the signal frequency. Why? Because different loads can work with different frequency. In this case load is a DC motor, which means that different DC motors works on different frequencies. Some DC motors work at lower frequencies, other DC motors work on higher frequencies, and that is the reason why it is important to have VF driver – to search and find the appropriate frequency of the chosen DC motor.

The following videos present how different frequency act on DC motor work. As you can see and hear, if the frequency is not adequate for chosen DC motor, it generates annoying sound. If you cannot hear an annoying sound generated by the DC motor, it means that the frequency is probably good. If you want to be sure that the frequency is adequately selected by the VF driver, check the DC motor temperature. If the temperature of the DC motor is also under normal condition, you can be sure that frequency generated by the VF driver is right selected.

Video#1 (jerky sound starts at ~1:30 minutes.) :  It is important for VF drivers to provide possibility to setup right frequency for DC motor, because it is not possible to drive each DC motor with the same frequency. Well, it is possible, but some DC motors will work perfectly, without generating annoying sound, while some DC motors will work like a jerk.

Video#2: If your DC motor sounds like a jerk, find the right frequency! ;-) Motor driver used for this example is: SERPENT I H-Bridge DC motor controller designed by zilsel-invent. 

At this point it is important to understand that there is a difference between frequency modulation and pulse width modulation of the square wave signal. When the frequency is found, square wave signal modulation in terms of changes the pulse width is another important modulation in order to control the speed of the DC motor shaft rotation. It is not possible to control the speed of the DC motor shaft rotation only with frequency since all frequency ranges (Hz, KHz or MHz) are not good for DC motor. For some frequencies DC motor can be overheated or to generate annoying sound. Beside frequency, pulse width changes of the square wave signal is also an additional important modulation, and it is called pulse width modulation or just PWM.

Difference between ideal and reality

To describe how electrical schematic (electronic device) work it is not important to deal with real components, since it is only important to understand how an electronic device work, it is important to understand the principles, so everything is ideal: operation amplifier, resistors, capacitors, diodes etc. But when you start with practical experience with electronics reality is something different. With simple words: components are not ideal, components are real. What does that mean? That means that all components are manufactured with tolerance. For example: 10KOhms resistor is not exactly 10KOhms resistor, it is 10KOhms +/- tolerance. The smaller the tolerance the resistance is closer to the target value of the 10KOhms. The same stand for capacitors (since capacitors are also manufactured with tolerances) as well as for operational amplifiers or any other available electronic component in the market - the reality is completely different.

Since we are dealing with practical electronics that means that frequency equation presented on the Figure #1.a is not good. The right way to present frequency is to place approximation mark instead of the equal mark (Figure #1.b), since it is not possible to do 100% precise frequency calculations with real components (do not forget about tolerance, you cannot know about real resistor values without measurements). You can only do approximated calculations and that’s it! If you really want to know what the generated frequency is, you have to do measurements, and that means that oscilloscope or frequencemetre should be used in order to know what exactly generated frequency is.

Figure #1. VF driver frequency. (a) ideal equation, (b) approximation

For example, VF driver revision 1.0 has three different points marked as “oscilloscope point” intended for signal tracking and measurements. By these three oscilloscope points it is possible to track and do measurements for the following signals: triangular, square wave signal and the amplified square wave signal since the operational amplifier output is buffered with a BJT transistor in order to provide higher current source. Anyway, without measurement points you will never know exact generated frequency, so each time when you do design of the signal generator be sure to place oscilloscope points.

How frequency of the VF driver is changed?

If we look under frequency equations we can see that frequency depends on four different values: R1, R2, R3 and C1. If we make changes of these values during runtime (during VF driver work) generated frequency of the VF driver is changed accordingly. In VF driver revision 1.0, the frequency is changed by changing the total capacitance of the VF driver since there are four different capacitors connected in parallel. All capacitors can be disconnected by using jumpers. If the jumper is removed that means that the total capacitor of the VF driver is decreased, and vice versa. If you place jumper, total capacitor is increased. The greater the total capacitor means lower the frequency and vice versa. 

Figure #2. VF driver with adjustable frequency. In this case generated frequency is controlled with capacitance C1, C2 and variable resistor R2. Total capacitance is determined by the following equation: C1 + C2 = Ctotal.

Beside capacitors, frequency is changed by using variable resistor R2. In case of the VF driver revision 1.0 it is implemented with potentiometer configured as a variable resistor. Pay attention here, capacitor and R2 are not the only values which can be used to change the frequency at runtime. Frequency can be changed by any value presented under frequency equation presented on the Figure #1.0. Play around with different values in order to generate different square wave frequencies. 

Related articles:    
Triangular oscillator - Fundamentals - I part  
Triangular oscillator - PWM square wave signal - III part
Triangluar oscillator - Operational amplifier slew rate - IV part 
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zilsel-invent assumes no responsibility or liability for any errors or inaccuracies that may appear in the present document. Specification and information contained in the present schematics are subject to change at any time without notice.