Column: Circuit drill
THD performance of a TTL crystal oscillator for various load resistances
By Sulaiman Algharbi Alsayed, Managing Director, Smart PCB Solutions T
he TTL crystal oscillator is one of the simplest circuits electronic designers can build; see Figure 1. However, despite its simplicity and stable
performance, it has a fundamental problem – it can’t generate a pure sinusoidal signal, just a slightly distorted one, regardless of the frequency; see Figure 2. Signal distortion can be measured by
the Total Harmonic Distortion (THD) indicator. So the question is, what are the best operating criteria that keep THD at its lowest level for a given TTL crystal oscillator circuit. We will find out with the following experiment.
Experimental setup For the experiment we used different values for the load resistor R5 (Figure 1), logging the output voltage signal generated by the circuit and its THD. We assumed the following:
• Troughout experiment, R1, R2, R3, R4 and the crystal were unchanged, to allow proper monitoring of the impact of load resistance changes on the output THD.
• Te power supply was also kept constant, to avoid interfering with the collected data. Figure 3 shows the THD values versus the
load resistance values. As it can be seen, the THD is kept low when R5 is between 3Ω and 86Ω; beyond this range (9.2kΩ-9.5kΩ), the THD is much higher. In addition, Figure 3 shows that the lowest possible THD value of the output signal can be obtained when R5 is 17Ω. We conclude that the load resistance (R5)
is better in terms of generated harmonics when it is between 3Ω and 86Ω, and at its best at 17Ω. But, this is not the end of the story. In
theory, the load resistance of any electronic circuit is sized to be equal to the circuit’s output resistance, to ensure the load receives maximum power from the circuit. Tis is fundamental for all circuits delivering power to their connected load, which begs another question: What is the optimum load resistance value within the permitted low THD range?
More to it To answer this question, the power delivered across the load resistance R5 was measured
at various resistance values. Tis power can be multiplied by the RMS value of the voltage and current across resistor R5; see Figure 4, which shows the output power varying with the value of R5. It can be concluded that the maximum load delivered across the load resistor, R5, is about 25.7Ω. Figure 5 summarises the findings
from this experiment. Unfortunately, the value of load resistance for the optimum power delivery differs from the load resistance for the lowest THD. Tis is an important finding, since we should not blindly design oscillator circuits only to maximise power delivery to the load, but we should also consider the THD performance. Consequently, as a circuit designer,
you need to pay attention to both factors in your design, and select the one most important for the application. Is a less distorted output signal more important than maximum output power, or vice versa? Tis experiment sheds light on one demanding decision circuit designers must make before assembling their circuits.
Figure 1: A typical TTL crystal oscillator circuit 08 December/January 2023
www.electronicsworld.co.uk
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