KISS 152
Oscilloscope: Interpreting Square Wave Traces
by Iain Churches
KISS 152
Oscilloscope: Interpreting Square Wave Traces
by Iain Churches
All right, you've read the manual and connected the scope up right and thought calmly about probing safely. But what do the squiggles on the scope mean? It's actually very simple if you keep your head and proceed logically, as Iain Churches explains.
INTERPRETING SQUARE-WAVE OSCILLOSCOPE TRACES
A square-wave is comprised of a a fundamental frequency with an infinite number of odd-order harmonics - 3rd and then 5th, 7th, 9th, 11th 13th, 15th etc in reducing amplitude:
In musical terms, this means that a square wave with a fundamental of 100Hz (approximately G2) will also contain a 3rd harmonic at 300Hz (approximately D4), a fifth harmonic at 500Hz (approximately B4) , a seventh harmonic at 700Hz (approximately F5), a ninth harmonic at 900Hz (approximately A#5) an eleventh harmonic at 1100Hz (approximately C#6), a thirteenth harmonic at 1300Hz (approximately E6) a fifteenth harmonic at 1500Hz (just below G6) a seeventeenth harmonic at 1700Hz (just below A6) and a 19th harmonic at 1900 Hz (just below B6) and so on.
Included later on this page are some audio samples of square waves at 1kHz, 100Hz and 5kHz. It is interesting to note that unlike a sine wave, the timbre of a square wave varies with frequency.
To reproduce the square-wave accurately, the amplifier must have an accurate frequency response between f/10 and fx10. So, by careful choice of the frequency of the fundamental. one can test both LF and HF response both below and above the audible range. Four frequencies 100 Hz, 1kHz, 5kHz and 10kHz are commonly used when investigating the performance of tube amplifiers.
The three basic parameters of a square wave are Amplitude, Period, and Rise Time (which should be so short as to be represented by the finest vertical line)
Imperfections in the trace produced by the amplifier, include overshoot, excessive rise time, phase shift and ringing. The term "damping" is used to describe how well the trace is contained between over and undershoot.
With a square wave of 5kHz or 10kHz one can examine the HF performance of a tube amplifier, and adjust CR step networks for the optimum trace and best stability.
Some care is required in the interpretation of the basic square-wave traces.
Oscilloscope pictures of those most usually encountered, together with an explanation of what they reveal is given below.
A very thin or invisible vertical line indicates a very fast rise time with no phase shift -- an ideal state of affairs.
Traces at 1kHz
A (near) perfect square wave at 1kHz
The vertical leading edge indicates the rise time, which in a perfect square wave should be very small indeed, as illustrated by the the thinnest vertical trace (occupying very little time on the horizontal scale) The left hand edge of the thicker horizontal trace shows the high frequency (HF) content and the right had edge indicates the low frequency (LF) content.
HF gain: Excessive (+3dB at 16kHz)
Rise Time: Good
LF gain: Good
Damping: Good
HF gain: Good
Rise Time: Good
LF gain: Excessive (+3dB at 125Hz)
Damping Good
HF gain: Excessive
Rise time: Good
Overshoot by 3dB and Ringing
LF Gain: Good
Damping inadequate
Traces at 100Hz
A (near) perfect square wave at 100Hz
HF gain: Good
Rise Time: Good
LF gain: Attenuated (-3dB at 125Hz)
Damping Good
HF gain: Good
Rise Time: Good
LF gain: Excessive (+3dB at 125Hz)
Damping Good
HF gain: Poor
Rise Time: Good
LF gain: Good
A wide band of 3dB attenuation 1kHz to 4kHz
Damping Good
HF gain: Poor
Rise Time: Good
LF gain: Good
A wide band of 3dB attenuation 1kHz to 16kHz
Damping Good
Traces at 5kHz
A (near) perfect square wave at 5kHz
HF gain: Good
Rise Time: Good
LF gain: Inadequate
Roll off at 6dB/octave from 500Hz
Damping Good
HF gain: Poor
Rise Time: Good
LF gain: Good
Attenuation -12dB at 16kHz
Damping Good
HF gain: OK
Rise Time: Good
LF gain: Good
Excessive gain at F (fundamental frequency)
Damping Good
HF gain: Excessive (+3dB at 16kHz)
Rise Time: Good
LF gain: Good
Damping Good
HF gain: Inadequate (-12dB at 16kHz)
Rise Time: Good
LF gain: Good
Damping Good
Iain's own amps are on his Audio Pages
The most up-to-date version of Iain's Square Wave page is here
THE VOLTAGES IN THIS AMP WILL KILL YOU.
GET EXPERIENCED SUPERVISION IF IT IS YOUR FIRST TUBE AMP
Text and illustration is
Copyright © 2007 Iain Churches
