A magnetic tape recorder's harmonic-distortion specification is very important. It usually determines where the record level of a recorder's electronics should be set. The record level is also used to determine the signal-to-noise ratio and frequency-response specifications. A typical harmonic-distortion specification might read "1% third harmonic of a 100-kHz signal at 60 ips." This means that the magnetic tape recorder has 1% third-harmonic distortion of a 100-kHz signal at 60 ips.
Harmonic distortion is the production of harmonic frequencies by an electronic system when a signal is applied at the input.
When an input signal goes through nonlinear electronic circuitry, the output signal will include some harmonic distortion (or unwanted frequencies).
If you analyzed this distortion, you'd see that a pattern exists. A pattern, whereby the frequency of each unwanted frequency is a multiple (X1, X2, X3, etc.) of the center frequency of the input signal.
There are two types of harmonic distortion: even-order and odd-order.
If the frequencies of the distortion are 2, 4, 6, etc., times the center frequency, it's even-order harmonics. If the frequencies of the distortion are 3, 5, 7, etc., times the center frequency, it's odd-order harmonics.
Odd-order harmonics are normally caused by the magnetic tape itself.
Even-order harmonics are normally caused by (1) permanently magnetized magnetic heads, (2) faulty circuits, or (3) asymmetrical or unbalanced bias signals.
As you might guess, even-order harmonics can be reduced by doing the right maintenance and periodic performance tests.
The primary harmonic distortion in magnetic tape recorder systems is third-order harmonics. If the level of third-order harmonics in a recorder increases, the level of distortion will also increase (figures 6-4A and B show this relationship). Two things that determine the level of third-order harmonics in a recorder are (1) the signal bias level, and (2) the record level. Figure 6-4A shows how third-order harmonic distortion decreases as the signal bias level increases. Figure 6-4B shows how the third harmonic increases gradually at first and then abruptly as the record level increases. That's why the third harmonic is used to determine the normal record level.
Figure 6-4 A & B. - Effect of signal bias level and record level on harmonic-distortion level.
Figure 6-5 shows a typical test equipment setup for measuring harmonic distortion. With this setup, the test signal from the signal generator is recorded and reproduced by the magnetic tape recorder at a normal record level. The amount of harmonic distortion is measured at the recorder's output on the wave analyzer.
Figure 6-5. - Test equipment setup for measuring harmonic distortion.
The technical manual for the magnetic recorder you're testing will tell you how to set up the test equipment. It'll tell you to set up the wave analyzer to measure a specific frequency. This frequency will be one of the multiples (X1, X2, X3, etc.) of the frequency the signal generator is outputting.
For example, let's say the technical manual told you to set up the signal generator to input a 10-kHz test signal into the magnetic tape recorder. Since you want to measure third-order harmonics, the technical manual will tell you to set the wave analyzer to measure the amount of harmonic distortion at 30 kHz.
It used to be thought that the only important specifications of magnetic tape recorders were signal-to-noise ratio and Frequency response. But now, with the need to record and reproduce more complex waveforms, such as telemetry and computer data, the phase-response specification becomes as important as Frequency response.
Phase response is the expression of the variation of the phase shift with respect to frequency. A good magnetic tape recorder will have linearly increasing phase response as frequency increases.
In simpler terms, good phase response shows that a magnetic recorder can reproduce a complex waveform (such as a square wave which has an infinite number of sine waves) without distorting it. Figure 6-6 shows both good and bad phase response.
Figure 6-6. - Pictures showing the effect of good and bad phase response on square-wave reproduction.
You cannot directly measure phase response. The best way to check the phase response of a magnetic tape recorder is to record and reproduce a square wave and watch the output on an oscilloscope.
If the output signal is symmetrical, like in figure 6-7,the recorder has good phase response.
Figure 6-7. - An example of good linear phase response.
Q.7 A recorder's harmonic-distortion specification reads 2% third harmonic of a 100-kHz
signal at 60 ips. What does this mean?