Truerta audio spectrum analyzer
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The zener diodes (D2 and D3) must be tested in this circuit and be discarded if they exhibit low-frequency “popcorn noise.” You will understand the name if you listen to it! The pink filter is the same as the model 109. I have no doubt that modern generators are as good as - if not better than - these vintage models.įigure 2: Model 131 random noise generation circuit, white noise output. The B&K used a pair of zener diodes for the noise generation in an otherwise all-vacuum-tube design, and the General Radio used a diode-connected triode in a magnetic field to generate the noise in an all-vacuum-tube circuit. The Allison model was solid-state in an encapsulated module, so I have no knowledge of the circuit.
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TRUERTA AUDIO SPECTRUM ANALYZER GENERATOR
My conclusion stated: “Within experimental limitations imposed by the measuring system, the noise sources evaluated were found to be sufficiently stationary for many laboratory uses.” In practice, this means that if I make a measurement today with a certain random noise generator and I make the same measurement tomorrow with the same generator, the results will be the same within a known percentage. I took data from three commonly available generators of that time: Allison Laboratories model 655, Brüel and Kjaer model 1402, and General Radio Company model 1390-A. In 1965 I did a study on the stationarity of laboratory random noise generators using long-time averaging (integration) of the mean and mean-square output voltages 2. Without all the math, this simply means that the mean and mean-square amplitude values don’t change, in the statistical sense, with time. The other feature that makes the output signal of a random noise generator useful is stationarity. If you sampled the noise waveform for a lifetime you still wouldn’t be able to predict the next value with any degree of certainty. However, you can’t know what the next amplitude value is going to be. To be useful for measurements you need a known frequency spectrum: white, pink 1, or other shape that has a known amplitude density - you hope it’s Gaussian (the bell-shaped curve) with a near-zero mean value and another item I will cover later. Generally, these would not be useful for making measurements.
TRUERTA AUDIO SPECTRUM ANALYZER TV
The sound of rain on a metal roof, the sound of a waterfall, static between radio stations, and the “snow” on a TV screen not tuned to an active channel are all examples of random noise. With 1/24-octave analysis and 10,000 point averaging. This figure shows the lower dynamic range of the swept frequency measurement. Figure 1: Comparison of a swept frequency response (upper trace) with a pink noise response.