(6) 1 kc fundamental
Second harmonic 8 times third Negligible hum
(7) 1 kc fundamental
Second harmonic 12 times third Negligible hum
(8) Second harmonic 2 times third, plus higher-order harmonics
(9) 1 kc fundamental
All hum and distortion products about equal. This is the output of a Type 1301-A Low-Distortion Oscillator at optimum adjustment. Total noise and distortion at this frequency was 0.05%.
manner. Hence, in evaluating distortion and hum in the order of .5% or less, the user can be misled as to both the quantity and the nature of the un-desired signal by-products. Means are provided on the distortion and noise meter, however, both to determine the magnitude of the residual voltages and to make them negligible as a factor in measurements of .1% or less.4
Residual hum (in the distortion-measuring circuit) causes a residual meter indication when no input signal is present, and is therefore easily recognized. It is most pronounced on the lowest frequency range and on the lowest (most sensitive) attenuator position. An adjustment is provided to minimize it, but it is still an important factor in making noise-distortion measurements of the order of .1% or less. To minimize error in analyzing the hum content of such signals, the hum should be evaluated at a signal frequency above 150 cycles, if possible, and with the Type 1932-A calibrated at its normal (1.5 volts) input level. In any event, whenever the distortion pattern exhibits a high hum content, the input signal should be disconnected momentarily to determine whether the hum is residual or is a signal by-product.
Residual distortion can be virtually eliminated simply by operating at a signal level 10 db below the 1.5-volt normal input. In practice, this consists merely of using a different attenuator setting from that normally used, and a corresponding meter scale. All General Radio Type 1301-A Low-Distortion Oscillators (rated at not more than .1% total distortion and noise) are tested by this method.
4The residual hum described here applies only to distortion measurements, because it is introduced in the distortion-measuring circuits of the Type 1932-A. In noiae measurements, when the noise constitutes the total signal being measured, thiB circuit is not used, and consequently residual noise and hum are negligible (more than 80 db below zero dbm level).
A further limitation must be considered when measurements are attempted in the presence of r-f voltages, as in demodulated carrier signals, beat-fre-quency oscillator outputs, and the like. No provision for radio-frequency filtering is made in the Type 1932-A, and such filtering, if necessary, must be inserted ahead of the input. The same considerations apply as for any high-gain audio-frequency amplifier.
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