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hewlett-pa: 320B; Distortion Analyzer 320

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Forum » Radios and other type of sets (Physics) etc. » MODELS DISPLAYED » hewlett-pa: 320B; Distortion Analyzer 320
Pius Steiner
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23.Sep.12 13:18

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"From HP Catalog 18A" Laboratory Instruments", Copyrighted 1945."

Available for any frequencies from 50 cps to 20 kc
Measures harmonic distortion directly

Compare distortion content of wave with fundamental voltage
Observe type as well as amount of distortion (with oscilloscope)
Speed production testing
Attenuate voltages and signals—range zero to 70 db



The -hp- Model 320 Distortion Analyzer is a simple and convenient device for studying and measuring the harmonic distortion in audio frequency apparatus. The instrument is particularly suitable for development work, in as much as the character and type of distortion can be determined at the same time that the distortion is being measured. In addition, the instrument is excellent for production work because it is easy to operate and provides a rapid and accurate check for normal operation. Essentially, the Model 320 is a comparison device; it is used to compare the distortion content of a wave with the applied wave itself.

When this comparison is made in conjunction with a cathode ray oscilloscope, the type of distortion can be observed readily. The various order harmonics will show up in the oscilloscope pattern together with hum and noise voltages, so that a great deal of information about the voltage being analyzed can be obtained. For this reason, the Model 320 is a valuable instrument, not only when used alone, but also when used in combination with other equipment which measures the total rms distortion directly.

If more accurate measurements are desired than can be obtained with an oscilloscope, a sensitive voltmeter, such as the -hp- Model 400A, can be used as the detector. With such a meter the distortion measurements may be made with as good accuracy as can be obtained with the more expensive distortion meters.

The Model 320 Distortion Analyzer consists of fundamental elimination filters combined with a calibrated attenuator reading in decibels.

When the voltage to be analyzed is adjusted to the proper frequency, the fundamental is eliminated completely by the filters. The residual voltage containing the harmonics can then be observed with an oscilloscope or measured with a sensitive meter. When the level has been noted, a calibrated attenuator is substituted for the elimination filter. The attenuator is then adjusted until the same indication is obtained on the detector as with the filter in the circuit. When this condition is obtained, the distortion level is the number of decibels below the applied wave indicated by the setting of the attenuator.


Two models of the -hp- Model 320 are available. The Model 320A is designed for measurement at frequencies of 400 and 5000 cps, while the Model 320B is designed for measurement at frequencies of 50, 100, 400, 1000, 5000, and 7500 cps. All filters are designed to "provide more than 60 db of attenuation of the fundamental, so that distortion measurements may be made down to 0.1%.


Frequency Range: The Model 320A is designed for measurements at 400 and 5000 cps. The Model 320B is designed for measurements at 50, 100, 400, 1,000, 5,000, and 7500cps. Filters for other frequencies can be supplied on special order.

Input Impedance: The input impedance is at least 20,000 ohms. A bridging transformer may be used to increase the input impedance.

Attenuator: An attenuator is provided to compare the amplitude of the harmonic voltages with the fundamental. It consists of a 60 db unit adjustable in 10 db steps and a 10 db unit adjustable in 1 db steps. Individual resistors adjusted to ±½%.

Distortion Range: The filter circuits will provide more than 60 db of attenuation of the fundamental. Distortion values as low as 0.1% of the fundamental may be measured with a detector of sufficient sensitivity.

Detector Sensitivity: The sensitivity of the instrument is determined by the detector. The detector should give a readable indication on 0.1% of the fundamental if harmonics of 0.1% are to be measured. The detector need not be calibrated because it is used for comparison only. The usual oscilloscope with a one stage amplifier is sufficiently sensitive to measure 0.3% of a 30 volt fundamental. For proper operation of the instrument the input impedance of the detector must be 100,000 ohms or greater. An amplifier may be used between the instrument and the detector to increase the sensitivity. The only requirements on such an amplifier are that it must pass the highest harmonic which is of interest, and that it have a high impedance input. It need not be free from distortion otherwise.

Mounting: The Model 320A and 320B are mounted in attractive oak cabinets with the panel finished in wrinkle gray. The Models 320AR and 320BR fit the standard 19" relay rack with ¾" spacing. The panels are finished in wrinkle gray with machine engraved designations.

Size, Net Weight, Shipping Weight
320A    13" x 9" x 8"   15 pounds 22 pounds
320B    13" x 9" x 8"   17 ½ pounds 28 pounds
320AR 19" x 9" x 8"   17 pounds 32 pounds
320BR 19" x 9" x 8"   19 ½ pounds 35 pounds

This article was edited 25.Sep.12 12:12 by Pius Steiner .