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hewlett-pa: 325B; Distortion Analyzer

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Forum » Radios and other type of sets (Physics) etc. » MODELS DISPLAYED » hewlett-pa: 325B; Distortion Analyzer
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22.Sep.12 12:15

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

Measures total noise and distortion
Measures distortion as low as 0.1
Measures voltage as low as .0005
Will not load circuit being measured
Available in standard FCC frequencies

Total noise • Frequency response
Total distortion • Amplifier gain
Hum level
Voltage level, audio and supersonic

A distortion analyser
A broad band 20 or 75 db amplifier
A pre-amplifier for an oscilloscope
A 10 cps to 100 kc vacuum tube voltmeter
A universal laboratory test instrument


MODEL 325B is a combination of three separate elements: a frequency elimination circuit; an input amplifier, stabilized at 20 db, and a vacuum tube voltmeter. Any of these may be used individually. Together they form a uniquely practical trio.
By eliminating the fundamental frequency of a wave, and measuring the residual voltage, the -hp- Model 325B Noise and Distortion Analyzer makes possible accurate measurements of total noise and total distortion of any given wave.



At balance its filter circuit has almost infinite attenuation at a single frequency, while other frequencies pass with little or no attenuation. As shown in the chart, (see figure 1) the attenuation at the second harmonic (2F) is of the order of 1/2 db, while at the resonant frequency it is almost infinite - from 60 to 70 db in practical circuits. This makes it possible to measure distortion as low as 0.1%.
The amplifier employs inverse feedback, with the result that the accuracy is virtually independent of line voltage and tube characteristics and gives a constant gain of 20 db. The amplifier employs inverse feedback, with the result that the accuracy is virtually independent of line voltage and tube characteristics and gives a constant gain of 20 db. The amplifier is flat from 10 cps to 100 kc so that harmonics as high as the 6th of 15 kc are correctly indicated. Because the input impedance is high, the 325B will not load down the circuit being measured.

The sensitivity of the vacuum tube voltmeter is increased 10 times by the 20 db amplifier so that the hum may be measured directly, and voltage as low as .0005 can be measured.
The vacuum tube voltmeter is identical to the 400A except that the frequency range is limited to 100 kc. It is calibrated both in volts and in db. Terminals are provided at the output of the vacuum tube voltmeter amplifier for observation of the waveform on an oscilloscope. These terminals may also be used to employ the full system as a broad band amplifier of 75 db gain.


The -hp- Model 325B covers the audio frequency spectrum, with filters for frequencies of 30 cps, 50 cps, 100 cps, 400 cps, 1,000 cps, 5,000 cps, 7,500 cps, 10,000 cps and 15,000 cps, all set within ±5% of specified value. These are the frequencies recommended by the FCC for checking broadcast transmitters. On special orders, filters for other frequencies from 20 cps to 20 kc can be obtained. The meter scale is calibrated in volts and decibels.



The Model 325B, when used in conjunction with one of the -hp- 200 series Resistance-Tuned Audio Oscillators, provides equipment to make most laboratory AF measurements, including distortion, power gain, and frequency response. It will not load down the circuits under test since the input impedance is 200,000 ohms. The voltmeter section has an input impedance of 1,000,000 ohms and is very conveniently used independently to measure a-c voltages from .005 to 300 volts. When used with a simple diode detector the Model 325B will measure audio distortion in the r-f transmitted signal of a broadcast station. It is also useful in production testing of high fidelity receivers.


Sensitivity: The Model 325B will give full scale indication on a distortion level which is 0.3% of the fundamental. Measurements to 0.1% may be made with good accuracy. The full scale sensitivity is 3.0 millivolts or 50 db below 1 mw in 600 ohms. Satisfactory readings to 70 db below zero can be made. The over-all gain from the input terminals to the oscilloscope terminals is 75 db.

Voltmeter Range: Nine voltage ranges are provided with full scale sensitivities of .03, .10, .30, 1.0, 3.0, 10, 30, 100, 300. The ranges are also calibrated in db and the range multiplier changes the sensitivity in steps of exactly 10 db. Zero level is 1 mw in 600 ohms.

Voltmeter Accuracy: The over-all accuracy of the unit for voltage measurements is ±3%. Line voltage variations from 105 volts to 125 volts or changing tubes affects the reading by less than 3%.

Voltmeter Frequency Response: The frequency response of the meter including the input amplifier is within 3% from 10 cps to 100 kc.

Filter Frequency: Filters are provided for the following frequencies: 30 cps, 50 cps, 100 cps, 400 cps, 1,000 cps, 5,000 cps, 7,500 cps, 10,000 cps and 15,000 cps. These frequencies are set to plus or minus 5% of specified value.

Distortion Measurement Accuracy: The filters are designed to eliminate the fundamental by more than 60 db and to attenuate the second and all higher harmonics by less than 5%. Distortion measurements in general will be accurate within ±3% for distortion levels above 0.5%. When the distortion is largely second harmonic the accuracy will be ±5% for frequencies above 7.5 kc.

Noise Measurement Accuracy: The accuracy of noise and hum measurements is essentially the same as the accuracy of voltage measurements. The measured value is the average value of the full wave.

Input Impedance: The impedance at the input amplifier terminals is approximately 200,000 ohms shunted by approximately 24 uufd. The input impedance at the meter terminals is 1 megohm shunted by approximately 32 uufd.

General: The Model 325B is complete with all necessary power supplies for operation from 115 volts 50/60 cycles. The various parts of the unit are arranged for maximum flexibility so they can be used separately if desired. Terminals are supplied to connect an oscilloscope across the metering circuit so that observations of the applied wave and residual distortion may be made if desired. The indicating meter is a 4-inch square type with a large scale for ease of reading. The meter and various range switches are calibrated in both volts and decibels.

Mounting: The unit is mounted in an attractive oak cabinet with the front panel 19" x 10 ½". Complete shielding is provided against pickup from either 60 cycle or r-f fields of reasonable magnitude. The panel is finished in wrinkle gray with all controls and ranges clearly marked.

Net Weight: 56 pounds. Shipping Weight: 115 pounds.