350A Attenuator

Attenuators, Voltage Dividers

The 350 attenuator was listed for the first time in the 1945 catalog page 28. It consists of two bridged-T circuits. One is a 100 dB attenuator, calibrated in 10 dB steps and the other is a 10 dB attenuator, calibrated in 1 dB steps. The design, made of individually adjusted resistors to +/- 0.5 %, results in less than 2 dB accumulative error at full 110 dB attenuation, and a usable frequency range of 100 Kc.

Two models were available. The 350A matches a 500 ohm impedance and the 350B matches a 600 ohm impedance. It is interesting to note that at this time the 600 ohm impedance was not yet the standard for audio measurement. It is probably the telephone industry which will set it some years later.

The HP 320A Distortion Analyzer

Distortion Measurement

Also introduced in the early 1940s, the Model 320 just added a T-notch rejection filter to the attenuator described above to make, (as stated in the 1945 catalog description) a simple and convenient device for studying and measuring the harmonic distortion in audio frequency apparatus.

We take the liberty to classify this device as a measurement accessory because at this level it is only a passive device. But the 330B and 325B distortion analyzers and the 300A harmonic wave analyzer, all listed in the same 1945 catalog are a clear testament to the early concern of HP to produce high performance signal analysis instrumentation. Four different instruments dedicated to signal analysis in 1945 are certainly a good explanation to the overwhelming leadership of HP in spectrum analysis which would soon begin in 1963.

The picture below recreates the amplifier distortion test set suggested in the 1945 catalog page 19. From left to right, a 200C oscillator is the signal source which feeds trough a 450A amplifier under test. At its output the carrier frequency is rejected by the 320 T-notch filter and the remaining harmonic spectrum RMS value is measured in the 2 Mc bandwith of a 400C electronic voltmeter.

330D Distorsion Analyzer

Distortion Analyzer

Combining all the instruments shown in the above picture in a single box would result in the 330B distortion analyzer which is presented in the 1948 catalog as follows:

The -hp- Model 330B Distortion Analyzer is capable of measuring distortion at any frequency between 20 cps and 20,000 cps. It will make noise measurements of voltages as small as 100 microvolts. A linear r-f detector makes possible measurements of these characteristics direct from a modulated r-f carrier. The convenience of operation, high sensitivity, accuracy, stability and light weight of the -hp- 330B make it an uniquely valuable instrument for broadcast, laboratory and production measurements. The circuit of the Model 330B consists of a linear r-f detector, a frequency-selective amplifier, a vacuum tube voltmeter, and a regulated power supply. The r-f detector includes a diode rectifier operating in conjunction with a resonant circuit which is tuned to the carrier frequency under measurement. The detector covers a range of 500 kc to 60 mc, and is varied by means of a tuning condenser and range switch which selects one of six bands. The detector may be switched out of the circuit when audio frequencies are used.

460BR Distributed Amplifier

Amplifier

Another birth in the measurement accessories category, the 460A wide band amplifier was the first of another long life product line. It was introduced in the 1950 catalog, page 62, but more importantly, it is the product described in the first Hewlett-Packard Journal, Volume 1, Number 1, published in September, 1949. The 460A is described as follows on the front page of this first HP Journal issue:

"Recent developments in nuclear radiation measurements have created a need for amplifiers having extremely wide bandwidth or speed compared to conventional video amplifiers. For example, certain photo-multiplier type radiation detectors have output pulses as short as 0.01 microsecond. The new Model 460A amplifier will amplify such pulses with very little distortion and provide suitable output for operating scalers, coincidence devices, or for direct viewing on a cathode ray tube. Although this amplifier was developed primarily to meet these requirements, other more general applications suggest themselves for amplifying signals anywhere in the range from audio frequencies to 200 megacycles. The amplifier can readily be used for extending by tenfold the sensitivity of a peak-reading diode voltmeter or for amplifying the output of a low-level signal generator. The distributed amplifier circuit provides a means for connecting conventional amplifier tubes in parallel so that their plate currents add but their capacities do not. Thus, the bandwidth can be increased beyond the point where the individual tubes have a gain of one, which is the limiting point in conventional cascaded stages."

The amplifier uses 13 x 6AK5 Vacuum tubes in a cascaded distributed arrangement whose simplified schematic is shown below. The specified gain is 20 dB with 200 ohm input / output resistive load with a .0026 µs rise time.

HP 428B Clip-on DC Milliammeter

The Clip-on DC Milliammeter

An instrument which could be also listed in the voltmeter chapter, the 428A Clip-on DC Milliammeter was introduced in the June-July, 1958 issue of the Hewlett Packard Journal.

The 428B measurement range goes from 1 milliampere to 10 amperes full scale in a 1, 3, 10 sequence.

The 428B Clip-on DC Milliammeter remained in the HP General catalog up to 1986 and it is still today a very useful instrument in the electronic workshop.

The HP 428 was such a successful product that it was chosen by Bill Hewlett for a special appreciation in the introduction of his book: "Inventions of Opportunity" edited in 1983:

"The instrument described in this article is an example of an old technique used for modern purposes. The principle is essentially similar to that used in a conventional flux-gate compass, i.e., a magnetic amplifier that produces a second harmonic component of the existing ac current. This second harmonic is proportional to the dc flux surrounding the conductor being measured. The use of negative feedback increases the accuracy and stability of the measurement. To illustrate some of the practical problems relating to the design of this instrument, the earth's magnetic field is about three hundred times stronger than the field measured by the instrument on its most sensitive range. The applications for this instrument had obvious advantages, for it could measure direct currents without the insertion of an external instrument or without the load on a circuit of a resistor across which a voltage could be measured.
Like many of our other products, the suggestion came from the outside. One of our senior sales engineers with aviation instrumentation experience had made the comment, "Have you ever thought of using the flux-gate compass principle to create a clip-on milliameter?" That was all that was necessary."