Analog Voltmeter

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Evolution of Analog Voltmeters
during the 1960 to 1980 Period






Choosing the Right AC & DC Voltmeters

In 1969, the Hewlett Packard general catalog listed a choice of 35 different analog instruments, and more than 10 different digital ones to measure Volts, Amperes, and Ohms. As usual, Hewlett Packard technology teaching efforts generated various publications dedicated to its AC & DC Voltmeter product line. The two Application Notes below were published to help the engineer in choosing the most appropriate instrument to fit his measurement need. Considering that most BSEE educational curricula did not consider measurement technology, HP filled a crucial need in educating engineers in the theory and practice of modern measurements with their well-respected Application Notes. Even for a "mundane" measurement of ac or dc volts, there was much to learn about the subtleties of what a measurement number meant. Was the readout derived from average, peak or rms sensing, that made a definitive difference?

Cover of the AN-60 - Apr. 1967


Which AC Voltmeter ?


Click on the link below for a PDF of the AN-60. Titled "Which AC Voltmeter ?," it was published in April 1967 to help the engineer choose the optimum AC Voltmeter for their need.


"Which AC Voltmeter ?" - PDF File (2.9 Mb)




Cover of the AN-69 - Aug. 1965


Which DC Voltmeter ?


Click on the link below for a PDF of the AN-69. Titled "Which DC Voltmeter ?," it was published in August 1965 to help the engineer choose the best DC Voltmeter for their need.


"Which DC Voltmeter ?" - PDF File (8 Mb)






What Makes a Better Voltmeter ?

During the reign of the Analog Meters of every type, Volt, Ampere, Ohm, etc... the last link in the chain, the interface between the parameter to be measured, and the human eye reading the resulting value, was a delicate electromechanical device: the galvanometer. It was the galvanometer quality which made the difference between a more or less accurate, comfortable, and pleasant to use instrument.

The story below describes the technology used by Hewlett Packard to manufacture, in-house, most types of galvanometer that were included in the manufacturing of analog meters.

Meter assembler, adjusts the axial position of a moving coil.
Instead of pivots and jewels, the moving coil in a taut band meter is
suspended on a platinum alloy band-eliminating friction and insuring
excellent repeatability. From MEASURE Magazine, March 1964.
Courtesy of the Hewlett Packard Company


The platinum twist for HP meters

Reconstruction of an article published in MEASURE Magazine, March 1964.

THERE'S MORE TO THE TWIST than meets the eye at Hewlett-Packard.

The twist in point is that performed by a tiny platinum band in the heart of HP's recently introduced taut band meter.

Creating better components, better designs, and better methods of quality assurance is a continuing and underlying philosophy at HP-and the HP twist is an offshoot of this effort, relating specifically to the meters used in many HP instruments.

The taut band meter story really starts several years ago when the company pioneered an automatic system to calibrate and photographically print a meter face to match the characteristics of each individual meter. The need was obvious. Having a standard scale for all meters means that each individual meter can be expected to have a small error in "reading." This is because each meter, no matter how precisely built, reacts differently to an electrical current. By recognizing the existence of variation, and printing individually calibrated scales for each meter face, extreme accuracies of better than one-half of one percent are consistently achieved.

After all of this work, the problem would seem to be dismissed. But following the HP philosophy, the quest for improvement continued and the HP twist-better known as the taut band suspension-emerged. The taut band principle has long been known. A galvanometer, for example, is essentially a taut band system with moving coils suspended vertically by a wire or band which supplies the restoring torque. But, it wasn't until recent efforts of a German manufacturer -Siemans and Halske- that a taut band of short length and horizontal suspension (the needed elements) became a practical reality. A licensing agreement with that company was obtained, and for over a year now the Loveland Division has been producing all the critical parts needed for taut band meters. This work is under the direction of John Lark, manager of the meter manufacturing department.

On a conventional meter, the pointer is attached to a coil which in turn is suspended by a pivot and jewel mechanism, similar to the balance wheel of a watch. Rough usage can cause the jewel to wear and even crack. Worse than that, however, the friction of the pivot-jewel mechanism affects repeatability of readings. That is, the pointer doesn't always return to the same starting point when the current is reduced. The taut band suspension has no pivots. When the meter coil turns with applied current, it "twists" the platinum band. As the current is decreased the band returns to its original flat position, thus returning the coil and pointer toward zero on the scale. The platinum band is kept taut by anchor springs attached to the band ends opposite the coil. As the old saying goes, it takes two to tango -or twist- which is only to say that the combination of photographically calibrated scales with taut band suspension is providing HP meters with extreme accuracy plus heavy-duty ruggedness. The result: more precise, reliable measurements for Hewlett-Packard customers.

Lower left: In a dust-free work space, Barbara Needles mounts a scale which has been individually calibrated to the meter's movements.
Lower right: A magnifying glass is used by Meter Technician Harold White during final electrical inspection of a unit.
From MEASURE Magazine, March 1964. Courtesy of Hewlett Packard Company


The HP 403A

The HP 403A AC Voltmeter,
The Second HP Transistorized Instrument


Just before the taut band meter technology described above, the 403A AC Voltmeter was the second transistorized instrument produced by HP, just after the 721A Power Supply.

The 403A AC voltmeter weighing less than 5 pounds and battery powered can be considered as the first HP handheld instrument, another benefit of transistor technology. Designed during the very last months of the 1950s it would be introduced in the 1960 catalog, page 59.

From 1 Hz to 1 MHz the measuring range goes from 1 millivolt to 300 volts full scale (12 ranges) in a 1, 3, 10 sequence.

Accuracy is within ±3 % of full scale from 5 Hz to 500 kHz. Nominal input impedance is 2 megohms. Battery life is 400 hours. More than 6 months normal use.



The HP 3400A


The HP 3400A


Introduced in the 1963 general catalog, the HP 3400A is an rms-Responding Voltmeter with high crest factor rating. This rating is the measure of the voltmeter's ability to read the rms value of waveforms that have a high peak-to-rms ratio such as low-duty-cycle pulse trains, electrical or acoustic noise, and voltages of undetermined waveform.

The Model 3400A voltage range extends from 100 microvolts to 300 volts and its frequency range from 10 Hz to 10 MHz. Its crest factor rating is 10:1 which enables it to read at full scale the rms value of pulse trains that have only a 1% duty cycle. At 1/10th of full scale, it will read pulse trains of only 0.01% duty cycle.

A block diagram of the 3400A voltmeter is shown below. The sensor is a heat-responding element called a theromocouple. The self-balancing thermocouple bridge circuit is preceded by attenuators and amplifiers. The thermocouple bridge establishes an equivalence between the rms value of the input signal and a DC feedback voltage in the following manner. The two matched thermocouples are connected with output voltages subtracting. Power applied to the heater of the signal input thermocouple generates a DC unbalance signal in the thermocouple outputs which is amplified and applied to the heater of the feedback thermocouple. The increased DC output from this thermocouple reduces the unbalance signal.

Since the dc amplifier gain is high, the unbalance signal remains very close to zero (<25 µvolts) at all times. The DC voltage applied to the feedback thermocouple therefore is proportional to the rms value of the voltage applied to the input thermocouple. The panel meter monitors the feedback dc voltage to provide the indication of the rms value of the input voltage.

Any parameter variations common to both thermocouples do not affect accuracy because of the high loop gain and the bridge configuration. Since the thermocouples are subject to the same thermal history, the voltmeter reads a new voltage accurately even while the thermocouples are settling to a new thermal level. Relatively fast response (<2 sec) is characteristic of this voltmeter.

The dc amplifier is a high-gain chopper amplifier employing a photoconductor modulator and demodulator. AC feedback around the amplifier reduces sensitivity to component variations in the modulator, demodulator, and chopper amplifier.

Tandem emitter-followers provide a low resistance drive to the meter circuit and to the feedback thermocouple heater, in addition to supplying the dc output at the rear panel through a 1 kilohm resistor. The resistor determines the output impedance and also isolates the output so that meter accuracy is independent of output loading. The output amplitude, corresponding to full scale meter deflection, is 1 volt into an open circuit or 1 ma into a short circuit.


Block diagram of HP Model 3400 A RMS Voltmeter.
From the Hewlett Packard Journal, January 1964. Courtesy of Hewlett Packard Company


The HP 3406A

The HP 3406A,
Broadband Sampling Voltmeter


The 3406A Broadband Sampling Voltmeter was introduced in the 1965 catalog, and was described in a long article published in the July 1966 issue of the Hewlett Packard Journal.

The 3406A operates on the principle of incoherent (random) sampling, thus providing and output usable for signal analysis. The sampling circuit output is available on the rear panel, giving instrument unusual capabilities for analysis of broadband signals.

The specified frequency range of the voltmeter is 10 kHz to 1 GHz, and the frequency response of a typical production instrument is flat within one percent over this range. Useful sensitivity extends from 1 kHz to 2 GHz or more. Voltage measurements are accurate within ±3% of full scale from 100 kHz to 100 MHz, ±5% from 10 kHz to 700 MHz, and ±8% to l GHz.

The sampling voltmeter responds to the absolute average values of unknown voltages, and is calibrated to read both the rms value of a sine wave and dBm in 50-ohm systems. It has eight voltage ranges from 1 mV full scale to 3 V full scale, and its sensitivity is high enough to measure voltages as small as 50 µV. Voltage scales are linear, and resolution is 20 µV on the 1 mV range.


Frequency response of typical production sampling voltmeter is flat within ±1% from 10 kHz to 1 GHz.
Useful sensitivity extends from 1 kHz to over 2 GHz.
From the Hewlett Packard Journal, July 1966. Courtesy of Hewlett Packard Company


The HP 400F


The HP 400F & 400FL


Taking advantage of the taut band technology for its galvanometer, the Model 400F was an evolution of the HP 403A described above.

The HP 400F was introduced in the 1966 catalog as a solid-state AC Voltmeter for measuring AC voltages from 100 microvolts to 300 Volts rms full scale. The main improvement on the 403A was an extension of the passband, covering a frequency range from 20 Hz to 4 MHz on the Model 400F & 400FL.

The HP 400F overall accuracy is 1/2% of reading on its 41/2" mirror-backed taut-band meter. The meter scale is individually calibrated with 100 divisions to provide greater resolution. An option 01 changes the galvanometer to arrange the dB scale uppermost for greater resolution in measurements where the dB readout is preferred.

The HP 400FL has all the characteristics mentioned for the 400F with 1% of reading accuracy on a linear 12 dB logarithmic scale. Its meter movement responds with logarithmic deflections.




The HP 427A

The HP 427A Analog Multimeter


Introduced in the 1966 general catalog, the HP 427A is a fully-portable laboratory-grade instrument, useful from 1 Hertz to 4 megahertz in AC mode, at levels to below 1 millivolt. It is also a sensitive DC Voltmeter and Ohmmeter.

THE EVER-USEFUL PORTABLE VOLTOHMMETER, as it was described in the May 1966 HP Journal, finds wide application primarily because it is a portable instrument requiring no external power. Unencumbered by a line cord, it is easily moved from place to place and it can be readily operated where ac line power is not available. The absence of a line cord has a further advantage in that a common source of ground loops is thereby eliminated.

The voltmeter circuits were designed for low current drain, total maximum power dissipation being only 400 mW. One 22.5-V battery thus provides over 300 hours of continuous operation or up to 700 hours of intermittent operation (a series regulator allows the instrument to function with rated accuracy on battery voltages as low as 15 volts).

In DC Volt Mode, the measurement ranges are from ±100 mV to ±1000 V full scale in a 1, 3, 10 sequence, and 9 ranges.

In AC Volt Mode, the measurement ranges are from 10 mV to 300 V rms full scale in a 1, 3, 10 sequence, and 10 ranges.

In Ohm Mode, the measurement ranges are from 10 Ohms to 10 megohms center scale in 7 ranges.



The HP 414A


The HP 414A,
Autoranging Voltmeter


The HP 414A was introduced in the 1965 catalog as a "Touch-and-Read" voltage and resistance Autovoltmeter, providing the "Touch-and-Read" convenience of a digital instrument with the economy of an analog instrument.

The automatic voltohmmeter selects the proper measurement range in less than 300 milliseconds after contact is made to the measured circuit or component. A lighted display above the meter shows the automatically-selected range, the units of measurement and, in the case of voltages, the polarity of the input.

The "touch-and-read" convenience of automatic ranging speeds up measurements on production lines, in point-to-point troubleshooting, in data taking, or in any test procedure that involves potentials of widely ranging magnitudes and differing polarities. DC voltages can be measured at sensitivities ranging from 5 mV to 1500 Volts full scale. Resistance measuring sensitivities range from 5 Ohms to 1.5 megohms full scale.

While not really very useful today, nor much of a great collector showpiece, the main interest of this HP 414A "AutoVoltmeter," as it was introduced in the 1965 catalog, is told in an article published in MEASURE Magazine in September 1965.

The article, titled "A Product Goes to Market," is a very interesting description of the Marketing step by step process followed by the product from its design to its introduction at the 1965 Wescon Electronic Trade Show.



Cover of MEASURE Magazine - Sep. 1965


MEASURE Magazine September 1965


Click on link below for a PDF of the "A Product Goes to Market," four page article published in the September 1965 issue of MEASURE Magazine.


"A Product Goes to Market" - PDF File (3.3 Mb)






The HP 419A


The HP 419A,
DC Null Voltmeter


The Model 419A DC Null Voltmeter was introduced in the 1965 catalog as a floating, high-sensitivity DC Null Meter which can measure voltages to below 1 microvolt and achieve virtually infinite input impedance.

The problems caused by noise, drift, and superimposed AC have all been carefully considered in the design of the high-sensitivity DC null voltmeter.

The Model 419A DC Null Voltmeter has 18 measuring ranges from 3µV end scale to 1000 V end scale. The Voltmeter operates from ac line power or from rechargeable internal batteries.

Using the HP 419A as a sensitive null detector during standard cell comparisons is another common application in a standard calibration laboratory. The instrument finds constant use in the HP primary standards laboratory at Loveland, CO. The set-up shown below was used for calibrating universal ratio sets, volt boxes, and Kelvin-Varley voltage dividers. It was described in the April 1966 issue of the Hewlett Packard Journal.




Typical low noise and high stability of HP Model 419A DC Null Voltmeter on 3µV range.


The 419A DC Null Voltmeter is part of HP standards lab set-up for calibrating universal ratio sets, volt boxes,
and Kelvin-Varley voltage dividers. Precision divider is composed of HP standard resistors, described below.
From the Hewlett Packard Journal, April 1966. Courtesy of Hewlett Packard Company


To be continued with the AC-DC Standards produced by HP during the 1960s . . .

The DC Room in HP's Palo Alto Standards Laboratory in 1969. It had temperature controlled within ±0.003°C.
Courtesy of Hewlett Packard Company



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