The HP 400 series AC Voltmeters
There might still be a place in the Lab for these Forgotten Meters
a tech note under construction ...
In a previous tech note, we discussed how many modern DMMs, such as the widely used 34401A 6 1/2 digit DMM, are not suitable for measuring AC signals less than about 1 to 5 mV. Since then, we have been investigating low cost solutions for low level AC measurements. One viable solution for low level measurements turns out to be a long forgotten hp AC voltmeter series. In particular, three models of the outdated hp 400 series AC voltmeters produced from about 1969 to 1983, are useable down to microvolts. These models are the hp 400 GL, hp 400 F, and hp 400 FL.
While the hp 400s are analog meters, most have at least an amplified AC V output on the rear panel. When properly restored, they can serve as a front end amplifier for most existing DMMs or digitizers. While lacking digital controls and autoranging, for some low level applications, an hp 400 can be used as an extremely high quality, ultra low cost preamplifer. For other applications not requiring data acquisition, the meter indication can still be plenty good enough.
The hp 400 GL, F, and FL models all include a solidly designed JFET front end using negative feedback for linearization and fixed gain. The three transistor front end amplifier is reliable as well as protected from over voltage (with some limits). The input impedance is high and the input capacitance is relatively low below 15 or 30 pf, depending on the range selected. The GL has a wider dynamic range for each scale setting at 20 dB per scale (such as 10 uV to 100 uV). The F and FL have more resolution, but narrower 12 dB scales, such as 25 to 100 uV.
The hp 400 F indicates down to "0" uV, however the noise floor on these three models is generally in the range of 2 to 5 uV. The reading (meter or rear panel AC out) is the vector sum of the input signal uncorrelated to the internal meter noise floor. That is, at the lower end of the lowest scale, the reading can be representd as SQRT ((V meter noise)^2 + (V input)^2).
Also, note that while the input bandwidth is relatively high, for example 4 MHz with the 100 kHz filter off, for high source impedances, RC filtering with the input capacitance can dominate bandwidth and limit it to far less than several MHz. On the other hand, keep in mind that 4 MHz was only offered as that range for which the meter was rated to give a properly calibrated indication. In fact, the hp 400 GL still indicates to well in excess of 10 MHz.
Another "quirk" of the hp 400 series is that they are "average responding" meters. This means that they are calibrated in RMS (root mean square) only for a sinsoidal input waveform. Most of today's bench meters with AC voltage scales are "true RMS responding", meaning they give a correct RMS reading for a variety of input waveshapes. In some cases, a simple correction factor, such as 1.13 for converting a noise reading to RMS, makes the difference unimportant.
Other vintage instruments usable to tens of microvolts AC include distortion analyzers such as those produced by hp and Tektronix.
hp manuals and related catalog pages are reproduced with Permission, Courtesy of Agilent Technologies, Inc.
COPYRIGHT © 2006 JOSEPH M. GELLER