GERTSCH RATIO TRANSFORMERS (original) (raw)

A Blast from 1958

A Ciphers By Ritter Page

Terry Ritter

2005 Feb 27

A ratio transformer is an adjustable precision AC voltage divider.

An obvious parallel would be a 10-turn precision potentiometer, which "divides" the voltage present at the outside terminals. These typically have a linearity of 0.25 percent, although some are rated as tightly as 0.05 percent. But a Gertsch AC Ratio Standard has a rated terminal linearity of 0.0001 percent.

A modern approach for an op amp feedback network would be to use precision matched resistor network, but even those have a typical division accuracy of about 0.1 percent. In contrast, a Gertsch ratio transformer instrument covers a full range of ratios and typically is 100 times as accurate.

Variations

Various construction selections are possible:

• Accuracy. Foremost is the number of decades of switch selection. This can range from two digits (in the case of a small coaxial design), to seven digits (in later ratio standards). Most designs tend to have five switched digits and a final potentiometer which could add up to two more digits of resolution (even though that would exceed accuracy specifications).
• Transformer Type. There are two types of toroidal transformer, and that choice limits both frequency of operation and voltage. (Very high division ratios can imply noise problems. For example, with a ratio setting of 0.000001, a 1V signal becomes 1uV, which would have to compete with noise. But with the same ratio, a 1kV original signal becomes 1mV, which should be well above the noise. So the ability to use higher voltages when they are available can be advantageous.)
  • The smaller, lighter toroids are limited to an RMS voltage of 0.35F, which is just 21 volts at 60Hz. The small toroids are most accurate at 400Hz, which would allow 140 volts RMS. While best accuracy in the small toroids is available below 3kHz, they are usually specified for 50Hz to 10,000Hz.
  • The larger, heavier toroids are limited to a RMS voltage of 2.5F, or 150 volts at 60Hz, which is also their most accurate frequency. Some of the large toroids have special insulation for up to 1000 volts, which would be usable at 400Hz and above. Best accuracy for the large toroids is available under 1kHz, and they are usually specified for 30Hz to 1,000Hz.
• Switches. Some models used pushbutton switches, but most used some form of decade rotary switch. These came in two varieties: The normal duty wafer switch, and the heavy duty switch with solid adjustable contacts that almost never wear out.
• Cases. Typically thick aluminum casing runs from tank-like heavy rack mounts, through heavy and medium bench top models, to semi-portable models with a combined handle and stand.

Limitations

Ratio transformers are signal devices, not _power_devices. While they are protected by fuses, they are transformers and the creative technician can burn them up. In some models, fuses may not protect an output potentiometer, which also can burn up. Then the faulty equipment might be put in a rack, never used again, and end up looking pretty good when it is sold. So caveat emptor!

One cost for ratio accuracy is a limitation to audio frequencies. DC cannot be ratioed in any transformer (though one could imagine various chopper arrangements to convert DC to precision AC for measurement). In the ideal case, a Gertsch ratio transformer would work on 400Hz or 60Hz power frequencies, depending on transformer size.

The frequency limitation is both a hassle and an advantage: We have a pretty good generator for 60Hz signals from the AC power lines, needing only a little power transformer to provide isolation for use. And 60Hz is a frequency that op amps and switching devices should have no problem handling in further processing, such as precision rectification, multiplication or synchronous rectification.

Availability

Ratio transformers were produced in a variety of configurations, and the rack mount versions can last a long, long time. The equipment uses no tubes, transistors or power supplies, so there is just not much there to wear out or die. Models with the cheaper switches could be a problem, since switch replacement was suggested as a factory activity. But the heavy duty switches on other models probably should not wear out and can be readjusted. In any case, no calibration is ever needed, so if the equipment "transforms" it ought to be working right, even if it is 50 years old. Unless, of course, somebody got into it.

I did buy a coaxial RatioTran (one not described in the 1958 catalog) that turned out to be faulty. The "coaxial" description means that two 10-position switches are controlled by two disks rotating on one shaft, which is also the control for the output potentiometer. The device had a noisy or intermittent output pot. Upon opening it up a burnt smell indicated that something had been overloaded. Presumably somebody had tried to multiply a voltage _up_without knowing that the last stage is typically a precision pot and without setting that pot to the end of travel. That could be a common problem, because the simple diagram on the panel generally does not show a pot. And a suitable precision replacement (10 turn 25 ohms, or 1 turn 5 ohms) could be very expensive even if one could be found.

Ratio transformer equipment is available at a wide range of prices, from 20to20 to 20to2000. The price seems to be set mainly by to a general lack of desire for the equipment, which comes from a lack of information on what the equipment is and what it can do. The rack-mount ratio transformer equipment is especially heavy (an RT-4R weighs about 40lbs), so shipping costs can be high.

Applications

There probably are a wide range of modern applications for precision ratio generation and measurement, but a few come to mind immediately:

• **Check and calibrate voltmeter range switches.**A 6-dial ratio transformer instrument with an input of 1V can dial out a full range from 0V to 1.1V in 1uV steps. That is roughly the same range of signal as a radio antenna input and a speaker output.
• Calibrate or select components for precision division ratios.
• Act as the adjustable component of a bridge. In this use they can check modern digital measurement instruments over a wide range, from a few precision references.
• Precisely measure amplification.

GENERAL LITERATURE

• Gertsch Standard RatioTrans. A very general introduction from the front of the catalog.
• INSTRUCTION BOOK. A brief survey of what a ratio transformer is, what not to do to it, and how to maintain it.

PRODUCT LITERATURE

• Low Voltage, High Frequency Models. A summary table for various models, including RT-1, RT-2, RT-5, RT-6, RT-7, RT-10, RT-11, RT-12, RT-13 and others.
• High Voltage, Low Frequency Models. A summary table for various models, including RT-3, RT-4, RT-8, RT-9, RT-14, RT-15 and others.
• Very High Voltage, Low Frequency Models. A summary table for models RT-14, RT-14R, RT-15 and RT-15R.
• Push Button RatioTrans. Specs for RT1-1R, RT2-2R and RT3-3R.
• Normal Duty Rotary Switch RatioTrans. Specs for RT-4, RT-4R, RT-5 and RT-5R.
• Heavy Duty Rotary Switch RatioTrans. Specs for RT-6, RT-6R, RT-7, RT-7R, RT-8, RT-8R, RT-9, RT-9R.
• 1000 Series Ratio Standards
  Front Page
  Specs
• RT60 Series Small, General Purpose Ratiotran
  Front Page
  Specs

TECHNICAL ANALYSIS

• Accuracy Analysis
  Page 1
  Page 2
• Low Impedence Voltmeter Calibration.
• Accuracy Calculations.
• Use in Bridge Circuits.
• Measuring Small Phase Angles
  Page 1
  Page 2

Terry Ritter, hiscurrent address, and his top page.