The AH 1100 capacitance standard frame containing from one to four AH 11A fused-silica capacitance standards provides reference capacitors of unexcelled stability. The inherent stability of this system when subjected to mechanical or thermal shock makes it the ideal transfer standard for capacitance. The built-in precision temperature controllers make it a simple, reliable system to use. These are the only standards currently sold that are sufficiently accurate to calibrate the AH 2500A capacitance bridge to the limits of its specifications.

The AH 1100 frame incorporates a panel meter with a three-and-one-half digit LED display to monitor internal parameters whose stability is critical to the stability of the capacitance values. This meter can read the most important power supply voltages at the press of a button. The meter also reads the difference in the readings taken by the pair of temperature sensors in each AH 11A. For convenience, these differences are reported in units of ppm of the capacitance value. Although the temperature of the AH 1100 chassis is not critical, its temperature may also be read by the panel meter. In addition, each AH 11A standard has an associated LED on the AH 1100 front panel that will flash if control of the oven temperature is lost, even briefly.

Features:

Any capacitance value in the range from 0.1 pF to 115 pF may be ordered.

Stability of larger capacitors is better than 0.3 ppm/year.

Temperature coefficient of the capacitance with respect to changes in ambient temperature is less than 0.01 ppm/°C.

Hysteresis resulting from temperature cycling is less than 0.05 ppm.

Hysteresis resulting from mechanical shock is less than 0.05 ppm.

AC voltage coefficient is less than 0.003 ppm/volt.

DC voltage coefficient is less than 0.0001 ppm/volt.

Power line sensitivity is less than 0.0003 ppm per 1% change in power line voltage.

Dissipation factor is less than 0.000 003 tan delta.

The user-selected capacitance value is set at the factory to a NIST traceable accuracy of 2 ppm at 1 kHz.

The built-in precision oven in each AH 11A uses a dual temperature sensor system which provides increased reliability and confidence.

The AH 1100 frame provides monitoring of critical temperature control parameters such as the differences within the dual temperature sensors in each standard.

Each AH 11A standard can be easily removed from the AH 1100 frame. Their small size and light weight make them easy to ship for calibration.

Shipping is simple since continuous temperature control of the ovens is not needed to maintain the stability specification.

The fused-silica element is hermetically sealed in dry nitrogen.

Three-terminal BNC connections minimize connector costs and number of cables.

BASIC DESIGN

AH 1100 Frame

The AH 1100 frame consists of a 3.5 inch (8.9 cm) high, standard width, bench-top or rack-mountable frame. This frame can hold up to four AH 11A fused-silica capacitance standards. The frame provides the electrical power to operate the precision temperature-controlled oven that is part of each AH 11A. The frame also provides the metering circuits that monitor internal power voltages and temperatures. A three-terminal connection to each standard is made independently from pairs of BNC connectors on the front of the AH 1100. The AH 1100 also contains extra space to accommodate several optional features that will be available in the future.

AH 11A Standard

Each AH 11A standard has, as its basic element, a fused-silica disk which is used to form a set of capacitors having values that are binary weighted. Factory selection of the appropriate capacitors from this set allows the AH 11A to be offered in a wide range of very accurately set values. Final trimming of the value is achieved by a small adjustment to the temperature of the oven. This method avoids the introduction of a separate trimming capacitor with its associated instabilities. Thus the dielectric of the AH 11A is very nearly 100% solid fused silica.

The fused-silica disk is hermetically sealed in a copper chamber that is thermally well insulated from the outside case of the AH 11A. The temperature of this chamber is measured by two precision temperature sensors, connected to two totally independent monitoring circuits. The average of the output voltages produced by these two circuits is used to control the temperature of the copper chamber. The difference in these output voltages may be selected for display on the front panel of the AH 1100 frame. As the standard ages, this difference should remain near zero. The smaller the difference, the greater the confidence in the stability of the oven temperature.

Each AH 11A incorporates the temperature measurement and control circuitry needed for its oven. Connection from the AH 11A to the AH 1100 frame is made with three connectors: two BNC and one 16 pin.