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I compared my old HP 5061A cesium standard against a much newer HP 5071A with high performance tube. Cesium standards are tweaked by adjust an electromagnet in the tube; this adjustment is called the “C Field.” In the 5061A you need to set the C Field manually, but the 5071A does it automatically. With the high performance tube, the 5071A’s absolute accuracy is +/- 5×10