High accuracy, 150 dB, attenuation measurement system for traceability at radio frequencies

The author describes the series audio frequency (AF) substitution attenuator calibrator which was designed and made at NPL, Malvern in 1988. It was designed to cover the range 0.5-50 MHz and, later on, the upper limit was extended to 100 MHz. The two synthesized sources employed are always operated 10 kHz apart and they are both locked to the same ultra-stable 10 MHz crystal-controlled oscillator; after mixing, an extremely stable 10 kHz intermediate frequency is always produced. The attenuation reference standard is a seven decade precision inductive voltage divider (IVD), which is inserted between two 10 kHz tuned amplifiers. At each setting of the device under test (DUT), the IVD and, if necessary, the gauge block attenuator (GBA) are adjusted so that the output voltage returns to the same value.<>