Electronic Input  vs. Mechanical Input Module       Amtronix Instruments, Inc.  11/2008

    The electronic input module is found on most HP 8920B and HP 8924C units. HP made this change sometime in 1996 to increase the reliability of the input assembly. The mechanical input assy uses 4 sealed relays for port switching and another 7 relays for attenuator  switching. The electronic input assy uses only 4 for port switching.  The performance of the electronic input differs from a mechanical input module in an HP 8920A. 8920B, 8924C unit in the following ways:

1)    The electronic input module is spec'd for use from 30 MHz to 1000MHz but actually maintains it's accuracy from just above 11 MHz to 1000MHz in generate mode and from 2 MHz to 1000MHz inreceive mode. When the unit is receiving, there is virtually no difference in performance down to 400 KHz. The Spectrum Analyzer is accurate down to 2 MHz and from there down to 400 KHz, the level indicated on the spectrum analyzer is lower than the actual level input. At 1 MHz, the Spectrum Analyzer will indicate about 3 db low. The received AM, FM or SSB modulations are still accurate.

2)     When the unit is in generate mode, the output level is accurate down to 10 MHz. Below 10 MHz, the output rf level is lower than the display indicates. This is due to the electronic switching in the electronic input module. At 5 MHz, the output level is about 6 db low. At 1 MHz, the output level is about 20 db low. The HP 8920, and 8924 units are still usable down to 400 KHz as the AM and FM levels are still accurate, it's just the rf out level that drops off. This will allow units with the electronic input assy to be used as low as 400 KHz

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