John Dunn shares how a client found a workaround by replacing diodes when an EMI susceptibility issue arose.
A measurement instrument that was under development was experiencing problems of radiated EMI susceptibility. The client had found a way to quick-check that by using a CB (citizens band) transceiver as a source of radiation.
First, the client turned on the transmitter and laid it down across the new instrument's circuit board. After which, the operation of the instrument being tested was disrupted.
Figure 1: The transmitter lain down on an instrument circuit board.
The cause of the issues was a pair of 1N4148 diodes that had been used as part of a biasing arrangement as shown in a simplified form in the SPICE models below. Though the diodes were well into their current carrying range, they were still performing as non-linear elements which would rectify incoming RF and cause unacceptable bias shifts.
Figure 2: A pair of 1N4148 diodes was caught as the apparent culprit.
Replacing the diodes with a new biasing arrangement that used only resistors got rid of the stray rectification issue and improved the radiated susceptibility situation.
Figure 3: Replacing diodes with a biasing arrangement using resistors.
First published by EDN.