[Boatanchors] Boatanchors Digest, Vol 84, Issue 49

[Drew P.]

On receiver front end protection, Howard Holden wrote:

"How about using a pair of diodes back to back across the input?"

Diodes are non-linear elements, and when driven into conduction by strong signals, will cause intermodulation.  That 50 Kw AM broadcast station a mile away would magically appear all over the band.  In this case, diodes would represent a possible solution if the offending strong signal were to be reduced, as with a high pass filter ahead of the diodes.

The back to back configuration would work only when taking advantage of diode conduction in the reverse direction, as with zener diodes. The conduction threshold voltage would then be the zener voltage plus the forward drop. This could be used to raise the conduction threshold to a voltage higher than constant "nuisance" strong signals, as in that 50 Kw flamethrower, while still providing a measure of protection against extremely strong signals.

More frequently (as in ARRL receiver project plans) the diodes would be connected in inverse parallel so as to utilize the forward conduction threshold as a limiter.  Intermod city.

Note that the protection ability is limited by the current handling capability of the diodes themselves.  A pair of 1N4148's across the connections of a separate receiving antenna ain't gonna do no good if said antenna is also being inadvertently made to serve as part of a yagi-by-circumstance being fed by a 1 gallon amplifier.

Unless, that is, if the diodes fail in a shorted mode.  Sacrificial protection.

Drew