[hpsdr] Advantages/Disadvantages of Pin diode switching?

[Greg - ZL3IX]

The important difference between PIN diodes and ordinary diodes is the 
bias current required in the ON state.  For ordinary diodes the bias 
current must exceed the peak signal current, otherwise the diode will 
switch off during the negative peak of the signal.  For 10 W in a 50 ohm 
system the peak current is 632 mA, so you would need upwards of 700 mA 
DC to hold the diode on.

That is not true for PIN diodes, which are designed for slow turnoff.  
Provided that the carrier lifetime of the PIN diode in use, is longer 
than a half cycle, you only need enough current to keep the on 
resistance reasonably low.  But beware, check the carrier lifetime 
carefully.  Use on the lower HF bands needs really long lifetimes.

Use of any diode switching in circuits intended for SSB, needs care, as 
there is great potential for introducing intermod distortion. Use a 
spectrum analyser or spare Mercury/Hermes module to check your 
transmitted spectrum.

73, Greg, ZL3IX



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