PIN-diode lowest operation frequency limitations
My thinking is that as long as you keep the PIN diode properly reverse biased the signal frequency should be irrelevant for conducting / blocking a signal.

I found this article where on page 2 is says:


  At frequencies much lower than f
  τ
  ,
  the capacitance characteristic of the
  PIN diode resembles a varactor
  diode. Because of the frequency
  limitations of common test
  equipment, capacitance
  measurements are generally made
  at 1 MHz. At this frequency the total
  capacitance, CT
  , is determined by
  applying a sufficiently large reverse
  voltage which fully depletes the Iregion of carriers.


Which seems to confirm my idea that as long as the reverse biasing is present and the signal doesn't momentarily forward bias the diode, the diode will behave as a (variable) capacitor.

In conclusion I think that the lowest signal frequency isn't a "hard" limit by itself. What does happen is that, as long as the PIN diode is kept in reverse bias, the reverse voltage will change with the signal and as the diode behaves as a varicap the capacitance will also change (with the signal).

Perhaps the lowest frequency is the frequency above which this effect (changing capacitance) becomes less relevant so that the diode can be seen as a (somewhat) constant capacitance.