Pete KE9OA wrote:
It was once explained to me that the longer the minority carrier
lifetime,
the better the low frequency response.
These folks explain it much better than I can:
www.microsemi.com/micnotes/701.pdf

wrote in message
oups.com...
If you are just switching bands, why does the carrier lifetime
matter?


A practical article about this is:
Tom Thompson W0IVJ, Exploring Intermodulation Distortion in RF
Switching and
Tuning Diodes, QST December 1994, pp 25-27

His tests demonstrate that PIN diodes intended for RF switching are
superior to PN switching diodes, especially at low bias. Surprisingly,
the common 1N4007 1A rectifier which has a PIN structure equals or
outperforms the MPN3700 and BAR17 RF PIN diodes for intermod at 5-10mA
bias but its reverse bias isolation would require two of them in series
to match these others.

The 1N4153 PN diode he tested had excellent isolation but lousy
intermod, great for turning something off but poor when turned on!
The author measured a 10-12 dB improvement just by raising the
forward bias from 5mA to 10 mA.

What was especially revealing in this article was the testing of a
varactor diode as the capacitor in a tuned preselector circuit
contrasted to a fixed capacitor switched by a PIN diode. The author
concluded that "using tuning diodes to tune high impedance circuits
in the presence of strong signals is not a good idea". I'll have to
go back to the library because a footnote to this sentence was that
the same issue of QST had an article by Ulrich Rohde, the highly
respected communications engineer, on a diode-tuned filter designed
to minimise IMD.

Tom