Super Diodes and detectors
in [Design]
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From: Graeme Zimmer on 21 Jul 2010 23:07 Hi, This post was prompted by the recent discussion about Diode-Connected Transistors. The Crystal Set nutters are always seeking the holy grail of a perfect diode detector (it must be passive of course). The problem is that existing diodes are lousy rectifiers at low currents (three steps forward and two steps back). What is needed is a diode which has a steep forward/reverse curve at very low currents (e.g. even with bias, a Silicon diode is worse than the old fashioned Germanium). The favorite at present is a zero-bias FET, using the high Q of a tuned circuit to develop a voltage swing to drive the Gate, which switches (varies) conduction in the (low impedance) Drain/Source path. Any comments please? Thanks ............ Zim
From: John Larkin on 21 Jul 2010 23:31 On Thu, 22 Jul 2010 13:07:44 +1000, "Graeme Zimmer" <gzimmer(a)wideband.net.au> wrote: >Hi, > >This post was prompted by the recent discussion about Diode-Connected >Transistors. > >The Crystal Set nutters are always seeking the holy grail of a perfect diode >detector >(it must be passive of course). > >The problem is that existing diodes are lousy rectifiers at low currents >(three steps forward and two steps back). > >What is needed is a diode which has a steep forward/reverse curve at very >low currents >(e.g. even with bias, a Silicon diode is worse than the old fashioned >Germanium). > >The favorite at present is a zero-bias FET, using the high Q of a tuned >circuit to develop a voltage swing to drive the Gate, which switches >(varies) conduction in the (low impedance) Drain/Source path. > >Any comments please? > >Thanks ............ Zim > I think the best low-level detector diode is a germanium back diode, essentially a tunnel diode with very low peak point current. People still make them. John
From: Graeme Zimmer on 22 Jul 2010 01:07 > I think the best low-level detector diode is a germanium back diode, > essentially a tunnel diode with very low peak point current. Thanks John, Yes, Tunnel diodes have a sharp knee and can work well with apropriate bias. There are many other tricks, such as using as high impedance as possible (to get higher voltage swing) and a Schottky (eg HP 5082-2835). The best "normal" diode so far seems to be the rather obscure ITT FO-215 some info on the FO-215 at http://www.theradioboard.com/rb/viewtopic.php?p=16891&sid=12ca5c44ec9c2b7b97a6af75bc6770e7 The FET idea can be seen at: http://www.arrl.org/files/file/Technology/tis/info/pdf/culter.pdf ............ Zim
From: Robert Baer on 22 Jul 2010 03:37 Graeme Zimmer wrote: >> I think the best low-level detector diode is a germanium back diode, >> essentially a tunnel diode with very low peak point current. > > Thanks John, > > Yes, Tunnel diodes have a sharp knee and can work well with apropriate bias. > > There are many other tricks, such as using as high impedance as possible (to > get higher voltage swing) and a Schottky (eg HP 5082-2835). > > The best "normal" diode so far seems to be the rather obscure ITT FO-215 > > some info on the FO-215 at > http://www.theradioboard.com/rb/viewtopic.php?p=16891&sid=12ca5c44ec9c2b7b97a6af75bc6770e7 > > The FET idea can be seen at: > http://www.arrl.org/files/file/Technology/tis/info/pdf/culter.pdf > > ........... Zim > > Would not a crystal headset be better for matching and sensitivity?
From: m II on 22 Jul 2010 02:51
John Larkin wrote: > I think the best low-level detector diode is a germanium back diode, > essentially a tunnel diode with very low peak point current. People > still make them. ...as visions of a straw covered alpine shack floor, with a germanium spinning wheel in the corner come to mind. Outside, the dark and foreboding clouds of the impending Silicon invasion are gathering. A silence has filled the Valley. mike |