Claudio, I am a little confused. It is true that the minority carrier
lifetime is short compared with the LF waveform period but surely this only
really has any effect if you are trying to use the PIN diodes as
attenuators. If you use them as switches and pass a few milliamps in the
forward direct it is not relevant. This probably explains why our
experiments on replacing the "ordinary" diodes with PINs in HF receivers
(FRG7700) with octave front end filters have been unsuccessful. Some small
switching or "signal" diodes worked better in terms of IMD that the PINs. It
is possible the reverse attenuation of signal diodes will not be as good
because their stray capacitance is usually higher, but if they are reverse
biassed to separate the depletion layers they work very well.....and two can
be connected in series to reduce the capacity. The cut off the high pass
filter in many cases is the main culprit as there are I believe no LW BC
stations in Japan or the far East. Increasing the series capacitors can
lower the cut-off but this may not be as easy with modern SMD construction
as it was with through board components in the FRG7700 and R1000 era. I find
a lot of this equipment also has a 20dB extra attenuation switched in below
2MHz, usually on the AGC line. (I cannot say about the 817 on this score)
Any modification of these radios to improve LF reception will require an
external LP or BP filter to keep out strong BC signals.
Cheers de Alan G3NYK
----- Original Message -----
From: "Claudio Pozzi" <[email protected]>
To: <[email protected]>
Sent: 10 January 2005 10:45
Subject: Re: LF: Re: Pre-amps on LF
Many RTX uses inexpensive diodes in the front-end design, for bandpass
filter and preamp and attenuator switching.
Also if they used more expensive PIN diodes those devices are not the
best switching devices for LF. A 100 kHz PIN diode switching devices
require a minority carrier lifetime of few ten's of microseconds. If
the life time is shorter the diodes acts as a rectifier, not as a
switch. I don't know many PIN diodes useful in LF band.
So I think that the problem is not in the mixer but in band switching
devices.
73 de Claudio, ik2pii
--
Claudio Pozzi - Happy Linux User - http://www.qsl.net/ik2pii
--
Email.it, the professional e-mail, gratis per te: http://www.email.it/f
Sponsor:
Migliaia di manuali e guide scaricabili su: Office, Webmaster, Internet,
Windows e non solo...clicca qui
Clicca qui: http://adv.email.it/cgi-bin/foclick.cgi?mid=2731&d=10-1
|
