> I am looking at a design for receiver protection in an
SO2R environment
> that involves a series resistance (#49 bulb) and stacks of
back to back
> diodes shunting the receiver input. The objective is to
protect front-end
> components against strong out-of-band signals, but in the
process I don't
> want to incur an intermod penalty under normal operation.
A bulb is a good choice Pete, generally better than any fuse
I've tested. They are faster to open (less thermal mass) and
resistance comes up much more rapidly in the process of
failing.
The clamp is required on today's receivers, because you can
easily toast an FET before the lamp would fail.
> If I stack 3 diodes in each direction, the forward
conduction point will be
> 2.1 volts. Is that high enough to avoid generating
intermodulation, except
> under circumstances where I wouldn't be able to hear
anything anyway?
I just was measuring something like that today with a 50 ohm
source and load and two tones near 5 MHz.
1N916's TOI -12dB and started to show IM at 0dBm each tone
(about 1/4 volt !)
1N4005 TOI -14.6dB and started to show IM at -3dBm
UF4004 TOI -15dB and started to show IM at -3.5dBm
1N5408 TOI -18.8dB and started to show IM at -5dBm
Stacking the diode pairs increased things about 6dB, as
expected.
I
> figure that much voltage at a 50-ohm receiver input
terminal to be
> something like S9+80dB.
As if S-meters mean anything. Next thing someone will claim
they are all 6dB an S unit. :-)
Actually IM starts at around 1/4 volt RMS, or about .3V
peak, with the BEST diode I tested (so far). Unless you're
running a very high impedance system or very high
frequencies, non-linear junction capacitance isn't an issue.
I'd back-bias the diodes. Remember the clamp voltage source
has to have VERY low impedance or the diodes won't limit
hard.
73 Tom
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