> Motorola claimed about 7 or 8 microseconds for their 1N4007. Now, I do
> not know if everyone else's 1N4007s are the same, and as generally, it
> doesn't matter for the 'correct' application, it may well be an
> uncontrolled variable parameter.
The 1N4007 has mediocre isolation characteristics at H.F. when functioning
as a PIN switch. Ten Tec used several 1N4007 diodes in its Omni Six T/R
circuit, but the result was that main-to-aux antenna port isolation was only
40-45 dB. Changing nothing but the 1N4007 diodes to Microsemi UM-2110
diodes yielded port-to-port isolation of better than 80 dB. The difference
is significant when you are trying to use a low-noise Rx antenna on the aux
port and a high gain antenna on the main port.
After the upgrade, a +40 dB/S9 SWBC signal is not even heard when engaging
the aux RX antenna (port terminated into a 50-ohm load). That same signal
with the 1N4007 diodes produced S9 signals. Optimizing junction bias on the
1N4007 diodes offered no improvement.
My understanding is that Ten Tec originally used PINs that offered good
isolation in their first production run (perhaps MPN3700), but due to the
high failure rate in that part of the circuit, they elected to use 1N4007
types which appear to be more robust for T/R purposes. The failures
stopped, but it came with a severe compromise in port isolation. The
Microsemi UM-2110 diodes are perfect for high-power T/R switching and are
optimized for H.F. The only drawback is their high per-unit cost.
Paul, W9AC
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