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RE: Preamp bypass circuit



Jon,

You may well already have thought of this, but in case you haven't...

Could you use the exernal preamp 12V which goes down the coax from the
FT-847 when you switch the menu (forgot which menu it is) from internal to
external preamp?

This works the opposite of your RF sniffer. It puts out 12V when in RX,
nothing when in TX. Thus the 'normal' mode of operation is to pass through,
no preamp. In RX, the 12V down the coax switches on the relays and thus the
preamp.

It means you can switch it in and out from the FT-847's front panel too.

It would mean changing your RF sniffer circuit into a DC sniffer - basically
capacitively couple the antenna and use an inductor to get the DC. The DC's
only a few tens of mA from memory, so you'd need a buffer transistor to
switch your relays - which you probably already have in your RF sniffer.

73 Howard G6LVB



-----Original Message-----
From: owner-AMSAT-BB@AMSAT.Org [mailto:owner-AMSAT-BB@AMSAT.Org]On
Behalf Of Duffin, Jon
Sent: 14 February 2001 19:15
To: 'amsat-bb@amsat.org'
Subject: [amsat-bb] Preamp bypass circuit


I have recently built up a GasFET preamp for use on the 2 meter band on
AO-10.  To switch the preamp out during transmit, I have built up a RF
sensed circuit which bypasses the preamp through a relay.  The problem is
that I can observe a breif RF transient that would go into the preamp (I
actually have not hooked up the preamp to the switch circuitry yet) before
the relay switches into the bypass mode.  I am fearful that this RF
transient would be enough to blow out the GasFET.  Does anybody have any
hints or ideas how to supress this transient so I won't be replacing the
FET?

My rig is a FT847, and so I though another idea would be to switch the
preamp out of the transmit path by using the 847's PTT accessory jack.  But,
I don't think that the results would be any different unless there is a way
to delay the power output stage from coming up at the same time as the
switching of the PTT accessory jack signal.

How does everyone else get this to work on their homebrew preamps?

Thanks
Jon
KK7RZ

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