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Re: Preamp bypass circuit



>From: "Duffin, Jon" <jduffin@dbxpro.com>
>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?

Jon,

This is a common problem faced by hams doing eme.  Using an RF detect
circuit you will always have a finite switching time and result in a short
exposure of the preamp to Tx RF.  At 50w you may get lucky? if the Rf
circuit is quick.  At >1kW there is no luck!  

Instead I suggest you control both the preamp and FT-847 PTT with a
sequencer circuit.  This means that your mic PTT and cw key will need to be
routed thru the sequencer as well, or an accidental transmission could happen.

The sequencer disables the preamp, switches coax relays, and enables your
xmtr in that order with small finite time delays to prevent one operation
happening out of sequence.  The sequence repeats in reverse order when
returning to receive-mode.  I really think satellite operators should
consider using a sequencer for controlling their equipment.

There is a lot of literature about them in the moonbounce community.
Easiest is probably to purchase one from Downeast Microwave:
http://www.downeastmicrowave.com
check under accessories.

To be absolutely safe you should separate the Rx antenna input in the
FT-847.  But I have been successful using a sequencer on my unmodified FT-847.

Ed


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