[NLRS] 857D

[Donn - WA2VOI/0]

See below....


----- Original Message ----- 
From: <tosca005 at umn.edu>
To: <Melvynkc0p at aol.com>
Cc: <NLRS at mailman.qth.net>; <cvvhf at cvvhf.org>; <wa2voi at mninter.net>
Sent: Tuesday, September 01, 2009 2:00 PM
Subject: Re: [NLRS] 857D

On Aug 31 2009, Melvynkc0p at aol.com wrote:

SNIP
>
>I could use one circuit of the stage 4 relay to control
>the ALC being fed back to the IF  Transceiver;
>Common to Transceiver ALC input
>N/C    Tied to a -4 volt source
>N/O    Tied to a -2.5 volt source

Mel:

My concern has to do with the ALC contol relay. In stages 0, 1, 2, 3, the
relay is in the NC position, and feeds -4v to the ALC of the radio to force
the power output level to zero watts (or close to it). Only when the
sequencer gets to stage 4, does the relay switch over to the NO position,
where it feeds -2.5v to the ALC, to allow 2 watts to reach the transverter
TX-IF input. I assume that the relay is an ordinary "break-before-make"
type, so there is an instant at the start of the transition between stage 3
and stage 4, where the relay is in neither the NC or NO position ("break
the NC connection before make the NO connection). I assume that for that
instant, there is NO negative voltage on the ALC input, and the radio can
put out full power.

SNIP

If there was a way to configure the ALC so that an open relay (i.e., no
connection at all, not the NC position of the relay) generates -4v of ALC,
and when the relay closes, the voltage rises to -2.5v on the ALC line, then
you would be better protected, because the maximum negative ALC voltage
would be present until the relay finishes switching into position.


******
One way is to use the relay to SHORT OUT a section of a voltage divider whioch 
provides the -bias thusly:

         GND
             |
             |
            R2
             |
             +---------+---------------ALC voltage
              |            |
             R1         |  NC  NO
              |            +-->|   <--- no connection
              |                   |
             +--------------+
              |                  C
              |
           -V
         (-4.0)

With the relay in the normal state, -V is applied to the ALC; when the relay is 
activated, the ALC sees the "correct" bias for the desired power level.  In this 
case, R1 and R2 are chosen so that -2.5 is available when the relay is OPEN. 
The current thru the divider should be >than the current required for the ALC 
bias (at least 10X greater).  Usually, this isn't a problem.

73 Donn
WA2VOI/0