[NLRS] 857D

[tosca005 at umn.edu]

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

>
>
>Hello Joe, I am using a Kenwood 9130 2M all mode...
>
>State 4  Transciever ALC forced to -2.5v=SSB, Operate,
>         cuts 5 watt power out down to about 2 watt.
>State 3  DEM Transverter to TX by PTT low
>State 2  DEM 3W amp On by Ctl Low
>State 1  Pick Microwave T/R relay to put in Xmt  mode
>State 0  Transciever ALC forced to  -4v=Cutoff
>         no power out  allowed.
>         Transciever in Receive  mode
>         Transverter in Receive mode
>
>Transition from State 0 to State 1 is initiated by
>Microphone PTT or Manual CW hold switch. Transitions
>to States 2, 3, and 4 are controled by the sequencer
>time constant. When the Microphone PTT is released,
>the sequence is reversed all the way back to State 0
>and I am again in receive mode.
>
>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.

You apparently have the radio configured so that "full power" is already 
limited to 5 watts instead of the 25 watt maximum of which it is capable. I 
believe that there is no corresponding switch or menu setting on the 857D 
that tells it to limit maximum power output to 5 watts. (I could easily be 
wrong about that, however.)

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.

~~~~~~

I am currently using an ICOM IC-706MkIIg as my IF radio for 10 GHz, though 
I plan to switch over to a Yaesu FT-817 soon. In my system, I am not using 
the ALC input at all, nor am I even using a sequencer. This is living 
dangerously I guess, but so far, I haven't toasted the 10 GHz transverter. 
This IF radio puts out a maximum of about 50 watts on 2M. I used a menu 
option to cut the power to as low as I can go, which I think is about 2.5 
to 5 watts. The transverter is configured in the 2-10 watt input mode. This 
radio is alleged to put out a full-power spike (in this case, about 50 
watts) at the start of transmit until the internal limiting kicks in and 
lowers the power as set (in this case, to about 2.5 or 5 watts).

In my experience, the DEMI transverters are pretty robust. I use a similar 
radio, the IC-706MkII, with a DEMI 222 MHz transverter. This radio puts out 
a maximum of 100 W on the 10M HF band, and the transverter is configured 
for an input of only 10mW! I am using the DEMI TIB interface in this case, 
which uses the ALC line to limit the output power of the radio. In this 
case, the negative ALC voltage is output "all the time", so the radio is 
limited to 10mW at "all times". Early in my assembly and testing, I 
accidentally disconnected the ALC input to the radio from the TIB, and the 
radio pumped 100 watts continuously into the transverter for several 
seconds, until some of the "magic smoke" escaped from the transverter. The 
good news was that the only component that I blew up was a 50 ohm resistor 
across the TXIF input line of the transverter. I replaced it with a new 50 
ohm resistor and was back in business. But it has been alleged that even 
this radio will pump out a 100W spike and then settle down to the 
ALC-configured power output level, so I guess that I am still living 
dangerously on the 222 band. However, I have never encountered any direct 
evidence that this is happening.

Obviously, not everyone is so lucky. The power spike at the start of 
transmit is a serious concern.

To test for this, one could take the output of the radio, feed it into a T 
connector, and have one side of the T go to a 50 ohm dummy load. The other 
end of the T would go to an oscilloscope input. Set the scope for 100 volts 
full scale to start, and transmit into the dummy load. If no ALC is 
presented to the radio, or if the ALC does not work, then one should see 
about 70v for a 100W radio, or about 50v for a 50W radio, or about 35v for 
a 25W radio. If the ALC is properly limiting the output power to 2.5 watts, 
the transmitted signal should be about 11v. If ALC limits the power to 2 
watts, the transmitted signal should be about 10v. P=V^2/R, R=50 ohms, 
V=sqrt(P * R), V=sqrt(P * 50). If the spike is present, and you time your 
scope trace properly (perhaps use the stage 4 output to trigger the scope, 
or else hit the mic PTT just as the trace starts over at the left edge of 
the CRT in free-running mode), the voltage will surge to a high voltage 
indicative of full power output, then quickly settle down to the 
steady-state power output as configured by the ALC. I guess that I have all 
the equipment needed to do this test, but just have never gotten that 
elusive ROUND TUIT. Since I never GOT A ROUND TUIT, I have not observed 
this behavior first-hand.

Another possibility to consider: the FT-857 family (and the FT-817 family) 
has a TX INHIBIT line available. When pulled "high", it inhibits the 
transmitter. When allowed to drop low, the transmitter transmits. Perhaps 
the approach to take is to set the ALC to limit the radio to the desired 
output power level continuously, and set the TX INHIBIT line to +V (value 
of "high" is not specified in my FT-817 manual) through the NC connection 
of the stage 4 relay. When the sequencer hits stage 4, the relay closes, 
and TX INHIBIT goes to ground, telling the radio that it is OK to transmit. 
Because the ALC will have been driven low the whole time, the hope is that 
the spike will not occur when the radio goes into transmit mode in stage 0, 
because the TX INHIBIT prevents ANY output from being generated until stage 
4 is reached. This is how I plan to interface my FT-817 into my 10 GHz 
system, once the need for a sequencer exceeds my "missing ROUND TUIT 
threshold" when I add the 8 watt amplifier to it.

I hope that helps. If someone runs the oscilloscope test on an FT-857D (or 
any radio for that matter), I'd love to see the results.

73 de WØJT