[Elecraft] KPA500 Inhibit Line

[Jack Brindle]

This actually should work. The #INHIBIT signal will disable transmission rather quickly, at least as fast as the key line.
The catch is that most tuners don’t detect high SWR and output their tune signal all that fast. In fact most use a relay,
adding 7-10 mSec of time to the overall time. You will probably end up with something like 15 to 20 mSec of
delay between key down and disabling of amplification. Cutting that down as much as possible is generally a good thing.

Now having said all this, this is exactly the problem that #INHIBIT is meant to solve. #INHIBIT is the fastest way to
disable amplification, causing the proper TR switching sequence along the way, exactly the same way the negating the
KEY line does. The general discussion above of reducing the ATU’s HI SWR / fault detection is important in that you
want to reduce the time the amplifier is subject to the bad conditions. We took great lengths to reduce this time using
the KEY line using the KAT500, unfortunately most ATU manufacturers don’t take quite as much care in this area.

So, the answer to your question is yes, you will get sever mSec of high power, something like 10 to 20 mSec.

Let us know how your experiments come out.

- Jack Brindle, W6FB
Elecraft Engineering


> On Mar 20, 2015, at 6:14 AM, j-meister at comcast.net wrote:
> 
> Setup is Yaesu FTDX3000 xcvr --> Elecraft KPA500 amp --> LDG AT1000ProII tuner. I am using a cable 
> between the FTDX3000 and KPA500 to transfer band data and the key line (KPA500 Aux Pin 10). I would 
> like to use the open-collector interface on the AT1000 (which is pulled low during tuning) to pull 
> the KPA500 Inhibit (Aux Pin 11) line low so that I am not running high power into the tuner during 
> tuning. (Note: I will also at the same time use the open-collector output to pull the correct line of the 
> FTDX3000 low to enter the low power CW key-down mode.) 
> 
> Any "gotchas" with this scheme? The KPA500 will be keyed up via Aux pin 10 at the same time it will 
> be inhibited via Aux pin 11. Is there a delay I have to be worried about? Is there an order of the 
> pull-downs that matters? Am I going to get a few msec bump of high power? 
> 
> I realize that one normally interrupts the amp via the key line, but it seems a little more straightforward 
> in my particular case to do it via the inhibit line. 
> 
> Jim, K2WU 
> 
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