Full QSK with the OMNI-VII and a linear amplifier

I've been told that timing is everything. Certainly with QSK it is!

If you use QSK with an amplifier you need to know exactly what the amp's timing parameters are. I'm talking about the time it takes to switch into transmit mode and then back into receive mode. Some amp makers probably specify these two parameters; mine didn't. I run an ancient Amp Supply LK-500ZC that was built in 1987. It's a great amplifier and was nicely built with Peter Dahl transformers and other quality components including a vacuum relay for QSK operation. But, no timing specs to be found.

What's a guy to do? Break out the scope and figure it out!

I hooked up my RF analyzer to the input of the amp and put it in VFO mode. That provided me with a continuous sine wave signal at a very low level. I hooked the output of the amp into a dummy load. I then plugged an electronic keyer into the amp's relay input jack. Input #1 of the scope was hooked to the key line while input #2 of the scope was hooked to a wire loosely coupled to the dummy load.

I then proceeded to lock the keyer into sending dits at 30 WPM. YES! I was intentionally hot keying the amp! Of course, the input was a few milliwatts and the output didn't register on the power meter so the relays were well withing their hot-switching spec.

Anyway, here's what the scope told me:

Leading or key-down delay

(First, I must apologize for the poor quality of the scope images. I really need a scope camera.)

The top trace is the key-line showing one "dit" at 30 WPM. The bottom trace is the RF coming out of the amp. Since the input signal is continuous, there is no RF output until both the input and the output relays have changed to transmit mode. Note the steep (vertical) rise and fall times of the waveform.

Anyway, here you can see that the delay between the key-line going to ground and RF coming out of the amp is around 8 to 9 millliseconds.

The next image shows the trailing edge or key-up delay. From when the key-line goes high (key-up) to when the relays switch back to receive mode is approximately 5 milliseconds.

Trailing or key-up delay

So, the next thing to do was to make some measurements of the new rig. Now, the OMNI-VII has a keying loop but I thought I'd start with just the normal amp relay output to see what was what.

The next image shows three scope traces. The top trace is the CW key-line going into the OMNI-VII. The middle trace is the amp key-line coming out of the OMNI-VII and the bottom trace is the RF envelope coming out of the OMNI-VII. The two vertical cursors are measuring the duration of the RF output which is a single 'dit' at 30 WPM.

OMNI-VII Timing at 30 WPM

From the above you can plainly see the 15 millisecond delay between key down and the output of RF. This is documented clearly in the OMNI-VII manual. You will also note that the CW key-line and the amp key-line both go to ground simultaneously. And, finally, you will note that the amp key-line stays at ground well after the 'dit' has been sent. WOW! With my amp you could simply connect the amp key-line from the rig to the "relay' input on the amp and have full QSK and be done with it. Pretty cool..

But, wait! There's more!

Since I had everything configured to make these measurements I thought I'd have a look at what the "EXT T/R DELY" parameter in the OMNI-VII was all about. What I found both surprised and pleased me enormously.

The EXT T/R DELY parameter is documented as providing "Hang Time" for a non-QSK linear. Further, the documentation specifies that a setting of 0% provides approximately 15 millisecond delay while a setting of 100% provides about 1 second delay.

With a setting of 0% I was measuring 8 milliseconds as shown in the image below:

OMNI-VII T/R delay at 0% (default)

Hmm.. Next, I tried a setting of 1%. Oddly, at that setting I was seeing the amp key-line go high approximately 4 milliseconds before the end of the RF waveform. (Apparently, I was so surprised that I forgot to take a picture at that setting.) Now, with my amp's 5 millisecond drop-out timing I could probably get away with that but why tempt fate? So, I tried a setting of 2% where I was measuring approximately 2.5 milliseconds of "hang time" after the termination of the RF envelope. Here is a picture of that setting:

OMNI-VII T/R delay at 2%

Perfect! Full QSK by simply connecting the rig's amp key-line to the relay input on my amp. Improving on it only required setting a parameter in the OMNI-VII's menu. It doesn't get much better than that..

Timing may be everything; but measuring it is absolutely vital..