One final problem in the Transmit / Receive switching consumed a good solid 2 days of
investigation, and was both highly educational, interesting and satisfying to solve.
This problem is shown here in a photograph of the oscilloscope screen, which shows a single
Morse code “dit” at 24 words per minute (duration 50 milliseconds). The lower (yellow) trace
shows the PTT signal from the QCX – 0V during Receive, and +5V during transmit (plus a bit of
RF interference that we can ignore). Note that the PTT is released a short delay after key-up
(something like 10 milliseconds), this allows the RF envelope shaping to decay the RF amplitude
to zero gradually, avoiding key-clicks.
With a modern digital storage oscilloscope we can record a complete slow sweep such as this with
a high sample depth (I used 40 million at 12-bit resolution). I can then freeze the display with a
Single trace storage, and use the timebase adjustment and horizontal shift controls to “Zoom in”
on the offending spike. This
amazing feat of modern
technology lets me take a
very close look at those
offending spikes, and better
understand their origin.
Here’s the zoom (right) with
horizontal scale 2us/division.
These horrible spikes occur
on the Transmit/Receive
transition in both directions,
Receive → Transmit as well
as Transmit → Receive.
The spurious burst of
oscillation is huge, around
200V! It is not at the operating
frequency. It would not be
50W QCX PA kit assembly
1.00q
62
