heard by the other station. The frequency is not stable or exact, but appears, based on the
timebase, to be something around 1MHz-2MHz, then a bit under 1MHz as it dies away. The whole
thing is all over in 20-25us. Though the other station would not hear anything on the operating
frequency, nevertheless the signal is a large spurious signal which could cause interference to
someone somewhere. And once we know about such a horrible thing, we can’t sleep peacefully at
night leaving it like that. And Pat Hawker G3VA (RIP) would surely be have been horrified had he
heard of QRP Labs doing such a thing.
The cause is easy to explain now, but was very hard to find, at the time. I tried adding things here
and there, and I could change the shape of the spike. Or move it around. Remove it at the end, or
the start of the transmission, but not both. But not easily eliminate it. A clue was that this spike is
nothing to do with the actual RF transmission – if it was, it would be possible to correlate the spike
to the start and end times of the RF envelope; but in fact the spike commences at the moment of
Transmit/Receive transition (PTT signal state change).
The actual cause, is positive feedback around the amplifier! I’m sure you’ve been at some public
event, where a speaker is holding a microphone. Someone has something adjusted wrongly, and
you get a huge loud squeal in the Public Address (PA, not *this* PA)… the squeal is again nothing
to do with the voice of the speaker. It is positive feedback – a loud enough sound makes it back
from the loudspeakers, through the losses of the air, and into the microphone – to be amplified
and go around the same loop fast enough to create a spurious signal, the “squeal”. Here the same
thing happens, but at RF.
The cause is that these PIN diode switches we love so much, are not perfect, instantaneous
switches. They don’t switch on or off in zero time. They don’t go from near zero resistance to near
infinite resistance immediately in zero time duration. In reality they go through some part-way state
for a fleeting moment before they settle in their new state. For a tiny moment, the three conceptual
switches in the project, “SW1”, “SW2” and “SW3” are all “partly-on” at the same time! This allows
RF energy to flow in a loop, back from the output of the power amplifier, to its input – getting
bigger each time around the loop, quickly building up into this huge spike of energy. Then as one
branch of the switch(s) become properly “Off” the oscillation dies out and normal service resumes.
The solution to this problem, is that we need some delays. Some sequencing. I had to make sure
that all of the switches are OFF for a short while, before the other switch(s) are turned ON. It is the
electronic equivalent of what in a more expensive mechanical switch, a rotary switch perhaps,
would be termed “break-before-make” - it ensures that the switch wiper is never connected to two
terminals at the same time. There is a “break” before it contacts the next output terminal.
In the circuit, this was implemented simply by the addition of two capacitors C5 (22nF) and C13
(1uF), and resistor R15 (previously the PTT control input was connected directly to the Q6 gate).
Consider the two circuit fragments containing Q6 and Q5:
50W QCX PA kit assembly
1.00q
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