QRP Labs QCX-mini CW, Руководство по сборке, Страница: 95 / 121

The standard, well-proven QRP Labs Low Pass Filter kit http://qrp-labs.com/lpfkit is used
here. To save space and cost, the components are installed directly on the PCB, not on a
plug-in board.
It is a 7-element filter design originally by Ed W3NQN then published for many years on the
G-QRP Club web site’s technical pages.
4.12 Key-shaping circuit
A hard-keyed CW transmitter generates clicks many hundreds of Hz away from the
transmitted signal that can annoy users of adjacent frequencies. This is purely a
consequence of the mathematics of the Fourier transform and is unavoidable. Any time you
switch a signal instantly on or off, you WILL splatter energy onto unwanted nearby
frequencies.
To combat this, any good CW
transmitter should include an RF
envelope shaping circuit to soften
the key-down and key-up transitions.
The ideal envelope shape is a raised
cosine, but this is difficult to
implement without significantly
increasing the complexity of the
circuit.
The simple key-shaping circuit used
here uses only a few components
but produces good results.
This circuit was derived from one published by Don Huff W6JL, see
https://www.qrz.com/db/W6JL/ though as he says, “ this integrator-type keying circuit is
found in many published homebrew designs over the past 40 years or so, so it is nothing
new
”. It uses a PNP transistor (Q6) and R-C integrator circuit. Don W6JL uses this key-
shaping circuit to drive a 600W Power Amplifier.
On key down the Q4 switch is “closed” by a high signal coming from the microcontroller. In
a really simple transmitter, Q4 could just be replaced by a straight Morse key to ground! But
in our case, the microcontroller implements automated stored message sending, beacon
modes, and Iambic keyer – so we need the microcontroller to be the boss of everything.
The microcontroller reads the state of the straight key or paddle, and processes it to
produce a key output. When in straight key mode the microcontroller transfers the signal
straight through from the key input, to the key output control line – but in other modes the
processor must generate the keying signal.
The component values set the rise and fall time. With the components shown, the rise and
fall time is about 5 milliseconds.
The following oscilloscope screenshots show a 40m band (7MHz) transmission, keyed with
a continuous series of CW dits at approximately 24 words per minute. The amplitude is
approximately 3.8W into a 50-ohm dummy load (with 12V power supply).
QCX-mini assembly Rev 1.05
95