Low Pass Filter; Key-Shaping Circuit - QRP Labs QCX-mini CW Mode D'emploi

4.11 Low Pass Filter

The transmitter output is rich in harmonics and must be followed by a good Low Pass Filter,
to attenuate the harmonics and satisfy regulatory compliance.
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
QCX-mini assembly Rev 1.05_Fr
107