CW (Morse Code) Reception in Radios

 Even though we have several modes in amateur radio now including the various digital modes and phone modes, still there are a good number of fans for CW or continuous wave operations. If you look at some online forums, there are even youngsters trying to learn CW, even though it is not mandatory to learn for an amateur radio license in many countries. Why is this charm for CW? As CW requires only lower bandwidth than speech, signal to noise ratio is better in the receiver. Human ear can resolve CW or Morse Code signals which are only marginally above the noise level whereas SSB or speech signals at the same signal level cannot be copied well. They say that CW has an advantage of about 20 dB over SSB signals. Narrow band pass filters can improve CW reception. Please note that when such a filter is being used, simultaneous SSB reception will be difficult.

CW transmitters are easier to make for a beginner as you need only lesser components. We started by homebrewing low power CW transmitters and later added modulator for AM or amplitude modulation. SSB transmitters are much more difficult to homebrew, though there are many who homebrew SSB transmitter and even software defined radios. My CW transmitter with 120 W DC input could virtually work the whole world using a simple dipole antenna under good band conditions. In countries where portable operations like Parks on the Air and Summits on the Air are popular, many operate with even hand held CW devices with telescopic antennas on HF bands. 

When I started listening to amateur radio signals long back using a broadcast receiver, CW signals would not be heard well as it was the carrier being switched on and off during keying. To get the musical tone in the broadcast receiver while listening to CW signals, a beat frequency oscillator or BFO is needed. BFO works at a frequency slightly away from the intermediate frequency of the radio and produces a heterodyne signal which is audible when the Morse Code signals are being received. Alternatively, the variable frequency oscillator or VFO of the transmitter can also work similarly if it is slightly away from the received frequency to produce a beat note in the radio. But then you will end up transmitting slightly away from the received frequency. Modern communications radios will overcome this problem with receiver incremental tuning and clarifier, which can tune to off frequency stations without altering the transmit frequency.

In my FT-710 radio, when I choose the CW mode, automatically filter settings change for optimization of CW reception. Two options, CW-L and CW-U are available. I use them according to the bands, with CW-L in bands below 10 MHz and CW-U in other bands, just like the use of LSB or lower sideband and USB or upper sideband. Received audio filter settings can be accessed from the CW settings after pressing the FUNCTION knob. Selections can be made from the on screen menu. LCUT frequency is 250 Hz by default on CW mode, but can be switched off or changed up to 1000 Hz in steps of 50 Hz. HCUT frequency by is by default 1200 Hz in CW mode, but can be adjusted from 700 Hz to 4000 Hz or turned OFF. It was possible to hear SSB signals in CW mode when the HCUT frequency was increased to 4000 Hz, but the quality was not as good as while on SSB mode. When the HCUT is kept at the default 1200 Hz, SSB signals were hardly audible. Both these filters are OFF by default in AM mode, while they are 300 Hz and 3000 Hz respectively in SSB mode. Different default values are there for FM, DATA and RTTY modes.