Solid State Shortwave Receivers For Beginners - The Listeners Guide

Apart from increasing the sensitivity of a receiving set up, adding
preselector also increases the selectivity when it is used with simple
designs such as those presented in this book. This is due to the addition
of an extra tuned circuit, and this factor is just as useful as the increased
gain. To obtain the best possible increase in selectivity a screened (coax)
lead should be used between the preselector and the receiver. Otherwise
pick up in the connecting cable will reduce the selectivity.
It is adviseable to use a fairly short cable to connect the preselector
to the receiver, and this should preferably be no more than about
300 mm long. This also applies to the untuned R.F. amplifiers. Using a
longer lead will result in some loss of performance due to losses in the
connecting cable.
Morse Practice Osciliator
The circuit diagram of a simple morse practice oscillator is shown
in Fig.35. This uses an NE555 timer I.C. in the astable mode. VR1
varies the frequency of oscillation and this is adjusted to give the
required audio pitch. A frequency range of a few hundred HZ to a few
kHZ is available. VR2 is the volume control, and the output can be used
to feed either a high impedance speaker (about 50 to 80 ohms) or any
type of headphones or earpiece. The morse key is simply connected in
the positive supply lead, and turns the unit on when it is depressed. No
other on/off switch is required, as no current is drawn with the key in
the up position.
86