This short-wave converter, which doesn’t have a single coil requiring
alignment, is intended to enable simple medium-wave receivers to be used
to listen to short-wave signals. The converter transforms the 49-m
short-wave band to the medium-wave frequency of 1.6 MHz. At the upper
end of the medium-wave band, select an unoccupied frequency that you
want to use for listening to the converted short-wave signals. Good
reception performance can be obtained using a wire antenna with a length
of one to two metres.
Short-Wave Converter Circuit Diagram
The converter contains a free-running oscillator with a frequency of
around 4.4 MHz, which is tuned using two LEDs (which act as
variable-capacitance diodes!) and a normal potentiometer. The frequency
range is set by adjusting the emitter current using a 1k trimpot. The
oscillator frequency depends strongly on the operating point. This is
due to the combination of using an audio transistor and the extremely
low supply voltage. Under these conditions, the transistor capacitances
are relatively large and strongly dependent on the operating point. The
second transistor forms the mixer stage.
If you calculate the resonant frequencies of the tuned circuits, you will obtain 6.7 MHz for the antenna circuit and 1.7 MHz for the output circuit. Additional transistor capacitance and the effects of the coupling capacitors shift each of the resonant frequencies downward. The tuned circuits are relatively heavily damped to obtain bandwidths that are large enough to allow the circuit to be used without any specific alignment. The results are good despite the low collector–emitter voltage of around only 0.6 V, due to the fact that only a modest amount of mixer gain is necessary. The entire circuit also draws less than 1mA.
If you calculate the resonant frequencies of the tuned circuits, you will obtain 6.7 MHz for the antenna circuit and 1.7 MHz for the output circuit. Additional transistor capacitance and the effects of the coupling capacitors shift each of the resonant frequencies downward. The tuned circuits are relatively heavily damped to obtain bandwidths that are large enough to allow the circuit to be used without any specific alignment. The results are good despite the low collector–emitter voltage of around only 0.6 V, due to the fact that only a modest amount of mixer gain is necessary. The entire circuit also draws less than 1mA.
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