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003
Very Wideband PC Radio
DTR
B. Kainka
*
zie tekst
see text
1
*
voir texte
(+10V)
PC radios are certainly nothing unusual. However, unless
you are prepared to spend a lot of cash you can’t buy a
wideband PC radio that receives short-wave signals — if
you want one that will not break the bank, you will have to
build it yourself. There’s no need for a battery or power
supply, since power can be drawn directly from the PC ser-
ial interface. The audio signal is fed into the PC sound card.
The circuit diagram in
Figure 1
shows this simple audion
receiver. The transistor in the common-emitter circuit
demodulates AM signals, thanks to its exponential charac-
teristic curve. Since the base–emitter junction is already
biased, RF potentials of a few millivolts are sufficient to
achieve demodulation. For this reason, the audion circuit is
significantly more sensitive than a simple diode detector.
*
siehe Text
R2
*
A1
R1
LINE IN
T1
C1
*
L1
47µ
25V
BC548
GND
014070 - 11
So where is the tuning capacitor? It’s not needed, since the
receiver has an extremely wide bandwidth and (simulta-
7-8/2001
Elektor Electronics
21
SUMMER CIRCUITS
COLLECTION
neously!) receives all strong signals ranging from the 49-m
band to the 19-m band. The coil is wound in two layers with
15 turns on a pencil. This yields an inductance of around
2 µH. The resonant circuit capacitance of around 100 pF is
composed of the base capacitance of the transistor and the
aerial capacitance. This places the resonant frequency at
around 11 MHz. The low input impedance of the transistor
damps the resonant circuit to the point that its Q factor is 1,
so the bandwidth is also around 11 MHz. The receiver thus
picks up everything between 6 MHz and 17 MHz. This
complete elimination of the usual selection leads to sur-
prising results.
Less is more.
For communications technicians, this means:
less selectivity = more bandwidth = more information.
Indeed, here you dive into a sea of waves and tones. The
special propagation conditions for short-wave signals
cause first one signal and then another signal to predomi-
nate. You hear messages in several languages at the same
time, music ranging from classical to pop and folk songs
from distant countries. Without the bother of the usual dial
spinning, you can roam at your leisure through the entire
short-wave region.
The supply voltage for the radio must be first switched on
by using a program (HyperTerminal is adequate) to switch
the DTR lead of the serial interface from –10 V to +10 V. If
you want to avoid this trouble, you can use a PNP transistor.
The alternative circuit diagram shown in
Figure 2
shows
DTR
2
*
zie tekst
see text
*
voir texte
(–10V)
*
siehe Text
R2
*
A2
R1
C3
LINE IN
T1
100n
C1
*
L1
C2
47µ
25V
BC558
10n
GND
014070 - 12
some additional improvements. The coupling capacitor pre-
vents the dc component from reaching the input of the
sound card, and residual HF components are shorted out
by the parallel capacitor. With these modifications, the radio
is also quite suitable for direct connection to a stereo sys-
tem, final amplifier or active speaker. In such cases, you can
do without the PC and use a battery (1.5 to 12 V) instead.
A downpipe from the eavestrough can be used as an aer-
ial if it is insulated at its lower end (where it connects to the
sewer system) by a rubber ring or concrete. If you are not
so fortunate as to have access to such an arrangement, you
will have to rig a wire aerial (at least 5 m long).
(014070-1)
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