Cosy MUTO, JH5ESM
23 May, 2006
Revised on 19 June, 2006
In 2005, I had organized a group purchase for European
transistors.
In this program, BC547, BC639, BD135, BD139 and TBA820M had been listed
and about 20 Japanese hams had joined.
In this article, a 10MHz CW QRP transmitter capable of 1[W] output
using European transistors is described.
The codename means “10MHz
TX,
BC639
push-pull
for the
final stage.” All the transistors came from Europe and thus
the
nickname “Europa” has been selected. She is now in
use.
Fig.1
10T639pp “Europa30”
schematic (click figure to enlarge).
The VFO consists of a varactor controlled “Super
VXO” and a class-A
amplifier followed by a 10MHz LPF (w/ 20[MHz] trap). In my case,
frequency coverage is 10.115 to 10.137[MHz].
The driver section is also a class-A amplifier, but with NFB
(negative feedback), stabilized biasing and collector keying.
The gain or drive level to the final stage is
strongly depends on
biasing. According to my experimental results, 1[W] output at nominal
supply
voltage (12[V]) without stabilizing driver bias decreases half at 11[V]
supply.
The rig is planned for field usage, the stabilized biasing at this
stage is adopted to overcome this drawback. The output characteristics
to the supply voltage will
be shown later in this article.
The keying is made at the collector supply line.
There are time
constant components (33[Ω] and 33[μF]) to suppress key
clicks.
The final stage consists of a class-C push-pull amplifier
followed
by cascading 3rd order LPFs with 20[MHz] trap.
BC639 (plastic TO-92 package) has enough capability for 1[W] output
without extra radiator.
It is expected that the even order harmonic made here
can be
canceled out. You will see a good output waveform later in this
article.
The considered pair receiver is DEGEN
DE1103
with external audio BPF for CW reception. It is quite an SWL receiver and has no remote capability so that we
have to make a tiny trick to protect our ears and its IF section
against large TX signal.
An SA612AN mixer converts 10[MHz] signals into 24[MHz]. The output frequency has been selected by my Xtal stock and 50[W] transmission test. Although the front-end of DE1103 is exposed to a large field of 50[W] signal, there is a very few amount of audible noise at 24[MHz] during transmission.
Supply for the VFO, the driver bias and Xtal converters are
stabilized by a 3-terminal regulator. Because I have a lot of low-drop
5[V] regulators in stock (my great thanks to Mr.Akishino, JA7HNV), I
picked up one for this TX to make an
approximately 9[V] internal supply by connecting 2 LEDs in series
between the regulator's GND terminal and the circuit common. The
low-drop regulators are very easy to oscillate, a large output
capacitor
should be connected between regulator's output and GND terminals as
shown in the schematic.
T/R control is made by the semi break-in relay
circuit. The delay
time is adjusted by 50[kΩ] potentiometer.
The RX antenna line is separated by 1SS53 diode
switch when
transmitting. 1N4007s may be considerable alternative, however the bias
resistor value may be selected by yourself.
The entire circuit is put on a 72×95[mm] universal
breadboard,
which is placed in a 150×100×40[mm] alminum
enclosure
including an AA×8 battery box. See Fig.2 for detail.
 |  |
| (a)
The
circuit board | (b) The
enclosure |
Some waveforms: VFO filter output, driver collector swing, PA
output
and TX LPF output are monitored and the output characteristics on
supply voltage are measured as shown in Fig.3.
 |  |  |
| (a)
VFO
output (0.2V/div) | (b) Driver
collector (2V/div) | (c)
Push-pull output (5V/div) |
 |  | |
| (d)
LPF
output (5V/div) | (e) Output
characteristics |
No visible distortions are
obserbed on her
output signal.
1.0[W] output power and more than 50[%] collector efficiency are
achieved. Even if supply voltage drops to 9[V], 0.6[W] signal can be
emitted.
