10T639pp “Europa30”

featuring European transistors and class-C push-pull final stage

Europa30 and DE1103 in the field

23 May, 2006
Revised on 19 June, 2006


1. Introduction

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.

2. The circuit and her implementation

Europa30 schematic
Fig.1 10T639pp “Europa30” schematic (click figure to enlarge).

2.1 VFO section

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].

2.2 Driver section

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.

2.3 PA section

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.

2.4 Xtal converter section

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.

2.5 Supply and T/R control section

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.

2.6 Implementation

The entire circuit is put on a 7295[mm] universal breadboard, which is placed in a 15010040[mm] alminum enclosure including an AA8 battery box. See Fig.2 for detail.

Fig.2  Implementation of Europa30 (click picture to enlarge)
circuit board
(a) The circuit board
(b) The enclosure

3. Measurement results

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.

Fig.3  Measurement results for Europa30. Time scale on oscillographs is 0.02s/div.
VFO output
driver collector
PP output
(a) VFO output (0.2V/div)
(b) Driver collector (2V/div)
(c) Push-pull output (5V/div)
LPF output
output characteristics

(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.

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