18T135pp “AinSebaa17”

PA stage photograph

31 May, 2007


1. Introduction

An 18.1MHz spot frequency CW transmitter of 1[W] output power is described. It features class-C push-pull power amplifier followed by two-stage LPF with 3rd order harmonic trap.

Ain Sebaa is one of the ST Microelectronics manufacturing site in Casablanca, Morocco, where BD135 used in this TX might be fabricated.

2. The circuit

Fig.1  AinSebaa17 schematic.  Click here to download detailed PDF version.

The schematic is shown in Fig.1.
It consists of three major parts: the main transmitter section, keying/break-in section and RX crystal converter section.

Q1 and Q2 are direct coupled crystal oscillator and buffer amplifier. Q3 provides driver amplifier where keying is made at this stage. Q4 and Q5 is class-C push-pull power amplifier. The configuration is very similar to Europa30 transmitter except the oscillator.
The oscillator and buffer power supply and driver's bias is regulated. Drive level to the PA stage is adjusted by emitter resistors at Q2 and Q3.
The output LPF has a transmission zero at 3rd order harmonic. This makes high quality (or in other word, “clean”) radio wave.

U1 is a simple crystal converter which converts 18MHz signal into 8MHz. RF signal can be attenuated by 10[dB] to avoid cross- or inter-modulations due to large broadcasting signals in 17MHz band and others. By considering impedance matching, 8:1 winding ratio for T5 is determined.

Keying and semi break-in section are made of two PNP transistors (Q6 and Q7).  Break-in delay is adjusted by 50kΩ pot.

The main circuit is implemented on 72×95[mm] universal breadboard and put into a 130×90×30[mm] aluminum enclosure including 9[V] battery, as shown in Fig.2.
Only power and calibration switches and a power indicator (LED) are located on the front panel. An external DC jack, a key jack, a BNC antenna connector, an attenuator slide switch and RX output jack are on the rear panel.
The attenuator is implemented on a DPDT slide switch.

Fig.2 AinSebaa17 implementation (click picture to enlarge).
top viewrear viewattenuator
(a) Top view.(b) Rear view.(c) The attenuator.

3. Measurement results

The output power is 1.13[W] under 12[V] power supply condition. PA collector current is 160[mA] and thus the collector efficiency is 59[%].
0.5[W] output power is obtained under 9[V] supply condition.

The output waveform and spectrum are shown in Fig.3. Each harmonic is less than -50[dBc], which meets Japanese regulation for 50[W] (and above) transmitter :-)
If you see a distortion in oscillograph, it is no good :-(

Fig.3 AinSebaa17 output signal (click picture to enlarge).
output waveformoutput spectrum
(a) Output waveform.(b) Output spectrum.

back to home