Friday, 26 May 2017

XENA- A Portable DSB/CW Transceiver

In my post of 24 th March, 2016 I shared a simple direct conversion receiver project built around a home-brewed mock I.C. This project grows out from that design. 'XENA' is a simple, portable DSB/CW transceiver for backpack use. As mentioned above this cute little project evolved around a single home made mock I.C. (made around components shown in a dotted square). It functions both as a transmitter and receiver mixer. For simplicity the design uses a ceramic resonator/crystal based dual band, super VXO for 80 and 40 meters (more about this later). The receiver is developed around ubiquitous parts generally available in one's collection and the complete project is developed in modular form with all modules made using ugly construction technique. The modular construction allows scope for future experimentation and further development of the project. The schematic of exciter cum receive module is given below:

Even to a crass view the circuit of exciter cum receiver module is too simple and hardly needs an explanation. I have added a multi-turn preset in the biasing arrangement of the mock mixer I.C. to aid precise mixer balance in order to achieve minimum carrier leakage during transmission. All broad band RF transformers are wound on pig-nose balun cores using 30 SWG enamelled copper wire.
The RF amplifier circuit is quite self explanatory. The schematic diagram of RF final is given below:
The three stage amplifier ensures almost seven watts of RF output. I used 2N4427 for the driver as it was available in my collection but many other suitable candidates like 2N3866, 2N5109 and 2SC1175 seem to work as well. The receiver band peak capacitor C21 is a 330pF type variable tuning capacitor. It eliminates the need of mechanical band switching arrangement in receiver front end using switches or relays etc. An attenuator ahead of it has been included for dire reception phases. It can be switched in during presence of receiver overloading.

The entire project is developed on 4"X6" PC clad pieces using ugly construction and each board is fixed to the sides of the enclosure body. The control circuit being in the middle. The control circuit provides the required QSK delay during CW operation, does all switching and provides side-tone during CW transmissions. I used a small piezoelectric buzzer, the type used in computer motherboards, UPS's and microwave ovens etc. to provide CW side tone, since the idea was simple to implement. The schematic for control board is given below: