40M DSB Transceiver

The circuit described here is actually a variation of the  MDT40 project by VK2DOB,  A simple double side band suppressed carrier (DSBSC) transceiver which operates on 7MHz. I find this project simple and a good start if you are interested into home brewing your own rig so I decided to make one.

In the original circuit, it uses a ceramic resonator as a common oscillator for both receive and transmit stage but I have difficulty in finding the parts here in Manila so I decided to use a vfo whose circuit was copied from KD7REM, a rocksteady vfo. Stability is considerable during start up which usually takes less than 30minutes of wam-up and can be used with a couple of QSO without correcting the vfo for a drift. T50-6 toroid was chosen as a core of my inductor where 25 turns of enamel wire was wound. RF feedback was taken at 5 turns from the cold end. I found by experiment that by keeping the capacitance large in the L/C ratio, the oscillator is more stable than the other way around. MPF102 is also used since JFET is more stable than its BJT counterpart.

The balanced modulator and the receiver pre-amplifier stage are constructed together in one board. There are four 1N4148 used in the balance modulator and please note that these diodes are carefully matched by their forward bias voltage. A 500 ohm potentiometer is used to find the null in the balance modulator. Carrier suppression is easily done when you already have completed the transmitter stage since this can be just adjusted by the power output of the rf amplifier stage.

The rf transistors found  in the transmitter stage are uncommon so I decided to redesign completely the entire stage using common transistors that are locally available here. 2SC2078 was chosen as the final stage since it is cheap and offers a 4W to 5W ouput at the HF frequencies.

Above is the prototype of my DSBSC transceiver that I made. It is housed in a plastic casing (black) and on the top (grey plastic casing) is the additional 15watts linear amplifier constructed using a push-pull IRF510 mosfet transistor.  ---73 de du1vss