Monday, 8 April 2019


"FIREO" is a simple QRP CW/SSB transceiver design developed around common  VHF/UHF subsystem IC MC3362. As the MC3362 chip already contains two Gilbert cell mixers that can be used for transmitter and receiver as well. The use of MC3362 can simplify design and aid compactness of the portable transceiver. But during development of the design the on chip mixer was found to be a bit prone to easy overloading during receieve, thus a discrete diode mixer is decided to be used for the receiver front-end. It resulted better performance as compared to the on chip Gilbert cell mixer. Both IF and Oscillator signal switching was done using BA582 PIN diodes in my prototype but assuming that such devices are not easily available to an average homebrewers, the switching in the final design is thus achieved through commonly available silicon switching diodes. A JFET IF amplifier built around J310 provides the some IF gain and AGC control to avoid unwanted ear-splitting. I don't like use of ubiquitous LM386N in serious receiver designs for its instability and noisy performance. I eschew its use, so a discrete AF amplifier aroung a low noise operational amplifier is designed. 

Studying its datasheet I found the MC3362 chip has an IF limiter followed by an on chip discriminator which can also be used as an SSB/DSB demodulator as well. Consequently I decided to make full use of available on chip resources and decided to use on chip limiter and discriminator for TX speech processing. The microphone signals are amplified to a reasonable level and are then modulated to a 455KHz signal in a singly balanced diode mixer wired around two switching diodes. The resultant DSB signal is fed to pin 7 of the MC3362, the IF limiter stage input. The DSB signal is processed by the limiter to achieve maximum average speech power for modulation, without any amount of added  distortion. This signal is demodulated through the discriminator and processed audio is fed  to the transmitter modulator.

The complete schematic of the exciter part of the transceiver is shown below:

The RF power amplifier circuit uses a linear pre-driver using a common BC548B. The driver stage uses two BC548B transistors wired some what as a blassted emmitter RF driver transistor. This stage provides enough gain to drive the final RF power amplifier using IRF510. RF PA protection circuit uses transistor Q16 and Q17 are used to shut down PA bias in case of excessive current drain. TX/RX antenna changeover function is accomplished by using common 1N4001 diodes. The schematic of RF power amplifier is shown below.

As mentioned above I am not happy with ubiquitious LM386N in serious applications. So I decided to use an AF amplifier built around discrete components. The major gain block is a low noise op-amp, TL071 which provides ample gain and drives complimentry pus pull stage. The amplifier produces around 1.5 W of clean audio which is more than enough. The schematic diagram of the AF amplifier is given below.

As for the requests, I am planning to arrange few kits of FIREO, those who need one, can drop me a line :