The Idea & Design
The circuit is quite simple and straight forward. I don’t’ claim that this is my design. The basic idea was from an article by W6IBC which employed two numbers of SA 612 and had very few coils & components. The circuit has been modified to ensure the availability of all components from the local market, simplicity of VFO design, deletion and addition of RF amplifiers to have better noise level, to provide digital read out, higher audio output and to have a RF amplifier train to deliver an out put of 1 watt from the exciter. I would like to thank OM Vaidyanathan (VU2PRV) who kindled the interest to me in taking up this project and helping in development of the circuit design.
The Circuit Description
The circuit employs two numbers of NE612 / SA612 and a crystal filter between the two ICs with an IF amplifier stage. Both the ICs have dual function in the Transmit & Receive mode. The IC1 doubles as receive and transmit mixer and IC2 acts as product detector and balanced modulator.
As shown in the circuit the IC U2, a mixer operates between audio and IF as a product detector in receive mode and as a balanced modulator in transit mode. This also acts as a BFO/Carrier oscillator with a crystal and a trimmer to adjust the BFO/ Carrier oscillator frequency.
The signal received from the antenna after the band pass filter is mixed with the VFO signal. The out put is amplified further by a single stage (2N3904) before the filter. A Cohn type 10 MHZ filter using 6 numbers of 10 MHZ crystals is used for good skirt selectivity. The filter circuit component details and values are given in a separate circuit.
As the IC U1 NE 612 requires very low VFO injection, a single stage BFW10 FET is used in the calpits mode. The VFO output is taken through a 6.8pf capacitor. It is preferable to have all the capacitors in the VFO section of styroflex type to minimize drift. An additional stage with BC549 transistor is employed to facilitate connecting to a digital read out through a 100pf capacitor from the emitter of the transistor. This eliminates the noise generated by pic IC of the digital read out. A separate small pcb is used for the VFO circuit so that it will be easy for mounting near the 330pf tuning gang. A stabilised DC voltage of 5 volts is powering the VFO derived from 78L05. The VFO coil comprises of 40 turns ( 36SWG) on a small IFT former.
The bi-directional signal flow through the crystal filter not only simplifies the circuit but also minimizes the number of components in this stage. A 74 HC4053 IC is used for change over of the bi-directional signal flow between transmit and receive modes. Even though the entire circuit operates on 12 volts a 7805 is used to power 5 volts to all the ICs except the audio amplifier IC LM380 which operates on 12 volts. A condenser mike capsule is used with a single stage transistor BC549 as preamplifier and the output is fed to the U2 NE612 IC. The audio out put from pin 5 of U2 NE612 IC is further amplified by a single stage transistor 2N3904 and fed to the LM380 audio amplifier IC. The speaker can be 4-8 ohms impedance.
The PTT switch energises when pressed a 12 volts relay RL1. Two sets of the RL1 relay contacts are used for change over of filter connections through 74 HC 4053 IC, and powering the audio stage and cutting the power to mike amp stage. When the relay RL1 is in the de- energized condition, it will be receive mode and when the relay RL1 is energized it will change over to transmit mode. The relay RL1 in the transmit mode also energises another relay RL2 which is used for Antenna change over. Both RL1 and RL2 are small PCB mounting type relays which are mounted and soldered in the exciter PCB itself.
The RF amplifier train consists of three stages using 2N2222A, 2N2219 and BD 139 Transistors. The out put from the BD139 transistor can be connected to a single stage or push pull amplifier as desired . The PCBs for the VFO & main exciter are available as a set and the push pull amplifier PCB set is available as an optional item. The power supply has to be decided depending upon the final power amplifier stage ie a single stage or push pull amplifier. How ever the exciter requires 12 Volts supply.
The coil details of T1 to T4, L2,T5 and T6 are given in the circuit. T1 to T4 are wound using the small IFT formers. The coil L1 at the output of 2N3904 is an inductor of 47micro Hendry. The 250 micro Hendry RFCs in the RF amplifier stage can be the readily available inductors of higher current rating or it can be prepared by winding 20 turns of 28 SWG wire gauge on a ½ inch torrid. The T5 consists of 30 turns of primary & 4 turns of secondary with 28 SWG enameled wire on a ½ inch dia torrid. The T6 is wound using a ½ inch TV Balun core consisting of 6 turns primary and 4 turns secondary winding with 28 SWG enameled wire.
It is to be noted that some of the 74HC4053 IC malfunction. Please use a IC base for all the ICS. So after completing the entire assembly and while testing the receiver the IC 74HC4053 can be removed from the base and jumpers can be used to the respective pins on the IC base like 4-3, 15-1 & 14-13.The specialty of this circuit is that there are no tuning of coils involved except the band pass filters,. The exciter with push pulls amplifier delivers easily 40 watts power.
The photographs of the rig constructed by me are given for reference. All the type of components used on the PCB , the front panel and push pull amplifier sections are clearly shown for easy understanding.
It is possible to use the same PCB for 3.5 Mhz or 14 Mhz by suitably modifying the BPF coils and adding suitable amplifier stages.