Wednesday, November 13, 2013

Direct Conversion Receiver

direct conversion receiver

80 meter oscillator & buffer

dc receiver front view

This is the working direct conversion receiver. I am using along with 80 meter xtal transmitter for test purpose. Initially it was made to test whether Diode Product Detector could be used or not. I was skeptical about diodes product detector but ultimately i beleived in these diodes.

It contains four stages.
1.   LO with buffer to drive the diodes. which requires 7dm power.
2.   Product detector of Four diodes, as shown in previous post.
3.   Active AF Filter, this is also shown in some post in this blog.
4.   Standard LM386 af amplifier.

All stages are operated from 12 Dc regulated voltage.It needs antenna (dipole)  and BPF at front end.

For 20 meter some sort of Rf pre-amp is a must. I had used the Cascoded RF amp with filter. LO was same but one stage of Transistor multiplier amplifier was used to drive the dbm. That was my best receiver to operate. I used it extensively  during the y2k as valves receiver required more power consumptions and this was just a small size receiver.

In the front panel, on left corner there is unmarked socket it gives the output directly from filter so that it can drive external AF amplifier.

Enjoy the simplicity.
Here is the circuit.

 The block diagram. This is the typical diagram. All the direct conversion receiver follows this stages.

Product detector. This is standard circuit. I was making the receiver and little skeptical of passive diodes. For the sack of go or nogo test connected the diode dbm and presto it worked first time. It lacked the audio so i put the resistor as shown and it became my favourite. It is given in the QRP Notebook also.

How to wind the core ? Make a trifiliar wire. Three wire is taken and twisted for 5 to 8 turns per inch by handdrill. then wound the 15 to 17 turns. One side wire end are labelled as 1,2,3 and their other ends are labelled as a, b,c. Test for dc continuity. 1-a,2-b,3-c and no other pair should show the zero. If it shows then wire enamel is broken and it needs to be rewound using new wire. Connect 2and a and your transformer is wound ok. Becareful with scrapping the enamel.

to be continue......