A Simple 6m Converter
The idea of the project is to build the most simple 6m receive converter possible. It sure is not the best, but nearly everyone can scavange the parts from their junkbox and built the thing in one evening. I built mine entirely with parts from my junkbox. This circuit is not of my design, it is made by LY3LP who's site you can find here:
http://sa555.blogspot.com He has loads of simple radio projects on his site. The documentation however is quite sparse.
The hardest part to find is the ferrite, you can use any ferrite made from 43 or 63 material. That is, you need a ferrite with wideband material. I used one I scavanged from an energy saving lamp and it worked quite fine.
The converter utilises a Polyakov mixer made from two anti-parallel germanium diodes, I used AA112, but any germanium diode will do. The weird thing with Polyakov is that it uses a local oscillator at half the frequency you would need with a traditional mixer. This is quite useful because 32MHz crystals are not exactly common and most likely 2nd or 3rd overtone. But 16MHz computer crystals are cheap as chips. As this converter lacks any type of filtering you can also use other crystals to get rx at the band of your choosing. You find the needed crystal with this equation:
Target frequency - target if / 2 = crystal frequency
In this case target freq is 50MHz, if is 18MHz, so:
50MHz - 18MHz = 32MHz
32MHz / 2 = 16MHz
This way you can adapt this circuit for some other project of yours.
To avoid complicating the circuit it does not have any type of filters or amplifiers. You can add those if you feel the need. The oscillator is a traditional colpitts with nothing fancy, the feedback caps not only should but must be of temperature stable variety like NP0 I used some styro caps I had in my junkbox. The absolute exact values are not that important so anything within 40pf of the values used should work. You can determine the correct operation of the osc by listening in with a radio, with a rf probe or by using a oscilloscope. RF probes are easy to build with the net' filled with plans. I used a BC547A transistor for the osc, but any common general purpose transistor (2n2222, 2n3904, 2sc1815 etc.) will do, just check the pinout.
The transformer between the LO and mixer has 4 turns in both the primary and secondary. Both are wound in the same direction with some 0.8mm emamaled copperwire Anything close to that can be used. <C2><A8>
The mixer itself has two diodes soldered back to back. Be careful while doing so, too much heat can destroy the diodes.
Manhattan style on a piece of FR2 "pertinax" pc board. But anything works, you could go out all Hans Summers and build it on a piece of Heinz bean can and it would probably work.
So there you have it leaves alot to be desired, but hey it works!
One could add a bandpass filer in front, along with a preamp, a diplexer and if amp at the output. But then it would not be as simple anymore. YU1LM has plans for a simple 6m bandpass if you want to add one: http://yu1lm.qrpradio.com/6m%20BP%20FILTER-YU1LM.pdf I'm using pair of 75ohm bnc connectors widened to accept 50ohm BNC's for both input and output, but a rca connector is also provided for output.
For testing you can either use a 6m rig, a signal generator or a 50MHz ttl oscillator module like I did. Either the ttl osc can or the crystal in the converter are a bit off in my test, but close enough to determine that it indeed works.