Simple direct conversion receiver - MRX-80

I have built my first analog receiver! This is maybe one of the simplest ones you can build ( a crystal radio might be simpler depending on how you count). The direct conversion receivers basically mixes the incoming signal with the local oscillator and the result is within frequency for our ears. The classic basic combo of ICs, NE602 and LM386 are used. The circuit is from the book "More QRP Power" from ARRL and is called the MRX-80. As you might guess it is for 80 m. 

 

The build is done inside an old box used for an Ethernet network hub. The BNC connection hole was already there but I had to to use a side of PCB material so I could mount the pots for gain and tuning. Yes, talking about the tuning, it is somewhat limited. I can change crystals and a diode is used as a varactor so a small swing of frequency is allowed, but I have not yet measured how big this is, maybe 1 kHz or so. The build is a mostly with leaded parts built ugly/Manhattan but all resistors are tiny 0805 smd, just because I wanted to try and see if it works. Maybe it does or probably not completely.  The build is not the prettiest, but if you see any bad choices of building other than aesthetic, please comment or if you have any other comments.

When I turned it on, the first thing I did was to measure the voltage out from the regulator. OK! I then turned my small Degen 1103 radio on and listened for the local oscillator. It was loud and clear so OK on that too! Then I connected my signal generator, but nothing heard, apart from noise. I started tapping on some connection and I heard them. So I guess the audio amp was working too.  Hmm, I started double checking other connections and soon found that I had connected earth connections between antenna and gain pot but not to the real earth on board. Then my test signal could be heard. When put in S9 mode it was OK, but not in S1. I wonder about what to expect from this simple thing. The real test will of course be when I try it with a decent antenna and see if I can hear any real on the air signals. 

I might have heard some today, or maybe not, because I never checked if they were present on another receiver. Next weekend I will go out to a 'quiet' place and listen. 

Below is a audio recording. Some signal can be heard, but QRM level is high so much noise. It does not sound as what I am used to what a radio should sound like. Either this is what a direct conversion receiver sounds like or I have some construction errors. I will bet on the last one. 

The sum up, a good build I think. Fun to do and easy. First small steps and more experience in radio homebrewing!

Update 2011-07-15:

The receiver has worked fine and the signal on the recording is SL0FRO by the way SM5-1252 Ullmar told me. 

Colt receiver part 1

I found a book as an E-book at the university library: Radio and Electronics Cookbook that was published by the Radio Society of Great Britain in 2001. I have skimmed through it briefly and it seems to cover both some  radio theory and some interesting projects. I do have a lot of  "maybe" and "should be" projects in my head (that I will cover in later blog posts) and now I think I decided, apart from getting the softrocks built (as they are already on my desk), I would like to build a hardware defined receiver.  A very basic one to really try to understand how a radio works.

In this book, I found one called the "The Colt 80 m receiver" that I liked, because it´s built in modular steps and quite thoroughly explained. It starts with the audio amplifier and then adds mixer (NE602) and VFO to become a direct conversion receiver. The last part is to add an IF stage to become a superhet.

What worries me a bit is that I can´t find any reference to this design when I google it. Or maybe blogs and so on wasn´t that common 2001 and earlier or this design is named something else elsewhere? A bigger problem is the availabilty of parts. Variable capcitors to be used in a VFO can´t be bought new, but I´ve have asked around in the club and I think that will be solved.

Then there are some specific inductors from Toko:

• Toko KANK3334

• Toko KANK3333

• Toko YHCS11100AC (tuned inductor)

For the two first ones I have found a guide at the GQRP club (that I by the way have joined; Sprat is a great magazine), so I can make those inductors with toroids. But the last one, I have no clue. Any ideas someone? There´s also a part number for a crystal filter, but I think I can use another one for 455 kHz.

We will see if there will be a part 2.