Direct-Conversion Transceiver


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1)   New kit from K1SWL & MidnightDesignSolutions
       by Mike Maiorana, KU4QO
       (posted 18-Nov on qrptech.io)

I got my "Old Friend" transceiver kit from MidnightDesignSolutions a few days ago, shipped in a padded envelope. I took a relaxing evening yesterday and built it, so I thought I'd give some feedback. The kit was very easy to build! Most of the through-hole parts were separated on individual cards, labeled with their value and reference designator. That made the inventory very easy to accomplish, with no searching for parts. All the surface mount parts were already assembled on the PCB (I was a little disappointed that I didn't get to build the SMD parts). The assembly is done in sections, with some sanity-check testing at the end of each section, which gives you confidence to continue. In total, the build time was about 3 hours, give-or-take. It worked perfectly on power-up. The calibration of the PLL was simple and easy to complete with a frequency counter. Even without calibration, it was within a few hundred Hz. Power out was right around 5 watts (maybe a shade less) with a 12VDC supply. Power draw was less than 1A on transmit, 130mA on receive. I haven't looked at the output spectrum yet, but I'll try that later. I don't expect to see anything unusual.

My kit was built for 40 meters. I connected it to my dipole last night just to have a listen. There were a TON of signals, and on a crowded band, the DC receiver shows some of its weakness with signals on both sides of zero. The audio peaks at around 800Hz (the sidetone frequency), but isn't razor sharp, so it doesn't have that nasty ringing like you are listening inside a tin-can. The front-end and mixer were solid, with no broadcast interference. I do hear the "tick tick tick" from the Wifi router in my office, but if I cover the board with my hand (or an aluminum tin) it vanishes. I think when this is mounted in a metal enclosure that will disappear. 

The tuning encoder they included seems like a high quality part. There are no detents, but you can feel the bumps from the internal switches ever so slightly while tuning. The software has "ballistic" tuning, where if you tune slowly it jumps 20Hz, but when you turn more quickly it jumps by 100Hz. Encoder tuning always feels a bit clunky to me, but this setup isn't bad at all, and is very simple. 

The audio out is at a fixed volume, which set me back a little bit. It is designed to drive a pair of low-impedance headphones. I used a decent pair of headphones for testing (Sennheiser HD598). The band noise level was a bit loud for me, but not offensive. Weaker signals sounded good. Dave designed in a limiter that prevents very strong signals from blowing your ears off. It works very well and is seamless, unlike some AGC systems that cause annoying pumping of the audio level. However, the really strong signals lose that smooth sinewave sound and sound clipped (which they are). Again, it isn't hard to hear, it's just different. One thing to keep in mind is that different headphones have different sensitivities. If you use a pair of very sensitive headphones, the volume may be a bit too high. Likewise, if you use a very inefficient pair, it may sound a bit too weak. All that being said, and keeping in mind the simplicity of this radio, it works fantastically. I love it.

Hats off to Dave and the guys at MidnightDesignSolutions for a great kit! 

73

Mike M.  KU4QO

 

 

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