I'm up to my elbows in the design work for the 2010 FDIM QRP Challenge, and I think I've come far enough along to give the rig a name. I'm happy to announce that I've dubbed this project Clackamas, in keeping with my tradition of naming my creations after Oregon rivers.
The receiver is pretty close to its final configuration (I hope), so I took it in to work so I could measure the performance using the very nice calibrated test equipment at my bench. This is my first time making this entire range of receiver measurements, and I have to say that it was quite interesting. I wish it was something that I had started taking more seriously a while ago. I have a wish list for test equipment a mile long, but I've realized that I really need to get my hands on a pair of good signal generators, hopefully ones that can give me an output down to -140 dBm.
I followed the procedures in the ARRL Test Procedures manual as closely as I could (do yourself a favor and save a copy of this highly useful document). The numbers came out pretty close to what I would expect, so I'm reasonably sure they are legitimate. I'm just hoping that I can win the challenge so that my rig gets taken to HQ for the real battery of tests! Below is the results of my testing, followed by a very brief commentary on the numbers:
IF Bandwidth: 462 Hz MDS: -126 dBm 3rd Order DR (20 kHz): 80.5 dB (S5 signal level per ARRL Lab Procedures) IIP3 (20 kHz): -5.2 dBm Blocking DR (20 kHz): 102.6 dB IF Rejection: 23 dB Image Rejection: 48 dB
I believe that the MDS, dynamic range, and IIP3 measurements are all acceptable for a receiver using 40 parts and a 7-part VFO. I know why the IF and image rejection is so horrible (it's because of a design trade-off), but I can't get into the details of that yet. The filter response is a bit funky, but I'm sure that my impedance matching isn't the greatest. On the air, it's my opinion that the rig sounds decent. Maybe I can record some audio this weekend and post it. Let me know what you think; does this sound reasonable to you for a compromise superhet? I'd love to hear your comments on this.