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.
Yes, its a post about another simple, low-performing direct conversion receiver. However, I think that this one is slightly unique. I was inspired to give this a try based on the Flea minimalist transceiver that was introduced on the EMRFD Yahoo group. These little rigs are fun to build in an evening, but just how usable are they? Would you feel comfortable giving it to a new ham and believing that they even had a small chance of success? For me, these Pixie-class rigs are nearly unusable due to the horrible AM broadcast interference that blows right through the rig. While a minimalist rig is an admirable thing, they are only useful in limited circumstances. I figure that a few things have to be added to these rigs in order to make them more than a novelty. KD1JV also shares this viewpoint, and has created his own answer to the Pixie.
I've started with a similar philosophy, but built the rig around a different topology. The basic strategy is to use a differential amplifier as an active mixer. The rig is designed for the 80 meter band, which is probably the easiest for homebrewing. The LO is a Colpitts ceramic resonator oscillator, but is not separate from the mixer. Instead, the oscillator is built around the third transistor which acts as the constant current source. I know that this is certainly not a new idea; it's used all of the time in NE602-based QRP circuits. However, I don't think this topology is seen very much in discrete component use. It saves quite a bit of circuit space and is composed of very common components.
The rest of the receiver is very simple. I placed a standard double-tuned circuit bandpass filter in front of the RF port of the mixer to filter out all of the AM BCB crud. The output of the mixer feeds a dirt-simple emitter follower to transform the relatively high collector impedance of the diff amp mixer to a low impedance output. I haven't designed the final AF amp yet, but I don't think it will take much to get the signal up to headphone levels. When the emitter follower output is connected to my test bench AF amp, I have to have the amplifier AF gain control turned nearly all the way down, lest the whole thing start oscillating wildly.
Tonight, I connected the RX to the bench AF amp and the antenna to see how it would work. Tonight was an excellent night to try, since we are right in the middle of Sweepstakes. Pleasantly, the receiver immediately came to life with a cacophany of CW signals in the unfiltered audio output of the receiver. I've attached a recording of the receiver output so that you can get a feel for how well it works for such a minimalistic design. The ceramic resonator osc tunes from nearly 3.500 MHz to 3.580 MHz, and I tune across the entire band in this clip.
All I have to do to finish the receiver is to add on a discrete component AF amp. I think that a single class-A stage of amplification will be enough to get the audio up to headphones level. After that, I'm going to try to tack on a transmitter by picking the VFO signal off of the other unused collector port of the diff amp. I think that I can get away with another emitter follower as a buffer, followed by a class-C PA. I'm shooting for around 1 watt of output power, which is enough to snag QSOs without too much difficulty. I think this could be a lot of fun to build as a kit. It's will be quite a bit more complex than a Pixie or Flea, but also quite a bit more usable. Stay tuned for further developments on this rig.