I enjoy listening to the classic rock format on WRMI radio, broadcast on 5050 KHz mid evenings, and 9455 KHz late night. With the DC receiver project winding down, I have enough left over parts to ( I think ) build a simple radio for listening to WRMI under good conditions.
Having inventoried what I have on hand, I think the "design" goals are going to be:
Simple, beginner friendly circuitry, with no complicated circuits, or unobtainium parts;
Self contained except for power supply, suitable for daily listening, convenient to operate, hopefully not a toy that one becomes disinterested in after a week or so of use;
Sounds good
I've always liked the oddball look of this case that someone pretty good at 3D design designed for the bitx transceivers:
BitX40 Ergonomic Enclosure by fire5ign - Thingiverse.
So, I printed it up, in a really bright color to go with the fun tropical / Caribbean Sea motif that WRMI is using. That takes care of the case.
I have the Si5351 based raduino built and debugged - it just needs some minor tweaks to the code, and the 1602 LCD with an I2C drive board fits without difficulty in the case as is shown. That takes care of the VFO.
Two out of three already done, that just leaves the radio part. I've been thinking about the best way to go about it, and I think a single conversion superhet is the best solution to meet the design goals,
For the front end, I'm choosing a single tuned circuit, because I think the chances of correctly tuning up a double tuned circuit are nil, without some type of spectrum analyzer. Tentatively, I am going with a 50 ohm input. An active antenna, or maybe even a regenerated ferrite rod, might be attempted at some point.
I pulled a previously wound T-37-6 coil, and gave up on counting the turns wound on it, and just used an LC meter to measure its inductance at 3.54 uH, then calculated the capacitance necessary to resonate at 5050 Khz as 280 pF. I picked a 220 pF cap that measured out at 210 pF, and a 75 pF cap that measured 69 pF, to get the needed 280 pF. I used a 5 turn link on the toroid to go from a 50 ohm antenna to the 1500 ohm input resistance of an NE602, and assembled and swept the filter with the red pitaya.
And it peaked low, by about 30 pF of stray capacitance, when I back calculated the peak. I substituted a random 47 pF cap for the 75 pF, reswept the filter, and it looked close enough. An alternate, easier, approach is to substitute a trimmer cap for the smaller cap, and just peak it for maximum atmospheric noise or signal strength.
I made a simple mixer with an NE602, and tested it with an RF input of 0.01 volts at 5000 KHz, and an LO input of 0.1 volts at 5500 Khz and looked at it with the red pitaya to verify it worked. The 500 KHz IF, 5000Khz LO, and 10.5 MHz IF are plainly visible. The smaller peak at 11 MHz is probably an intermod artifact of the two IF's. The 0.01 volt RF input is very strong compared to an over the air signal.
All of this is being built on the board that I abandoned when I went a different direction with the DC receiver. Only the regulators were salvaged. The NE602 is using the 8 volt regulator. That's it for now. This is a low priority project, and updates / progress may be infrequent.
73.
Win W5JAG
I tried the new front end out last night, BF1105 RF amp and HP IAM 81008 active mixer, and, wow, it was a hot mess of unwanted AM detection.
But, really sensitive - at dawn this morning, it was easily hearing both WWVH and WWV on 5000 KHz.
All of the unwanted detection is surely in the BF1105 - dual gate MOSFET's are notorious for it.
Back to the drawing board.