I have an interest in extremely low-frequency (ELF) radio — transmission frequencies below 300kHz. Of particular interest to me is WWVB, the official government time station operated by the National Institute of Standards and Technology (NIST). It transmits on 60kHz. The NIST also operates WWV, which transmits on 2.5, 5, 10, 15, and 20MHz, but since those are HF shortwave transmissions, I don't pay much attention to them. All that RF stuff is too tricky.
But 60kHz — that's practically audio. So here's what I'm wondering. Could I make a WWVB receiver that is not a conventional superhetrodyne receiver? Instead, I think the classic tuned RF (TRF) receiver might be practical. This type of receiver was popular from 1915 to 1930 and preceded Armstrong's invention of the superhet. The TRF was not as selective as the superhet. Further, tuning multiple, cascaded stages was an iterative (and therefore tedious) process.
In spite of these issues, it would be an interesting design to investigate. Here are the details. We should need only one inductor: the loop antenna. This loop (aperture facing toward Fort Collins, Colo.) would have a parallel tuning capacitor selected to resonate with the loop's inductance at 60kHz. This tank circuit feeds a gain-and-tuning stage built around an op-amp configured as a VCVS bandpass filter (BPF) or possibly a three-op-amp biquad BPF.
I would design the filter to have as high Q as is practical without breaking into oscillation. The NIST would not appreciate someone splattering all over its transmissions. If more gain were needed, I could add a second stage just like the first. If I used the biquad topology, I could tune it by simultaneously tweaking two resistor values per filter. This is a good application for a dual (or quad) digitally controlled pot; I believe Maxim has these in its portfolio.
After the gain stage, I'd install the detector — probably an ideal diode stage built around another op-amp. After the detector, I'd add the decoder circuitry to extract the time and date information. Since that's all digital, and since that's not my forte, I'd leave that portion to someone far wiser than I am.
Would this work? Is it practical? Let me know.