The One Chip Wonder circuit has been performing so well, that I began to move on to detecting and digitizing the signal. Diode envelope detector? Nope, not quite enough input level. Op-amp precision rectifier circuit? Well, there was something at the output, but I couldn't figure out what it meant. I recorded some audio with audacity and checked the FFT of the signal. Nothing unexpected, a 2500-ish Hz tone and noise. Ok, a tone decoder then. Not quite. The tone is always present, sometimes at full amplitude, somethines reduced. The output went high, and then stayed there. What now? Well, we could number crunch an FFT for 0.1 seconds of signal ten times a second... Umm, no. Enter the Goertzel Filter, think of it as a simplified FFT that only returns the result for one frequency. A miniscule amout of code later, and pre-recorded .WAV files are turning in to square waves with mind blowingly perfect 0.2, 0.5, or 0.8 second intervals. This is what I want!

Simple as the calculation is, I wondered if strategically chosen frequencies could simplify things further. Sure enough. When the sample rate is six times the frequency being tested, a term usually called "coefficient" becomes 1.0000, this eliminates most of the multiply operations entirely. Sweeeeet! After crunching some numbers, I arrived at 2.4 kHz for the frequency and 14.4 kHz for the sample rate. That would put the oscillator at 57.6 kHz to tune in WWVB's 60 kHz. That will work, but I would have to order an ADC because the ADC0804 in the parts bin won't quite handle the needed 14.4 kHz sample rate. Looking again at the algorithm, a sample rate that is just three times the frequency sought turns the coefficient value in to -1. Still no multiplies and we just swap subtract and add operations. So, frequency 2.4 kHz, sample rate 7.2 kHz, and oscillator 57.6 kHz. Do these numbers look familiar to you? They're baud rates! I can use a baud rate generator crystal (1.8432 MHz) and flip-flops to derive all of the frequencies needed to run the system.

With a crystal controlled frequency source, I can use a very narrow bandpass filter to home in on the 2400 Hz difference frequency coming from the mixer. But what sort of mixer this time? The final stage of the oscillator divider is a 74LS74 with its' handy complementary outputs, I have an idea. Unlike previous circuits, what if we switch to ground rather than effectively floating the signal path with a "OFF" switch or MOSFET?

Here is the new circuit:

Quisk is reporting up to S9+31 with the ethernet cable loop antenna. It is easy to see the 1s, 0s and marks in the Audacity captures. Swapping the loop for a 60 cm. piece of wire (1/8300 wavelength) drops the signal to S9+21 with very good audibility. I think it might be time to stop using bits of wire and go look for a normal sort of antenna.