WWVB Receiver and Testing Transmitter

As I've been making steady progress on my Z80 computer, I've put a lot of thought into something to actually do with it when it's done. As tempting as it is to load up CP/M, add a video interface, and use it as my main machine, I think porting Firefox is too ambitious. For now, I've decided to add some more 7-segment displays and an RF front end to decode the WWVB time signal from an atomic clock in Colorado and hang it on the wall. If I'm still feeling ambitious, I'll add an Ethernet controller and write a driver and NTP daemon for it. Of course, this means I need a working receiver front end for 60kHz.



The WWVB signal is quite simple as far as decoding goes: the carrier has "full power" and "low power" states. The transmitter drops the carrier by 17 dB at the start of each second, and the length of time before it returns provides a single trinary value: Mark (0.8s), Not Set (0.2s), or Set (0.5s). A single digital line that goes low or high to follow the transmitter power state is quite easy to achieve with a fully analog circuit: after a bandpass to select only the 60kHz carrier, we send the signal into two peak detectors - one with a long time constant (low pass filter) to track the average signal power over several seconds, and one with a shorter time constant to track the per-second changes. Passing these two signals to a comparator, we get our nice single-bit TTL indicator of the current signal state. Plus, the longer time constant peak detector acts as an auto gain control, so no alignment is required.



While I will make my own receiver as described above for the final version, I ordered a SYM-RFT module for testing. It includes an LED to indicate received carrier status and outputs the inverted carrier power state compared to the average carrier power. I quickly breadboarded it up to an FT245R USB chip and wrote a few lines of C to poll the power status and measure the length of state changes. Unfortunately, New England being quite far from Colorado, the signal is below the noise floor and has thus far been highly unreliable. The module simply flashes chaotically in the presence of any electronic noise like my monitors, and does nothing in isolation.



I built the above two-stage preamp from two 2N3904's to attach to the 60kHz antenna that came with the SYM-RFT module to see if I could see any carrier on the oscilloscope. While I did get all sorts of noise and impulses, there was very little if any energy at 60kHz. Drat.

Luckily, I have the tools on hand to recreate the signal so I can at least test my receiver and code. I pulled out the FPGA RF DAC board I put together a while back, and a page of Verilog later I had a compliant WWVB transmitter:



After a small battle with PLLs to get a slow enough clock for 60khz, both the SYM-RFT and my USB worked fine.



jackc@ichor ~/Projects/wwvb_usb $ sudo ./wwvb
MARK    : 950 ms
NOT SET : 239 ms
SET     : 503 ms
NOT SET : 194 ms
SET     : 581 ms
MARK    : 991 ms
^Cjackc@ichor ~/Projects/wwvb_usb $