Keeping Time with Turing

A better antenna, folded cascode input amplifier, and a little bit of code yields the current time!

The antenna is simply 4 turns of unshielded, twisted pair cat-3 wrapped in a 40 cm. (15 in.) loop, held together with wire twist-ties. The individual wires are connected to form 28-turn continuous loop, in parallel with a 0.01uF capacitor to tune the loop to 60kHz. The remaining 4 turns are made in to a second loop, where one end is the input to the receiver and the other is connected to an end of the 28-turn loop and ground.

As I twiddled with the simulation of the new input amplifier, the gain was heading toward 40dB, not bad for a 2N3904 / 2N3906 pair. When build time came, the constant-current source in the simulated circuit was replaced with a resistor on the breadboard, with little change in performance.

Trading the full-wave rectifier for a half-wave with a peak detector allows for even more gain. During testing, I had to dial the signal generator to the minimum output to not have clipping at the peak detector. Neither my oscilloscope nor voltmeter could measure the input signal.

Turning the peak detector's output in to a nice 3.3V logic was quite an exercise. Initially, I used a fixed threshold, which worked well enough. Considering the varying nature of the input signal, a variable threshold for the comparator seemed to be necessary. A few days in that rabbit-hole, and things are no better. Finally, I settled on using a simple resistor-capacitor, with a long time constant, to provide the varying input for the comparator.

Here is the current design, with a handy output for 3.3V logic:

I'm still considering turning the gain up little more, along with adding a gain control circuit to the mix. That's for another time, I want to build a Turing machine!

Turns out that what I thought would be a quick test, became anything but. After writing a preliminary WWVB decoder and display driver, I hitched the receiver up to a Z8 dev-board. Suddenly, there is no signal. Eventually, I worked out that the scope gave the receiver a very nice earth ground, and that using a 5V wall-wart for power took that ground away.

Because I am prototyping this on a solderless breadboard, I understood that the circuit's behavior would be a bit off from the calculated value. With the signal at 60kHz and using relatively low resistance values in the filter, a few picofarads shouldn't make a huge difference, right? There was some, so a few changes were made and some more gain was found.

There still remains to choose an antenna. A long-wire antenna is usually the easiest to implement, until one realizes that the wavelength of a 60kHz signal is 5km. Yeah, that's over 3 miles of wire! The Mountain House is on a fair-sized patch of nowhere, but even a quarter-wave would go way out of bounds. A ferrite rod from an A.M. radio could be pressed in to service, but would need re-winding. That could be a lot of effort for an insufficient result. A promising alternative is a loop antenna, specifically a "small" loop. The definition of small in this context appears to be an antenna that is less than 10% of the wavelength. I'm starting with 100 m. of wire...

This project seems to be at the point where I could tack on a Z8 and call it done. I may hook up the Z chip as proof-of-concept for the input section, but the goal is still the Turing machine decoder. All that is needed to build to this stage is a bunch of passives, a transistor, a quad op-amp, and a comparator.

Here is what is on the proto-board at the moment:

The circuit starts with a quick and dirty 2.5-ish volt tap that becomes the op-amp ground reference. Next up is a fairly basic transistor amplifier. Most any general purpose transistor will work for 60kHz WWVB signal. Now the hard part, two sections of the op-amp form a 60kHz filter with a lot of gain. I have to  thank Analog's Filter Wizard for taking care of the heavy lifting. The nice looking sinewave at the output of the filter is fed to a precision full-wave rectifier and output filter capacitor, like it is a DC power supply. That DC voltage causes the comparator to switch the LED on and off as the WWVB carrier changes.

Well, why not? I'm going to hunt down a dev-board...

To say that this project is underway may be overstating the status, a little. The receiver is blinking to the tune of WWVB, more or less. A better antenna, more gain, and more selectivity would be nice additions. First is to upgrade the TL072 op-amps to a device with a bit more gain-bandwidth and possibly one specified for 5V operation. But, it blinks!