That big grandfather clock in the library might be an impressive piece of mechanical ingenuity, and an even better example of fine cabinetry, but we’d expect that the accuracy of a pendulum timepiece would be limited to a sizable fraction of a minute per day. Unless, of course, you work at CERN and built  “the most accurate pendulum clock on the planet.”

While we’re in no position to judge [Daniel Valuch]’s claim, we’re certainly inclined to believe him, mainly because the 1950s-era Czechoslovakian pendulum clock his project was based on, the Elektročas HH3, was built specifically as a master clock for labs, power plants, and broadcast use. The pendulum of this mid-century beauty is made of the alloy invar, selected for its exceptionally low coefficient of thermal expansion. This ensures the pendulum doesn’t change length with temperature, but it still only brings the clock into the 0.1 second/day range.

Clearly that’s not good enough for a clock at CERN, the European Laboratory for Nuclear Research, where [Daniel] works as an RF engineer. With access to a 10-MHz timebase from a cesium fountain atomic clock — no less a clock than the one that’s used to define the SI second, by the way — [Daniel] looked for ways to sync the clock up to it. Now, we know what you’re thinking — he must have used some kind of PLL to give an electromagnetic “kick” to the bob to trim the pendulum’s period. Good guess on the PLL, but the trimming method is a little cruder — [Daniel] uses a stepper motor attached to the clock’s frame to pay out or retract a length of fine chain into a cardboard dish attached to the pendulum’s rod. The change in mass changes the pendulum’s center of gravity, which changes its effective length, and allows the clock to be tuned a couple of seconds per day.

It seems like [Daniel] is claiming that his chain-corrected clock won’t drift more than a second from the cesium clock for 158 million years. Again, we’ll take his word for it, but it’s a wonderfully ad hoc approach to tuning the clock, and we appreciate its simplicity.

At the heart of many amateur radio and other projects lies the VFO, or Variable Frequency Oscillator. Decades ago this would have been a free-running LC tuned circuit, then as technology advanced it was replaced by a digital phase-locked-loop frequency synthesiser and most recently a DDS, or Direct Digital Synthesis chip in which the waveform is produced directly by a DAC. The phase-locked loop (PLL) remains a popular choice due to ICs such as the Si5351 but is rarely constructed from individual chips as it once might have been. [fvfilippetti] has revisited this classic circuit by replacing some of its complexity with an Arduino (Spanish language, Google Translate link).

A PLL is a simple circuit in which one oscillator is locked to another by controlling it with a voltage derived from comparing the phase of the two. Combining a PLL with a set of frequency dividers creates a frequency synthesiser, in which a variable frequency oscillator can be locked to a single frequency crystal with the output frequency set by the division ratios. The classic PLL chip is the CMOS 4046 which would have been combined with a pile of logic chips to make a frequency synthesiser. The Arduino version uses the Arduino’s internal peripherals to take the place of crystal oscillator, dividers, and phase comparator, resulting in an extremely simple physical circuit of little more than an Arduino and a VCO for the 40 metre amateur band. The code can be found on GitLab, should you wish to try for yourself.

It would be interesting to see how good this synthesiser is at maintaining both a steady frequency and minimal phase noise. It’s tempting to think of such things as frequency synthesisers as a done deal, so it’s always welcome to see somebody bringing something new to them. Meanwhile if PLLs are new to you, we have just the introduction for you.

If you’re dealing with RF, you’ll probably have the need to generate a variety of clock signals. Fortunately, [Jason] has applied his knowledge to build a SI5351 library for the Arduino and a breakout board for the chip.

The SI5351 is a programmable clock generator. It can output up to eight unique frequencies at 8 kHz to 133 MHz. This makes it a handy tool for building up RF projects. [Jason]‘s breakout board provides 3 isolated clock outputs on SMA connectors. A header connects to an Arduino, which provides power and control over I2C.

If you’re looking for an application, [Jason]‘s prototype single-sideband radio shows the chip in action. This radio uses two of the SI5351 clocks: one for the VFO and one for the BFO. This reduces the part count, and could make this design quite cheap.

The Arduino library is available on Github, and you can order a SI5351 breakout board from OSHPark.