Above photo: John Harrison’s H1 marine clock, 1735. An early example of a GPS clock, if you like. Note the four brass spheres (one not visible) that replace a traditional pendulum helping the clock to keep an accurate time while a ship is being tossed about at sea.
A conventional analogue clock, like the H1, uses gears to ‘slow’ down the action of the pendulum so that the clock can display minutes and hours.
The Need For Less Speed
We left part 3 with an accurate but stupidly fast pendulum – swinging nearly seven billion times per-second. While very precise it’s very difficult electrically to work with, also it’s an awkward number to divide.
Fortunately, electronics provides a way to slow this signal down to something more manageable. Unfortunately, it’s very complicated and so I’ll offer this simple description.
You have your main pendulum, A, and that swings at, say, 1000 times per-second. You then make a second (dummy) pendulum, B. You connect pendulum B to A through a series of levers and gears to make it swing every second. Pendulum B is swinging slower but locked to the timing of pendulum A.
This is what happens inside of the rubidium oscillator. The clever electronic engineers have connected the seven billion swings of the atomic oscillator to a dummy quartz oscillator. This swings at ten million times per-second (10MHz). Even though it’s a quartz oscillator it now keeps with the accurate time of the atomic oscillator.
Okay, but we cannot read the time-of-day directly from ten million ‘clicks’ every second. And that’s what the X72 is giving us. That’s it.
To turn that ten million clicks per-second in to knowing when to avoid BBC Question Time means that we have to some how divide it down into seconds, minutes and hours. We also have to be able to count these signals and display them in a human readable form.
In the olden days you would have designed a memory decoder, interrupt processing logic and input/output buffers etc. Kids these days don’t know they’re born! Now with the advent of small all-on-chip micro controllers it’s like a walk in the park to interface with our atomic clock module.
But, even with such clever microchips, ten million ‘clicks’ every second is still too fast. We need to electronically ‘gear’ or divide that signal to a lower speed.
In the above photo I’ve taken the lid off of my ‘atomic alarm clock’. The microchip marked with a green dot is the micro-controller (CPU). It does all the work – reads switch settings, scans the LED display etc.
The thing marked with blue dots is a little ‘buzzer’. It is an alarm clock after all.
Under the circuit board is the atomic oscillator itself. Note that it is fastened to a copper ‘heat-sink’ plate. These modules run hot, hence the cooling fan at the back of the case.
The chips marked with red are our ‘gears’. They bring the signal down to a speed that the micro-controller can read. It’s very simple:
The ‘slowing down’ or gearing is done with ‘binary counters’. That is what most of those chips are.
Think of it like this: the X72 ‘injects’ a signal of ten million ‘clicks’ every second into the ‘divide by 16’ chip. All it does is to count 16 ‘clicks’ from the X72 and then inject 1 ‘click’ into the divide by 100. It does that over and over. 16 clicks in for one click out.
That ‘click’ then enters the divide by 100. It counts to 100 and gives 1 click to the CPU. It does that over and over again. 100 clicks in for one click out.
The overall effect is that the 10 million ‘clicks’ from the the X72 have been divided by 1600.
The resultant signal is now slow enough for the CPU to count. For every 6250 ‘clicks’ that the CPU sees it counts 1. The CPU is now counting seconds. It is now a simple matter for it to count minutes and hours.
The CPU then takes those counts and displays them on a LED display.
For a bit of fun (marked with a yellow dot) I added an ‘arduino WiFi’ board. You can set the clock/alarm from your phone/tablet/PC.
Future Of Miniature Atomic Clocks
Time and tide waits for no technology and atomic clocks aren’t exempt.
Hot running rubidium vapour clocks are already being replaced by CSAC or Chip Scale Atomic Clock.
They can achieve the accuracy of a vapour cell clock and run cool(er) with the use of a laser that replaces the hot vapour lamp as in the X72.
It’s only a matter of time before atomic clocks become the norm.
© Doc Mike Finnley 2018