This brings back memories from my childhood. My dad, Dr. Leonard S. Cutler (1928-2006) led the team that developed this clock. He is the man with his back to the camera in the photo at 1:38. Back in the 1960s and early '70s he sometimes took me to Hewlett Packard at 1501 Page Mill Road in Palo Alto, California on the weekends. As a young boy I got to see the research and development of the Hewlett Packard cesium clocks. Later on, development of the cesium clocks was moved to Hewlett Packard's Santa Clara plant. Eventually Agilent sold their atomic clock business to Symmetricom. After my parents passed away, I inherited their house, and in one of my dad's old storage rooms is a four-drawer file cabinet that still has most of his notes and documentation for the 5061 and the later 5071 cesium standards.

You should do a series interviewing some old engineers from HP, Tektronix, etc. to know the stories behind these designs. Probable a few may still be around. Great work and keep bringing these engineering marvels back to life and for us to enjoy them.

23:02: Ultra-precision instruments in the foreground, bench grinder in the background.

Back when HP made the best instruments in the world.

Really interesting and well explained. I never knew how complicated the process is!

Wow, I worked at HP Santa Clara Division for 25 years, R&D, Building these Units all the way up-to the smart hp 5071a cesium atomic beam, It was Ethereal! I enjoy every moment. Felipe Espinoza

Wow, that brings back a lot of memories! I was a development engineer in the Precision Frequency Standards (PFS) group at HP Santa Clara (CA) division then. That's where they were developed and produced. It was really interesting. I previously had worked on Cesium clocks at MIT when I was a physics student, then interned at a competitor company in New York and there learned more about both Cesium clocks and crystal oscillators. Worked on the successor product 5062 (? my memory ?) also.

The comments on these video's are fantastic. Love atomic time. Grew up with friends whose dad's worked at the Dept. of Commerce in Boulder, Colorado. The best was my neighbor, who was an engineer at HP Loveland. This guy unloaded a basement full of electronics on me when I was about 12 years old, because he was moving to a new home. Lifelong radio hobbyist and electronics tech because of it.

The 80 version of the HP Cesium clock was used in military satellite communications terminals to provide both timing and frequency references. I used to daily do comparison time offset measurements between the clock in my terminal and the clock on the distant end terminal. We'd send those values off to the Naval Observatory weekly and periodically receive correction factors to load into our clock. The corrections were intended to eliminate the time offset between clocks. It was a blessing when Techtronics came out with scopes that would allow us to accurately measure the offsets vice having to use multiple freq counters to extrapolate the offset values. Those were great clocks not a lot of trouble with them.

Coast Guard LORAN-C stations used CAQI-5061A (CAQI=HP, some sort of manufacturer designation) for so many years... Can't tell you how many daily log readings I took on them over the years- Each station had an "Operate" or #1, a "Standby" or #2 feeding an associated timer and other gear that could be "dirty" switched in the event of a failure or "clean" switched by aligning the phase of the output 5mhz, and a "tertiary" that could be patched into either #1 or #2 position in event of a failure. Each freq standard had a log book with daily readings that travelled with it- on the rare occasion of a failure it would be returned and repaired, and the log book would stay with the standard. Each secondary station's standards were compared to the master station's standards and drifts were handled with phase microsteppers, which if I remember correctly adjusted to six decimal places below a nanosecond, and longer term were compensated with "C-Field" adjustments. It was some time after 2003 - 2005?? when the stations timing and control equipment was "modernized" that the 5061s were replaced with 5071s ? I retired from my last station in 2008 and the program ended in 2010. I feel a little bad I don't remember more, but it is amazing the things that stick in one's brain. I believe the story that had been around forever was that free-running, in a perfectly stable environment, the 5061 would drift about one second in 30,000 years. :)

Nice to see she still works. A year or so back I bought one of its great offspring an Ebay Rb oscillator. Also have a couple of GPS disciplined oven crystal oscillators running at 10Mhz. It cool to put them up on two channels of the scope and watch the phase difference. When you first turn on the Rb oscillator and it's warming up the phase sweeping back and forth looking for the lock. Then it gets closer and closer and finally it just snaps into lock. Just love the idea of owning something containing a "Physics Package". What I found interesting is what I call phase breathing. Over a period of 10's of minutes the phase error may be rock stable then it will breathe and slowly drift then be stable for a while again then drift back. Almost never slips more than one clock cycle at 10Mhz. What I would like to know is how much of this slow drift is from the GPS vs the Rb oscillator. With just two instruments I can't tell who is drifting. Because the drift is relatively short term its most likely the GPS. Got enough coax to run me a 5MHz reference to my Sunnyvale lab? Craig

Lived with these as a Loran technician for the U. S. Coast Guard. I was the project manager in the early 2000s for the upgrade to the Agilent clock.

Fascinating video. That's the first explanation of a caesium clock I've ever seen that I fully understood from beginning to end.

Was surprised to see the name Agilent inside that box. I guess they serviced this sometime after 1999?

This was very cool! The first thing I noticed when the cover came off was the "Agilent" logo, and I was confused due to the age of the instrument. When the Lissajous figure was stationary, and the scope traces were precisely in phase - wow, that was something. The scientific explanation sounded pretty solid to me, but it has been 37 years since I had physics and 39 since I had chemistry.

The nod to _Cody’s Lab_ gets my hearty approval. It’s nice seeing excellent science educators acknowledging each other. Now I need to go catch up on Cody’s recent videos.

My physics lessons at school were one of the more interesting parts of my education - however - this content is on another level - fascinating and I would have thought perfect to trigger new interests from anyone who watches. Should be part of curriculum's everywhere.

First result for cesium clock, 100% the exact level of detail i was looking for!

This was fantastic. I never knew how these things worked and I believe you did a fantastic job explaining it. Suggest a Kickstarter when this clock runs out of cesium. I'd kick in.

Beautiful video. I have watched it whole and I rarely do that with other videos. Especially the ones involving quantum physics.

Amazing to consider that this box of tricks was built a mere 50 years or so after Rutherford formulated his model of the atom (around 1911). Thanks for the video!

VERY nice! Takes me right back to my career working in electronic calibration laboratories in the 1980's and 1990's. Just discovered this channel and ... Subscribed!

One day when I'm very old, I'm going to build a replica of one of these Caesium tubes and the necessary (and very tricky) magnetic C field system. I've got most of the hard vacuum kit, I can do a reasonable job with vacuum glassware, and the microwave and PLL are very simple these days. Caesium ampoules are easy enough to get hold of and I have some ultra-stable and well aged 10 MHz quartz double-ovened oscillators that can manage a few parts per billion stability without locking. I've already got some Rb references and several Quartz oscillators that are double-ovened and kept in a vibration-isolated rack case in a temperature-controlled enclosure. Also five GPS-locked systems. It's interesting to see the occasional GPS phase glitches. An amusing issue happens when two references are very VERY close to lock, sometimes they jump into phase-lock where one is injection-locked by tiny leakage signals from another, same as when two pendulum clocks lock when mounted on a slightly compliant mount. For proper time/frequency geeks, the Time-Nuts email forum is an absolute must.

I can't upvote this enough. Thank you for the clear and concise explanation!

What a fantastic video Marc! Excellent discussion and explanation of this type of clock

"in soviet Russia you do not measure atom, atom measures you"

I learn so much from you and your friends Marc and with such ease from your superb descriptions. This is the next best thing to being in your lab with you. All the best.

Love the hyperfine transition discussion. So fun. Thank you.

Thanks for posting Marc

I have such a hell of time following exactly what is being talked about in these videos. Im a musician, not a scientist. However, I find this stuff super fascinating and despite my issues fully understanding everything, I watch all the way through. Great work :)

The Navy's AN/BSQ-4 Precision Frequency Standard contained two of these clocks. They were so reliable we almost never had to do anything with them but monitor them.

I saw your interview on Gizmodo, very cool! I also saw the 1401 you restored at the CHM, really awesome.

I've always wondered how atomic clocks work, and your explanation of how they utilize the Stern-Gerlach apparatus finally put it all together for me, MANY thanks !!!

Very cool! I've read the manuals for the HP cesium clocks and thought those were fascinating pieces of equipment. It's too bad the cesium tube has a limited life span, but glad to see you found a working one.

I never understood the principles of the caesium clock until today. What a wonderful, clear, explanation. Thanks, mate, from Chris, in Tasmania.

This is great. Looking forward to the expansion of the finer concepts

Thank you, no really, thank you for showing us this your channel is now officially my favourite channel

I used to play around with precision clocks. Every time you plot them on a scope together, and see them hardly drift, it's a magical moment. That you can do these things in your own home is incredible. (I was using a rubidium clock, but those have about the same accuracy as a cesium clock from that era.)

Thank you for this amazing video. You are very good at explaining things!

Beautifully explained, thanks!

Leo Bodnar makes some very good GPS clocks. I treat the other ones as suspect. Now, going back in my personal time machine or at least memory I visit 1966. I was working at a University of Michigan lab. We were measuring partial pressures of various atmospheric constituents at high altitude. We were using an Omegatron, a U of M invention I believe. The measurement was a similar tune for peak. It was at low enough frequencies to use little IRIG electronically variable frequency oscillators. They tuned it to peak in the lab. Um, later on they tuned it off peak a little. More on this later. The use model involved tossing a payload into a Nike Ajax nose and seeing what happens. They quickly learned that their magnets did not take well to the shock of the launch. (It's later.) They attempted to compensate for this by tuning off frequency a little hoping they'd arrive at the peak after launch. They decided that would not work. So they adopted the same sort of strategy as used in the 5061a. They used got fair accuracy, not what they intended. That is where I entered the picture. A little analysis later I figured out their demodulator developed biases that were not nice constants due to the sinewave sweep over the peak. The even harmonics were doing this. Betcha many reading this know what I did next. I changed the 30 Hz (if memory serves) sinewave sweep to a squarewave step to either side of peak. The second harmonic errors vanished, of course. The calculated error of the level of the peak fell to so close to zero I never published the real figure. I published figures for wildly imprecise components. Set it up in the lab and the error in free fall was reduced to parts per million from parts per 100s. I saw applications for this. I may be clever with electronics; but, I could not and still cannot sell everlasting solar powered refrigerators to people living in the Sahara desert. The person I worked for dismissed it with a ho-hum. In found somebody else had finally suggested this in the 1970s while reading a Proceedings of the Frequency Control Symposium. I felt simultaneously vindicated and cheated. I also swore off bothering about patents after that. The patent I do have, under a married name, was an accident. I was blind sided. Later on I was responsible for the frequency synthesizer design for the Phase IIb GPS satellites, the ones with two Rb and one Cs frequency standards. But that's another story. {^_^}

Had one in a military installation. It was set using the audio shown on an oscilloscope to sync it's 1 second pulse with the WWV tick. We had a delay we used for both the delay of the receiver used, and propagation delay from Fort Collins Colorado, so the clock would tick just before the signal on the radio was received. This was in the late 1970's before GPS time distribution became the new standard. Looks like we got one after they were out a while and proven in the field and the price reduced. On one shift, one of the new guys made the mistake of changing it for daylight savings time.. It was supposed to remain on UCT or GMT. When it was first installed, and set, it was installed on the dayshift, and I had the swing shift. I had to call my supervisor as it went into alarm for low battery. It was jumpered for 240v, but was plugged in on 120v, so the battery died on shift. It was New, Expensive, and thus I was not permitted to touch it. On watch just checked for the drift between the clock and WWV. Not touching it, I did check the setting for the line voltage. It did die on shift for low battery because I was not permitted to touch it. I got to work with one when it was cutting edge and just out on the market. Nice piece of history.

I remember having one of those at Bell Northern Research in the early 90s. IIRC it was used as a reference clock for validating the VCXO frequency used by telephone switches for trunking.

Chapeau Monsieur curious Marc! This is by far the best explanation of this fascinating piece of equipment I´ve ever seen! If I could i´d give 5 thumbs up!

Thank you for your efforts to explain this experiment.

Thank you , this was excellent theoretics and lab work.

I am astonished by the sheer amount of cool toys you guys get to play with! Good work!

That looks like a day spent well!

This chanel is getting better and better! :)

Heady was an understatement but still very cool stuff. Keep it coming.

Wow love the content!

Locked to atomic time. Huzzah! Coolest sound effect in a UKposts video ever. Kudos!

Thank you for another very interesting video. Looking forward for other ones. Please stay safe.

My goodness, this is quality content, ! i subscribed.

Great overview of the clock’s operation. A few years ago, I ran across some open courseware videos, from UC Berkeley as I recall, on an attempt to make an atomic clock on the chip level in MEMS technology. I’m not sure if they managed to make it work. Isaac Asimov’s physics books provide an interesting overview of the basic theory behind quantum mechanics.

what a beautiful piece of tech!

Amazing science, I never got properly before. You are a great teacher

We had one of these cesium clocks on our satellite communication's facility back in the eighties. Filled with dozens of cables connecting all the distribution amps together, all of which didn't like to be touched.

Really great content ! The build quality of old HP Gear is really outstanding. Some Versions had a Patek Phillipe dial at the front, was this an Option ?

Fantastic stuff!

wow supplier catalogues that list prices, that's novel. I'm used to "SEE DEALER FOR A QUOTE" on everything

My friend wanted on the clock team in 1974 for the Air Force PME Lab AKA PMEL which worked in.

Always a pleasure to seen some vintage HP goodness. These guys were the top of the best in groundbreaking lab tech. The fact HP recessed to a second zone home PC and disposable inkjet printers manufacturer puzzles me. Rise and fall of empires... Anyway Agilent was spun off HP in 1999 so that caesium tube is pretty recent.

Brilliant video!~ Thanks

We used a variation of this model as part of the VERDIN VLF communications receiver back in the 80's. We never had one problem with it in the three years I worked on the system.

I’m literally glowing from all this lovely new knowledge in my head 🤓

I might need to watch this one more than once.

What an absolute beautiful pice of equipment.

The most satisfying 8 zeros I've ever seen on UKposts.

Great video. Do you have to set the height above sea level on the Caesium clock?

I understood this very well ty for explaining!! :D

I love the fact that there's an atomic clock and a bench grinder on the table at the same time! Probably not that ususal of a setup 😁 This made me understand how a cesium clock actually works. Amazing explanation!

Géniale cette Video Marc merci beaucoup ! from Paris

This stationary Lissajous pattern was just gorgeous.

when you say that your mind will be blown and when I watch it it's like it makes total sense to me

Marc, you always pause just briefly before you say "H.P." it's almost like you're preparing to speak a powerful magic word or utter the name of God or something. ;) Thanks for the Quantum Mechanics explaination, you managed to explain it better than a lot of the physicists I have listened to. That oscilloscope was... ... ... too modern! ;)

Thanks for the physics lesson portion of the video. At 3:40 I was surprised to see that the unit on the left spelled the chemical element as Caesium. I've never seen it written with the A before.

Instead of Lissajous, better use Time Interval measurement on your 5334B, between 1pps of Cs clock and GPS clock (or OCXO), and use TimeLab over GPIB control to collect the phase shift. Data collection and averaging over several hours / a whole day only will mitigate any GPS signal jitter, and only then will give you the necessary resolution to calculate the accuracy of the 5061A. GPS receivers usually need several hours, or 1-2 days to really get stable, i.e. to learn the behavior of their internal OCXO (if available) vs the jittered GPS time information. So your momentary snapshots of phase (out of the Lissajous figure) is probably useless. Cheap GPS units probably never manage to achieve a short term stable time signal, (use ADEV analysis) compared to e.g. the Trimble Thunderbolt.

When I was an undergraduate physics student in Chicago in the mid1970's, my (very nice) parents bought me an HP35 calculator. One very hot day, the case melted inside my car. I took it to the nearest HP repair facility (in Niles, Illinois); they swapped shells for me at no charge (but broke one of the battery compartment clips and destroyed the back decal, unfortunately). Afterwards, the repair tech took me into the back to show me their brand-new HP cesium clock. Amazing! A year later, I did a summer internship at Argonne National Laboratory, where I put together a data acquisition system from shipping containers full of fresh HP2016 parts. They had allocated the full summer for me to do so; I had it up and running (BASIC interpreter and TREK73 game) by the end of the week :-)

Great video thanks.

10:10 Totally clear, in a "I could not repeat it, and I could not point at the parts, but it makes sense, and meshes with the tiny amount I understand about Relativity, and the even tinier part I understand about Quantums" way.

The degree that humanity has come to understand the physical world never ceases to amaze!

Great explanation. It is amazing this device was commercially available only a few years after the LASER was invented! Nowadays atomic clocks take advantage of lasers for optical probing of hyperfine transition in Cs vapors. Atomic beams are no longer used. The clock can therefore stay operational for many years without need for "refueling".

Sublimely awesome. These units must be hard to come by.

Well, my brain definitely does hurt now! But only because this was so fascinating I couldn’t stop watching!

Fascinating stuff. How much signal interference do the patch leads and the wiring of the clock give in your measurements, if any, and would shorter one's or different conductor materials and construction make a difference, as in audio equipment, where you can tell between different cables????

FYI the power connector shown at 3:15 is a Mil C 5015 connector with what I think is the 14S-1 connector arrangement. Due to their robustness and utility they are still being made and are available from the likes of Digi Key.

That was pure geekville and I loved every second.

I was going to mention the tube replacement! Score! Agilent is a pretty recent name...It came about in 2000. The test equipment division was spun off again as Keysight in 2014.

GREAT VIDEO !!!!

> But what makes it tick is just mind blowing. I see what you did there.

23:02 you can change the rotation of the image on the scope clockwise/anti clockwise just by thinking about it, try it!

I had a couple of friends who worked for HP in the Rohnert Park area, it seems like it was the microwave test equipment division mainly, and I got the tour on one Saturday night, walking through production and testing, into the anechoic chamber, etc. Later they showed up to work one Monday, to the "Company With No Name", because HP had spun them off, but without a name! They were no longer HP, just "...." Not even "The Artist Formerly Know As HP" LOL After some discussion, they decided to let the employees choose a name, and so suggestions were made, and then they started voting, and finally ended up with Agilent. So yes, that was certainly last worked on since then, so that '89 date isn't the date of the tube.

Awesome Marc etal.

REAL ELECTRONICS TEST EQUIPMENT in hand. Your da Man. Really enjoying this stuff Sir. Did you Calibrate all of your test gear while this was in hand ?

That's twice I've seen Marcel in a video and twice where I thought: "what's the singer and bassist from Carcass doing here?". He's hiding his Scouser (Liverpudlian) accent very well, lol Nice of him to bring some 'Toys' around for you to play with, Marc.

How much runtime do you get on the cesium source? And how much (time) do you think it has left?

In US Air Force Precision Measuring Equipment Laboratory we had one. It had to have a yearly touch up on the frequency due to the slight drift, that difference between -11 and -12.

I don't know about the 'usual' PCB-faced GPSDOs that are on ebay like that one, but I have a very slightly fancier one that exhibits instability for a bit when the output 'load' changes significantly, to the point where I could connect/disconnect a device on the timing circuit and watch it hunt on the scope.

Very cool!! I’m subscribing