I Thought we'd take a look at what are the more interesting things you can get on eBay and you can build up for your lab. It's a Rubidium frequency standard in this case the Fe 568a. You can get them pretty cheap on eBay So I thought I we'd do a tear down i' build one up, see how it works, and maybe build it into a little uh, custom case and hook it up and use it as a lab frequency standard primarily for my frequency counter, but you can use it for anything. Let's go.

So what exactly is a Rubidium frequency standard? Well inside this little cam which you can get for about 50 or 60 bucks on eBay used of course. Uh, you can't buy and brand new for that? Not a chance. They're usually you know, thousands of dollars, but because these are used in uh, cellular telephone towers and things like that to maintain an accurate frequency reference when they, uh, you know, discontinue or they decommission those frequency tows, tons of these things are left over. You can get them really cheap.

and what's inside this can? Well, it's basically an atomic frequency standard and you've heard of that before. You've heard they're used in GPS satellites and all sorts of advanced communication systems. Well, uh, there are a a few ways to actually get an atomic frequency standard. You can use Hydrogen Cium and the other one is Rubidium and that's a very popular one bit of a poor man's frequency standard.

This isn't going to uh match a GPS satellite, not by a long shot, but it's pretty darn good and it's going to be a super accurate frequency reference for your lab. So let's take a look at what's inside it. It's a bit complicated, but stick with me and here it is: I Know it looks complicated, but it's It's not that bad and this may not be 100% accurate, so don't about the details. Okay, if you want to find the real details, go look them up.

But I'll give you a basic rundown of how a Rubidium frequency standard Works shortened to RB that's the chemical, uh, you know, uh symbol for Rubidium. Now what we have here is what's called a physics Package I Love the term physics package because it it has a bunch of physics inside a little metal can that basically does some magic here with some Electronics with a frequency lock locked Loop and FLL you might have heard a PL phase lock loop. This is a frequency locked Loop where it basically has an internal o Ator Um, just a regular Crystal oscillator inside. uh, your standard.

It can either be a oven controlled one it might be that, or it might be you, a voltage controlled Crystal oscillator, a voltage controlled oven one, or digitally controlled or whatever. There's several ways to do it uh, but let's just assume it's a voltage controlled Crystal oscillator working at a uh fundamental uh frequency which is a multiple of the Rubidium uh transition frequency of roughly 6.83 4 GHz that's the transition frequency of the atoms and if you want to find out the exact details of the hyperfine transition of Rubidium, the Rubidium atom and all that sort of stuff, you can go look it up, won't go into it, doesn't really matter, but basically, uh, the physics package up here helps Um Ser It works as a Servo and it servos this Uh amplifier and keeps it locked to the transition of the Rubidium atom in inside this frequency package and then from that, it can just generate your Uh frequency standard out. In this case, 10 MHz is a pretty industry standard Uh reference frequency, and any good lab if they have a frequency standard will have 10 MHz output and that's with a lot of uh, Well, that's what most Uh frequency counters um will have an external frequency input for the reference oscillator and it's almost always 10 mahz. So uh, that's what we're going to get out of our little uh, uh Rubidium Frequency Standard today.

These are what it puts out: 10 MHz but it's disciplined around a much higher Uh fre, an intermitted frequency of 50 odd. In this case, it's 50255 MHz. But other Um implementations of Rubidium frequency standards can use different frequencies and different techniques as I said, Adcs and Dax and digital control. but uh, basically what it does is it Um has a discharge lamp up here, which actually this Uh takes quite a bit of uh, Power Which these Uh Rubidium frequency standards do actually require quite a bit of power to actually operate.

you know, in the order of 10 plus Watts or something like that. So uh, it has a Uh, a Ridium 87 discharge lamp. If you want to know what 87 is, go look up your physics textbooks and uh, it has a resonance cell which is a little cavity. Just a little cavity which um, the uh, Rubidium gas atoms will be inside there and they get all excited when you apply um a certain frequency to them.

and uh, this resonance cell can get upset by external magnetic influence as well. so it's usually in a Uh Mu metal shielded can so that external magnetic fields don't affect it. and there's some other things that they can do to try and reduce the effects of external magnetic fields as well. And then there's a photo cell on the output with a Uh Transm amplifier to detect the amount of light coming from the discharge Uh lamp.

And the whole idea is that uh, when the resonance cell is exactly at 6.83 4 and there's digits beyond that which is the Uh hyperfine transition frequency of the Rubidium atom, they all resonate. All the atoms inside there resonate and there's less light going through. or it blocks the light going through to the photo cell and that generates an error voltage out of the amplifier. and then it can Servo that and keep it locked in.

and uh, they might sweep it by a couple 100 htz or thereabouts around the transition frequency from this RF generator here. But that's basically what it does. It just uh, blocks out when it hits that resonant point, the uh, less light gets through the photo cell and boom and then it can adjusts the frequency and it settles down and locks in. So that's why these things might take um, a few minutes, or some might even take out tens of minutes to actually lock in and stabilize.

But once they do, they're incredibly accurate. So it's a basic rundown of a physics package. and how a Rubidium frequency standard works. Pretty neat I Love it.

Magic! So why does all this frequency stability matter? Well, I'm glad you asked. Let's take a look at it in terms of, say, a basic frequency counter. and let's take a look at some crystal oscillators now. Uh, I've got four different types here.

One is you standard quartz crystal which you're probably familiar with one of these little Crystal oscillator cans, or just one of these little, you know, regular, uh, quartz crystal oscillators you've used in your projects before and they're pretty darn ordinary. Uh, they're in the order of uh, 10 to 100 parts per million? Uh, tolerance? over over a standard temperature range? basically? Uh, this tolerance, um, table here takes into account uh, temperature. basically. So one, what's one part Per million? Well, one part per million is typically specified on the data sheets as 1 * 10^ of - 6.

10 ^ of 6 is a million. There's one part in 1 million and so on. 10 Power - 7 10^ - 8. So these standard quartz crystals are pretty crappy.

They're 10 to 100 PPM and the other thing with any oscillator. whether it's quartz or any anything else, they're all pretty. much all of them are based around Art Quartz. Whether or not their temperature compensated or their rubidium compensated as we've seen, they're going to have not only a tolerance over temperature, but they're going to have an aging Factor as well as they get older and that, um usually is specified per year, but can also be specified.

Uh, they can give you additional figures per month and per day as well, because you might want just a very shortterm shortterm stability over at one day. But let's just take look at some typical figures for a year. These things. Not only are they not that great, just, um, basically.

uh, at any one point in time over a year, they're also going to age 5 to 10. PPM might be a typical figure per year. so they're just going to get worse and worse. Now, if you look at a digitally uh, temperature compensated Crystal oscillator, that's where it's a regular Crystal oscillator, but they've actually measured it.

They've characterized its performance, and they've actually programmed in compensation values over temperature. So hence, digitally temperature compensated Crystal Oscillator and they're pretty much you know they're going to be like an order of magnitude. uh, better. 0.5 to 5 PPM uh one with one PPM aging order magnitude better stuff.

And then you get onto an oven controlled Crystal oscillator. That's where they actually keep a a regular quartz crystal like this at a very specific temperature. So the ambient temperature the Uh temperature in your lab which drifts all the time, doesn't matter, so they they can get very stable. Um, in terms of you know, 0.1 uh PPM 10^ minus 7 over the span of you know, a year.

that's the Aging sort of thing. and the tolerance is pretty darn good too. And then you take a huge step up to the Rubidium type stuff 10 the^ of -1 we're talking about with 10 the^ of Min - 9 aging. Absolutely incredible.

And of course, these are crystal oscillators. They come in all different types of manufacturing. what's called Cuts They cut the crystal in a different way. you know SC cut AC cut crystals and that can affect the Uh temperature characteristics and the Aging characteristics and the shock characteristics and all sorts of vibration characteristics.

Everything for these crystals. I Won't go into it, but let's take a look at how it relates to a frequency counter. So here's a typical uh, good old school uh frequency counter. It's a Philips PM 6672.

You can pick these up on eBay Going to have to do a tear down of this one? Um, and it's got uh, well, basically. um, it comes with several different types of oscillator options. When you buy frequency counters like this, whether they're the one hung low cheapies on eBay or they're the good ones. you know, name brand ones like this, then you're going to get different types.

and if you even this a good one like this, if you buy it just with a standard oscillator option, it's just going to have one of these regular, you know, uh, quartz, un temperature compensated oscillators in it'll be you know, a good one. A pick of the bunch. but you're still only going to get um, you know, 5 to 10 PPM Well, what does that? Uh, how does that translate to how accurate it is on the display here? Well, let's take a look at it. This is measuring a 10 MHz uh frequency.

Okay, so and it's actually displaying not in mehz displaying in KZ there. but it's 10.0000 megahertz. Now what's 1? PPM Well, if it was 1 megahertz, then one Hertz one, One millionth of that one part per million. it' be this least significant digit here.

But because it's 10 MHz it's going to be this digit here. So if you had a 1 PPM accurate crystal in there for example, then it would only be accurate to that digit there So the least significant digit there is absolutely useless. Plus, you've got aging. On top of that, you got temperature and all sorts of other stuff.

So you know if you've only got a one PPM crystal that's not that great. But typically you might get a 10 PPM Oops. you've just jumped up to uh, this digit here and the last two digits are useless. not to, um, take into account aging.

so it's pretty horrible. So generally for a frequency counter, this is a regular 8-digit one. you might have a n-digit one. You're going to want a pretty stable oscillator in it.

so if you buy one of those, you know, $100 or $80 One hung low brand frequency counters on eBay They just going to have a cheap as crystal in it and the last two or three digits aren't going to matter a rat's ass. Now here's the oven controlled uh oscillator inside this frequency counter. It's got this option. It's got the specific PM 9690 /1 option and I've had that on and you, trust me, that is actually quite warm.

That's why these things can take 10 or 20 watts of power. Um, just to keep them at a stabilized temperature inside. And as I said, inside there, they've just got like a a regular uh crystal in there, but it's going to be a really stable one. They've chosen it for a really good uh cut, but because it's kept at a constant temperature temperature, then, uh, once you let this thing warm up to temperature, you have to do that with these other control ones.

They got to warm up to temperature first, but once they're there, they're incredibly stable. This one. Um, if you look up the spec sheet for it, it's uh, you know, got a tolerance of in the order of uh, you know, 0.1 uh, PPM or uh, aging of 0.01 PPM. So pretty darn good.

You'll notice too that it's got an adjustment trim pot in there. and uh, these things aren't magic. Of course, you've actually got to trim them to the correct frequency, but once you do that, they don't drift much. So having that oven controlled oscillator in here with 10 the power of uh, minus Uh 8, um AG In then we're talking about the one digit past the least significant digit here.

In terms of Aging In terms of accuracy, we're talking about the least significant digit there, so it pretty much matches the capability. sort of. ovenized oscillator matches the capability of a typical 8-digit frequency counter like this. So, um, pretty much you're going to at least want if you got a if you got a good frequency counter even, or if you got a nine-digit one For example, with an extra decimal place, you may actually, uh, well, you at least want a oven controlled uh oscillator? Uh, with that sort of stability in that sort of Aging for this.

But we're going to blow it out of the park by using a Rubidium Uh standard and we can just feed the 10 MHz reference signal in the back. and Bobs Your uncle and that Rubidium Frequency Standard will absolutely guarantee that, uh, regardless of aging and temperature, and all sorts of stuff, what the frequency you get on your frequency counter is going to be exactly correct to? well, it's going to be plusus one, uh, least significant digit and that can be important for all sorts of Niche sort of, uh, stuff. You work on ham radio as well. Having an ultra stable frequency source is a big thing in the ham radio, uh, circles.

and there's some other timing applications, timing counting applications in the lab where, uh, you know a really good uh laboratory 10 MHz reference Rubidium standard. It's worth its weight and gold and you can buy them for 50 bucks so you know it's well worth building one. So let's take a look at this one we've got from Frequency Electronics Inc eBay is absolutely flooded with these things. so I'm sure you'll have no problems getting one for you know, 60 bucks? maybe? uh, in including postage.

or there might be postage on top of that. But these things probably cost thousands of dollars. Maybe, well, maybe not $10,000 but you know they would have cost, uh, several th000. Uh, brand new.

and they're a brilliant Rubidium Frequency standard. This is the Fe 568a Um, there are several uh versions of it and they're all used. You won't pick up one brand new I Don't think they're all uh, pulled out of old. uh you know, GSM cell phone B stations or something like that.

So um, pretty much uh, they'll all be secondhand. but if you buy from a good eBay seller, they will have actually uh, tested them now. Um, this one actually. uh, comes with the pin out.

It's got a standard D9 on here. it came with, you know, the D9 connector and a bunch of uh, flying wires there. and uh, pin one is the input it um from 15 to 18 Vols uh DC um I Don't know whether or not it can go below or above that, but I'm going to stick within that uh, it's got two ground pins. It's got a frequency lock um output so that uh, you can drive a a a lead indicator direct you put in a box amount of lead on on onto there and I believe it's active low and that will indicate that uh, it's um, it's warmed up and it's locked in and your frequency output is spot on.

It also requires a second input of 5 Vols on pin four there and then you've got your RF 10 MHz out and it is actually RF it's a sine wave um and it's uh 1vt uh Peak to Peak into an open load or 0.5 volts Peak to Peak into a 50 ohm load. So uh, let's uh Power it up and uh, see how much power it takes. Okay, let's power this thing up and see what we get. Uh, it's got a 15 to 18 volt uh input which I'm measuring the current here I'm using 15 volts on the input and it's got a 5V DC input as well.

uh, which I'm measuring the current on the fluke here. so uh, we expect there to be a bit of a power Spike at power on until it, uh, settles down. Um, so it should have. It should start up at a higher current and then slowly settle down to a lower current.

So I'll switch those supplies on both at the same time and see what happens. There we go wh 1.6 odd amps on the 15 volt Rail and basically just 100 odd milliamps, just under 100 milliamps on the DC Rail and haha we have Taada our output signal already and there it is and it is 10 MHz at 1.3 volts. uh Peak to Peak cuz there's no load on there. no 50 ohm load because if we actually switch on the 50 ohm impedance load, of course it will, uh, drop down.

but there you go, but uh, hasn't locked in yet. I'll have to um probe the logic output and see how long it takes to lock in, but the frequency has actually um outputed stra straight away. Okay, let's try that again. Uh, in this case, the Uh yellow uh Channel 1 waveform is our oscillator output.

uh Channel 2 1 V per division. that's our Uh lock output. So I expect a logic level output I believe it's active low once it's locked, so it should, we should, uh, actually see it jump up and I'll switch them both on and we'll time that and see how long it takes bang. It's actually High There you go.

So it hasn't actually uh locked in yet. So we're talking 1, 2, 3, 4, 5 volts and boy, there we go. There's some uh, something happened to the RF output there, but it hasn't locked in yet. We should.

After a minute or so, we should see that. uh, Green Line the Channel Two line there drop to logic low to indicate that it has locked in. Bang There it goes, and it's now locked in. and uh, what values have we got down here on the meters.

We have about 76 6 Uh amps there, 77 amps on the 15v rail, and 90 odd milliamps on the 5V rail. So as you can see, it actually draws a fair bit of power. so that's almost 12 w total power consumption for the device. and it only took about 35 odd seconds to lock in, so it wasn't that long at all.

and that case is getting pretty darn warm too. And uh, if we try and attempt to probe that we're talking, you know it's well over 40. and I've only had it on for like 5 minutes. So these things can actually get quite darn hot.

So you wouldn't want to mount this thing in like a sealed plastic Jiffy box or something like that. It just gets stinking hot. What you want to do is Mount one of these in a a diecast um alloy uh case or something like that, one of those extruded aluminium cases or something that, uh, you can get the heat out of this thing I don't know what uh, temperature will do do to these things. uh, long term and well, I don't really want to know I think so um yeah, it would be much, uh I think it'd be very beneficial if you kept these things as cool as possible.

And there you go. We're almost getting 50. There seems to be a hot spot over this side of the package over here, but it'll Yeah, they get pretty darn warm now. There are a couple of Uh options on this, uh specific model.

One is to actually get an RF um output uh Sep RF output connector here and also there's an option to get uh, digital frequency control so that I believe it's like some sort of Serial input Rs232 type input and you can send commands to it to actually uh, generate uh a specific frequency from0 to 10 or 20 MHz or something like that and that would be really nice. but I don't believe? Uh, this model uh actually has that. It's just the fixed 10 MHz output, but if you can get that one with the digital uh frequency control, that would be really, really nice. You could turn one of these into a really nice, fully programmable uh lab uh Rubidium frequency generator with a keypad and a microcontroller.

and LCD and you could set the frequency to anything you wanted and it'd be terrific. but we've only got this fixed 10 MHz one here. Oh well, yeah.