Hi welcome to Tear Down! Tuesday You don't have to worry about me getting lost in this video because I've got an EB or an electronic position indicating radio Beacon one of these emergency beacons that you carry when you're hiking or you're out boating or something like that. If you get lost, get in trouble. you know you're about to die. Flip the antenna, push the big red button and hopefully somebody comes to rescue you.

Well, that's the plan anyway. I Actually know quite a few people who've been rescued by one of these things for in under. You know, when they're hiking or canyoning or doing various things like that. Very popular little devices and life-saving devices.

This is the older model eerb. this is the minisat alert uh, personal eerb from Kti. It's an Australian company designed and built here in Australia designed for our local regul ations cuz all countries have different regulations for these things. Um, this is actually an EB uh, but they're also known as uh Plbs or personal location beacons EBS are traditionally designed for the Marine uh environment.

They can be self like floating, self- rting with an antenna and self saltwater activated and all that sort of stuff. This one is designed to carry in your backpack when you're hiking or something like that. or maybe on a little boat or something like that. It's not a true Marine one, it's a Personal Locator Beacon Effectively so this is the older model5 Mahz there it is.

125 MHz SL 243 mahz are analog system. They've actually phased this out now. so I think if I activated this um I'm probably not going to get rescued here in the lab. In fact, the signal probably wouldn't get out of the lab here.

You have to have pretty good conditions for a, you know, fairly Open Sky for these to be picked up by the satellites. Now this one's compatible with the Cpass and Saret Uh satellite system and it's the older analog system. The newer digital system is Um 406 mahz and that sends out a digal digital signal in addition to a Uh. If you've got a GPS equip unit, it can send out your exact GPS location.

But this one. Um, even though it's analog, it uh I think the Search and Rescue helicopters when they come out to rescue, you still have a 121 mega5 megahertz uh radio location finder on them so that they can pinpoint your location. Because when you activate one of these and you signal is picked up by the satellite and they triangulate your position, it is very rough. You know they can't actually get your exact location from that.

So when they call the helicopter, they fly out and then they use a Uh radio finder in the helicopter to narrow down your exact location. So that's why these that's with these older analog ones, but the newer digital ones, they can send out your exact GPS position. Bang! They know exactly where you are, sends out a digital code um that you pre-register and they know who owns it and they can contact your family to get additional info of exactly where you've gone and all that, what equipment you're carrying, all that sort of thing. This is the older analog one we're going to tear it down.

Could be quite interesting I wonder what's inside this thing? It's waterproof. It's shockproof. It's got 10year Lithium battery in there H But I hope I will try not to set the thing off. Let's go.

and I've got to thank Julie Burton who's a canyon in uh friend of mine for giving me this unit for the tear down. She doesn't need it anymore. it's an old one she used to carry on her Canyon in and uh, hiking trips so thank you very much. Julie Now as I said, it's a Kti manufactured by Kinetic Technology International here in Australia made in Australia not Austria and uh, its battery has expired July 2011 but it's still going to work because uh, basically these have a 10-year Lithium battery in here I Think this has a Ciz a Csize Lithium battery in it and they will typically have a 10-year shelf life, but they specify the replacement time as half of that or 5 years so this one would have.

The battery would have been installed and it was probably manufactured around 2006 or thereabouts, hence the Uh July 2011 replacement date and uh, it is the Miniat Alert person. Personal Eerb 121.5 MHz 243 MHz analog system and there isn't really much to them at all. They are very simple units. They've got some instructions on the back here how to operate, break the seal by raising the aerial and then fully extend upright.

Press button until red light flashes on land, place in a cleared area away from people and obstacles that C attached Beacon to your buoyancy vest and there you go. And of course it's got a test mode which we can try out because it doesn't transmit in that mode and as you can see, it hasn't been used. Julie's never used this thing Brak seal to raise aerial because the whole idea of these is hopefully you never have to use it. Um, cuz if you have to use it well, you know it's going to cause a lot of grief to uh, somebody, especially yourself if you've done an injury or something like that so it has never been uh broken because if you want to use it properly, you just extend the antenna up like that and push the button and it will transmit.

But it has a test mode. If the antenna's down like this, we can just press it. Here, We go there, we go. So presumably it does, um, like an actual uh, self test and maybe uh, you know, actually transmits internally, but not to the antenna.

It disconnects that of course because you don't want this thing trans actually transmitting to the antenna just for the uh test. So maybe it's got a system in there that, um, actually transmits the signal something that reads it back and then verifies it. So I'm guessing that there's a micro uh controller in here or something that, uh, that's fun. There you go.

and uh, so yeah, it's probably got a micro in here to do that Self test functionality cuz I'm sure the self test is just more than flashing an LED you know, and the batter's working I I Think it's probably a bit smarter than that. Um, to make sure that it's still operational. Anyway, it's got a lenyard here on the bottom. You open it up so you can attach a lenyard.

Oh, warranty, void if broken. We love that stuff. and uh, and apparently you can. These are repairable and you can replace the batteries in them.

So presumably there's a screw under there. If we undo that and that, we'll probably pop that bottom off. Probably o-ring sealed around there. But only one way to find out.

Let's void this warranty, you bloody. Ripper Here we go. There, We go. There's the screw.

t Uh, let's open this sucker up and presumably there will be a, uh, some sort of way to detect that that antenna is down as well. Um, because, ohy, there we go. Oh, look at that. Oh man.

check that out. Okay, so that obviously goes right up through there and goes into that metal thread. I Think I can see like a a metal threaded uh insert up there. so it goes all the way through and probably keeps it all together like that, so that's rather interesting.

This one doesn't. Oh yeah, there we go. Yeah, two of them got two long rods. Oh that.

Yeah, there we go. We got some o-ring seals on there and presumably it comes apart. Maybe I have to break off this tape around here or something. Oh, there's the antenna.

By the way. it is an extendable antenna like that. so I'm pretty sure we won't uh, transmit on this thing if we just disconnect the antenna and it's not going to get out of the office anyway. I think even if the antenna was extended, so we should be able to measure the output of this thing.

So let me let me actually take this tape tape off cuz I think that's probably holding the two halves of the case together Tada be able to. It's a bit springy, bit springy, but it is coming loose. Yeah, see the copper shield in there? Well, I Was expecting this board to be bigger, but check it out, it's not that big at all. All circuitry is under that.

uh, what looks like copper? Uh Shield there. and there's the looks like two C- size batteries down in the bottom. Yeah, here it comes. it's just going to pull out.

That's it. too easy. And there's our Piso Trans juicer down there that makes the uh beeping noise when we test the thing. But jeez, there's not much to it.

And I was spot on with the date there it is. May 2006 5 years replaced 5 years after the date of manufacturer. So we've just got two SE cells. Looks like it maybe has some protection stuff in the top.

Let's have a look at the board, which is rather interesting. The first thing to note is a standard PC test socket. Here these are for 2 mm uh probes. you can you can buy these from Digi Key: They're just off the shelf stuff.

I've looked at these looked at using these before and they're designed for 2 m. Like actual. uh, like they're than the exact width of your multimeter probe so it can just plug directly into that. So and look at that spring there.

They've got this little. well, it's not. Actually, it's kind like it looks like it's a spring, but it doesn't do anything in that respect. that is clearly the sensor that is using to detect whether or not that antenna is up or down.

So maybe it's some sort of, uh, you know, capacitive, uh, coupling thing or or something like that because when you raise that antenna, it's not going to physically, uh, move that spring of all and it is electrically connected. Is that is that grounded? There looks like that point is grounded. They may still be using that as a capacitive coupling system through to the antenna. So the circuitry May sense the antenna and uh, measure that first before.

uh, you know, switching on a solid state relay or something like that to Output the RF signal to the antenna. so there's something going on there. It's rather unusual. and there's our 121.5 MH Herz Crystal which is used for both frequencies.

Of course the other one is, uh, just double that frequency and yes, it is held down with uh, some sort of epoxy adhesive or something like that cuz this is designed to be uh, shock prooof so we can easily just slide that off there and bang. We've got our board out of there and of course that is all o-ring sealed down in there cuz this thing is waterproof to like 3 m. so it's got various O-rings over the place and uh, that's rather nicely designed and we've got now board to take a look at that. We have to get rid of that shield.

no problem. should be easy peasy to get off. just heat up the copper. I could cut it of course, but easier just to heat it up with the iron and then just pull it off.

No drama. Got to have that third hand and I'm trying to get it on camera here, so ah, hate trying to get get things on. camera never works. Tada And here's the board.

and as predicted, there's the microcontroller. It's a motor roller MC 68 HC 705 with its own local oscillator is at 20 mahz I think or is that could even be two? No, that's 2 mahz. There you go. it's running at 2 2 MHz and uh, the rest of it is pretty much all analog Slrf stuff.

There's the uh, there's the button of course, which, uh, the test/ activate button from the front, and here's the antenna output up here. and the Crystal actually connects to the other side there. So there's the two points for the crystal. So there's going to be an RF oscillator ba around here somewhere.

Based on these transistors, these are all going to be Uh transistors of some description and they've got to modulate the Uh signals well, as well as uh, double it because it has to uh, be doubled to that 243 mahz as well. So that's clearly all done in the analog domain. which I guess is not really surprising because um, there's not. You know.

Why do it? uh, digitally? really? I Mean the new Um 406 MHz digital PLS uh. Plbs of course, would be much more uh, complicated and high-tech in terms of uh uh, digital control and stuff like that. but in the end, it's just got to have 121.5 mahz um oscillator. It's got to have a doubler in there to uh generate the alternate Uh frequency and some sort of modulation thing probably I don't know happening around here.

Perhaps that uh, this would be the RF transmitter I would suspect based on the size of the device, it's physically larger. These things output I believe an RF signal level of about 150 M or something like that. So um, you know it's not terribly, uh, high in the scheme of things, but they obviously need that slightly larger output transistor there to do that. and the rest of it is I Would presume that's modulation because that sort of comes from the microcontroller down here into this area around here.

so it's probably got some switches and some probably another uh local oscillator or something to modulate the Uh signal on top of that. But uh yeah, there you go. It' be interesting to see the schematic of this thing. You could, of course reverse engineer it.

It's only a very simple double sided board, but it would take a significant amount of time So, but if one has a schematic for one of these things, um, please post in the comments cuz it would just be interesting to see the Uh Arrangement that they've used to get one of these things working. so there's not much really left to tear down on this thing. That's pretty much it. so we may as well activate it.

Let's go. So what I'm going to do with this is I'm actually going to, uh, eventually, like disconnect the antenna cuz I don't want to accidentally transmit 121.5 MHz at full power even if it is inside the lab here cuz I believe Uh, commercial airliners can still pick up um, this distress uh frequency I'm not sure if they still actively monitor it or or not, but they have the capability to do that and have done in the past in case the satellites don't actually uh capture them. but um, what we can do is we can hook up a scope to the antenna output here and we can capture um, we can have a look at the waveform and uh, capture the signal. So what I'll do is I'll do it first with the antenna down and we'll see if it's actually transmitting anything to the antenna at all.

and then we'll actually disconnect the antenna um and then activate it again. Um, and then it should do its full activation and we should be able to capture the Uh output signal and take a look at it now. I Just uh noticed this I'm pretty sure. as I mentioned, this is the output power transistor here driveing.

We've got a trim cap here just driving the antenna here. but look, then we've got it. AC coupled. The output to the antenna here is AC coupled and it goes into this separate transistor Arrangement over here.

which as I said, that metal bar there, um, somehow capacitively detects whether or not uh, that antenna is raised up or not. So I think that's what that circuitry there is doing at actually detects whether so the microprocessor before it uh starts to transmit. it probably reads that the antenna it checks to see if the antenna's down. If it is, it goes into test mode and it probably does that via that circuitry there and it looks like it just goes straight back into a pin of the micro.

Okay, so what I've done is I've reverse engineered this little section around here with what looks like a two transistors and that AC coupling cap to the antenna there, and unfortunately, it's not very exciting at all. Here's the antenna. It's got a Um inductor on the output. This is the RF transmitter section and it's picks off that AC couples it.

There's a diode clamp there to ground. there's a voltage divider there driving an MPN transistor with a capacitor on the output going directly to the pin on the microcontroller, which presumably of course, has an internal pull up resistor. That's all there is to it. They've clearly split this board into two halves.

Here's the RF uh oscillator stuff around here. all in this section and then the RF transmitter up here. and it's clearly divided by these digital lines running through here like this, and the only coupling between this part of the circuitry on the left here and the RF part on the right is the Um part we mentioned with that detection circuitry with that AC coupling cap picking off the signal from the antenna and the other one is this little resistor in here which couples this part which I believe is the modulator over to this RF section over here. And that's really.

there's another control signal coming in here like this in there. So there's a digital control signal there, and that probably shuts the transmitter off and on or does something. Perhaps it's another. Maybe it's a digital Um modulator, some sort of uh, test thing.

Maybe it's got something to do with uh, injecting a signal to test that the antenna's up or something like that. I'm not entirely sure. and there's probably something else which looks like it run another control signal runs into here. Something like that.

that's probably to switch between the two frequencies or something. So there needs to be a control line coming from the micro. You can see that there and then this resistor going into this part of the circuitry around here. And as I said, this is the oscillator part of it here.

So that's probably the control signal scope time. Okay, I've hooked up the scope to the antenna output here and watch this. I can make it. Come and go.

There We go. That's my 50 HZ AC coupling from the antenna. I Put my hand on the antenna there and there we go. Isn't that fun? Absolutely nothing to do with the operation of this thing, but just thought I'd show you that.

It's neat. All right. let's see if we can capture something in test mode here. I Have no idea what it's going to, Uh, give out or do anything at all or how to trigger.

That's why you got to experiment with these things to find out. So I got the channel one connected to the antenna output there. Single shot. Uh, capture.

We've got 100 milliseconds per division um, horizontal and 1 volt per division vertical set to trigger just above that. So eh, let's give it a go. Let's press the button and uh, see if we can trigger something something. Oh, there we go.

We got something. There we go. We got a spike and then Boop another little another little. Spike So that's clearly above 1 VT per division.

So let's set it to 5 Vols per Division and try that again. Shall we? Here we go: Let's set the trigger point a bit higher. There we go about 2 volts or something and Trigger bang There we go. So at 5 Vols that's the battery voltage.

Clearly it's applying something to the antenna there. So let's do that again. Let's go for a longer time. Base: 500 milliseconds per division.

Let's try it again. Push the button. Bang bang bang bang. There We go.

We got a bunch of pulses. We got that first one positive, got another one going negative there. I'm that's a huge Is that something? Yeah, something's going on in there. We've got an RF packet there we go.

The sampling is quite poor cuz the memory depth isn't uh, long enough here. This thing hasn't got enough memory at 4 Meg uh, 4 Meg sample memory. but that is, uh, we can have a look at that in more detail later cuz that is quite high so we can set our trigger level higher than that and capture just that RF packet. All right.

So we got the whole thing on the screen here and this is like the switch on pulse presumably when it like switches on the RF stage or something like that. So that's just a Spike as we saw it's you know, there's nothing much doing there. but then we've got this RF pulse here. We've got another one here here here here.

so it does five RF pulses by the looks of it, and then presumably switches the antenna off again. That's what happens in test mode. All right, let's see if we can capture this RF waveform. So I've set it to 5 Vols per division.

Now the trigger level um, at about 10 volts. So because we know that that start and end pulse only went to 5 volts, so we want to trigger above that only on those RF pulses I've uh Gone faster on the horizontal 100 microc uh per division. So just as a rough uh guess. so let's got on single shot capture mode.

Let's do that. I'll press the test button bang. We captured it too. Easy.

There you go. so it looks like it's about 10 15 volts. This is one 1 megga ohm input impedance by the way. I haven't got this like 50 megga ohms input impedance.

So there's the waveform. If we zoom in and uh, we'll turn on some measurements. Bingo! 125.5 I Think if we got in there, they're a bit more precise. The frequency counter on this thing isn't the greatest from the measured Uh data.

So there you go. but that would be 121.5 of course. And if you have a look at the Uh length of the burst here: 5 microc per division 5 10 15 20. it's about 23 micros or thereabouts.

RF Burst at 121.5 mahz. let's capture that again at a much F a Time base 10 nond per division bang. It's still only. it's still saying it's 124.5 MHz It's not 121.5 That's interesting, so maybe it offsets it.

Looks like that is the true frequency there. so it it is offsetting that I guess so it doesn't um, accidentally transmit on the 121 .5 so it tests it at 124.5 So with this coupling circuit here I Think what's probably happening is that when that uh grounding bar is close to the antenna, of course it's got more capacitive coupling. So when it does those um test burst Transmissions There it, uh, there is some capacitive coupling and the amplitude is enough to either um, presumably, uh, not or switch on or switch off this transistor which the microcontroller can detect at those lower Um 23 microc uh time burst time periods which we looked at. So it turns on the RF transmitter and sees and this circuit just picks up um a a the coupled AC coupled signal from the antenna, which the antenna's characteristic is going to change depending on whether or not the antenna's vertical or whether or not it's laying flat right against this grounded bar here.

Now what I'm going to test is that modulation Point down? Well, what I thought was a modulation Point coupling over to the RF Um stage. So I'm going to probe that at the same time as doing the test signal. So let's give that a go. Let's uh, run single shot capture mode and press the test button.

bang and way there the wooo it's way up there. let's uh, change channel looks like we got to go down in channel two. so there you go. Let's let's try that one more time for the dummies bang.

There We go look at that. that is 5 Vols per division. so that point sits at five. H It could just be coupling over.

so I'm not sure what's going on there, but uh, clearly that could just be coupling possibly between. Yeah, obviously it's going to do that. It's probably going to couple over. So I'm not sure whether or not that green Channel 2 signal is controlling that or vice or it's just coupling over.

It seems to be sitting at 5 Vols and then just and then just couples. Oh, it seems to step down a bit there. so let's go out and uh, at a longer time base and let's try and capture that again. and there we go.

I Capt that at a long time base and you can see what's happened. There's a slight change in that point there after the RF burst, so there's it's. kind of doing something in there. I'm not exactly sure what I'm not exactly sure what that's telling us at this stage, but oh well, it's interesting.

Now what I'm going to try is probe the output of that circuit that I reverse engineer which goes back to the micro there and I know that this is a uh, you know, a 5V um logic input signal. So I can set my uh vertical scale to 1 V per division. So I'll probe that and let's have a go and see what we can pick up once again when we do the test thing and single shot test. Boom test mode.

Bang There we go. So aha, UHA Can you see what I see? It goes from zero volts to 5 Vols and back down there. But there's also those green pulses that correspond with the RF burst. So let's once again zoom in on this.

There we go. There's some Ah that's just coupling. That's I think we've coupled in some uh noise there. that's our antenna Earth lead there just picking up some of the RF but that is supposed to be a digital signal if I probe that properly.

Anyway, Let's uh, single shot. Capture that again. Go into test mode. Boom.

There we go. So it drops down low. so that's so that's actually detecting. So that's pulling it low.

So it's detecting that there's that RF signal there. You see how it only goes low. Let's have a look here. see how it only goes low.

After the yellow, the yellow RF amplitude there has ramped up to enough signal level that it actually detects. and then bang. it goes low. And trust me, all that waveform there is just RF pickup That's a that green signal is actually a digital signal.

And just for those who don't believe me that that green signal there is being picked up by this antenna Earthly because that's pretty much what it is when you're talking about RF stuff. Oh, will get rid of that bugger that off and we'll use our little um, high frequency ground Probe on there. They're a bit tricky to sort of hold in place so I won't Probably won't be able to get it on camera at the same time. But uh, let's give that a go with exactly the same conditions and see what we get.

Actually, it's not too bad at all. Look at that cuz they've got that convenient uh via there that the ground connects into. so let's keep that on there. and then we'll single shot capture this waveform again and single shot turn.

push the test button and there we go. Much reduced. We're still picking up some crap on there of course, but as you can see, it's much cleaner now. and if you didn't believe me before, you will believe me now that where you can see where once this RF amplitude ramps up to to a certain signal level that switches on the transistor.

here. once it gets to a certain signal level that transistor switches on, it's just an open collector output with a pull-up resistor there. so it switches low like that once it gets to the level and that the microcontroller can then detect that there's RF on the output so it can detect it's testing there that the RF transmitter works. And now let's probe this point in the circuit where a digital line comes from the microcontroller somewhere into that RF section.

So I expect that to be some sort of gating pulse for the uh RF oscillator. Let's give it a go and there you have it. it is some sort of gaing pulse for that RF uh transmitter there. Once again I've got the antenna Earth lead back on so we're picking up crap here.

It's actually a digital. It's truly a digital signal here. This is a pain in the ass trying to probe. Um, you know, if you're looking at signal Integrity in an RF design like this.

Probing is absolutely everything. I Mean look at that. You would think that there's something wrong with your design when you get all this on it if you don't actually know what's happening with your probing. Anyway, it's clearly some sort of gating pulse, but there's a long time there by a long time.

I mean like you know, 78 micros or something before it by the time that goes low before the RF transmitter actually switches on. But it it is certainly some sort of gating type pulse. and there's another digital line which comes across over here like this. I'm not sure which way it goes, of course, but uh.

I'm going to probe that under the same conditions again and see what we get. Press a test button. Bang. No, we're just getting noise on there so it doesn't seem to be doing anything at all.

Really, that's quite boring. Not at all happy with that. There we go. We got it.

doing something. Now There we go. Okay, it's doing okay. We've got a much longer time base there, so it's it does something over here when it switches on and then and it's really it is doing so.

It is some sort of test signal which is rather interesting. Let let's have a look at the frequency of that 1.47 KZ there. Now this is interesting when the you can just see the yellow RF pulse there. switching on.

this changes frequency from Chang this frequency from actually it's going down. it's going down. Hang on I don't have to probe that anymore. What am I doing I No 700.

Look, it's going up in frequency. see as I go along as I scroll this back to the start. if I go all the way back. look it's going up in frequency.

So that's the modulation tone. There you go. That is the modulation. the decrease in modulation tone Bingo from the microcontroller.

So the micro controller is doing that in software. It starts out at about 1.4 khz. So so Bingo we have found the modulation signal for they use this for the Uh homing. It's a decrease in modulation signal on the carrier.

It starts off at 1. 1475 khz. Let's call it 1.5 and then it slowly decreases in frequency. It's going down and down and down 1 KZ and then it goes all the way down to when the RF switches on Bang There it is.

That's where the r you can see the yellow pulse there. That's the RF burst we've been looking at so it gets down to 700 HZ So it goes from 1.5 Kilz down to 700 HZ That's all being software T driven software timing in the micro and then bang after the Uh RF burst. there then decreases again and of course it goes through that five times. And but this is only the test mode by the way.

so it, uh, would expect to see operational differences. Um, when we're actually transmitting a real signal so um, that's probably all signals I was hoping to find that one. so um, that's probably all the signals I wanted to probe at I'm going to for now anyway. So let's go into let's um, uh, basically get rid of the antenna so it will think that you want to transmit for real and let's actually transmit for real.

So here it is, let's break the seal. How easy it is. It is it to break the seal on this thing. Ah, too easy There we go.

lifted up our antenna, you extend it, and if we press that button bang, it's going to transmit for real because you can see the uh, you see the metal clip in there so it's not going to get that coupling anymore so it's not going to detect that signal using that uh coupling I believe that so it may still use that for something. But anyway, it's um, it's going to go right. I'm not in test mode anymore I Am transmitting for real so it won't stop after like, you know, 5 seconds or whatever it has been now and it'll transmit at full signal power and uh, give the proper homing signal and all that sort of stuff. So what I'm going to do is I'm going to disconnect the antenna of course, even though I don't think it's going to pick it up in the uh office here.

there's all you know. it's got a metal roof and everything. so you know I don't think the satellites not that they can pick up this anymore I believe. But just to be on the safe side, I will disconnect the antenna well I don't actually have to disconnect the antenna.

What I'll do is I'll just take the entire board out of here and uh, bang. we no longer have the antenna connected so that shouldn't. Um, it'll be transmitting, but uh, it really won't be going anywhere. Be horrible.

horribly inefficient by orders of magnitude. All right, let's transmit for real, shall we? I'm in trouble I'm near death and uh, I've got to hit that uh Magic button on my eerb. Let's give it a go. I'm probing the uh antenna output.

We can probe other stuff as well. So here we go. There we go. It's it's now blinking and flashing so it is definitely transmitting.

So let's give it a go. Here we go. Ha. look what we have on the scope Tada We have bursts.

We have RF bursts. Check it out. look at that. So let's let's capture a whole burst.

There we go that one. of course there we go. 121.2 MHz that will actually be 121 .5 if it's the software frequency counter. The automated cursor based thing isn't the best, but that will certainly be 121 down at the lower time bases there.

So bang, There we go. it might be Uh Spectrum analyzer time actually and I just did a quick uh check for what the modulation requirements are for this thing, and apparently for EBS it must go uh, modulate either downwards in frequency or upwards. It can be either anywhere in the range from 300 HZ to 1600 HZ, but it must have at least a spread in there of Uh 700 Hertz modulation and uh, if it's a Plb instead of an Eerb. Apparently that has a requirement of only modulating upwards in frequency instead of downwards.

And of course, the 406 mahz uh digital models um can still optionally have this 125 Uh 121.5 MHz modulated carrier in here as a Uh homing Beacon that is still Uh used by the Search and Rescue helicopters. All right, it's crude measurement time. I've got a uh, just a flying wire here, not actually directly connected, but just Uh coupled nearby to the antenna connector. And of course we've seen on the oscilloscope it's transmitting in those Uh bursts.

and I've got the Ryo Spectr metalizer set up, so let's check it out. And we've got the Riyal DSA 815 Spectr Metalizer here and the dreaded Auto button on oscilloscopes don't like them, but on spectrum metalizers, they not bad for just getting a first whack at a signal there. and bingo there it is where at a center frequency of 1217 mahz and as you can see, it's jumping off. It's not the usual, you know carrier.

Nice clean carrier with the sideb bands that's chopping back and forth because look at what's happening on the scope over here. it's jumping all around so we can uh, get a better look at that if we go into Um single shot capture mode instead of just continuous mode. So if we hit the trigger sweep button here, it gets us into the Uh trigger sweep menu and mode. We're in continuous mode at the moment.

But let's go into single shot capture mode and Bingo! We've captured that and you can see our carrier here as you can see. Center Frequency 121.5 3 Mahz There it is. So we've got that. No problems.

let's go into trigger sweep again. Let's turn that back to continuous. Okay, it's jumping all around the shop. It's not the easiest thing to measure.

Now let's go into Span and let's take it out and see if we can get the 220 odd Meaher Hertz frequency. What is it? 243 Yes. 243 MHz Double. Um.

because I Don't think this thing there it is bang Because this thing there's the Uh 200. There's the alternate frequency of 243 MHz Now I Don't think it's actually got a Um I Don't think it's got a frequency doubler in there. it. It just relies on getting the first harmonic there from the signal.

So I Don't think it's actually got I Don't think it actually jumps frequency to 243. Mahz. it doesn't double. it, just reles on the fact that that is a harmonic of that signal and you can clearly see it there.

And if we go back into the center frequency, we can bring that over. I Can measure it again of course, but uh, where is it 240 Bang There it is 243 megahertz or thereabouts, so that there's our two signals which it's outputting. Now let's see if we can have a look at the Uh modulation on this thing. So what we can do here is actually go into what's called zero span mode and that should allow us to look at bang.

look at that we've that. You can see it modulating. You can see the frequency there modulating. and if you remember from before from the oscope shot, that's essentially the same waveform we got for the actual Um digital modulation signal.

All right, I'm going to use the Uh demodulation function of the DSA 8815 here. so I've got it hooked up to some headphones here and let me put the mic near it and you'll be able to hear the demodulated signal so you can clearly hear the Uh modulation repetition rate there of between 2 and 4. Hertz is what it uh information that I found and uh, that's clearly between 2 and 4. Hertz and uh, you can sort of hear like a sweeping whoop whoop whoop within those within that repetition rate.

Now, it's usually quite difficult to detect. um, these uh, burst RF uh signals on a spectrum analyzer like this because there's all these frequency components that jump around in there and they're not actually uh, real. So what we need to do is go into the Uh bandwidth detection over here and change the video bandwidth like this. and if we low of that, we should find that Bingo our signal or our carrier pops out of that just nicely.

There we go. when we decrease it to a video bandwidth of 300 Hertz 1 KZ It really. you know it starts to play Funny Business and go crazy like that. but if we turn it down, then we can actually see the carrier signal itself.

And if we change the span, of course we can uh oh no, going the wrong way, change our span. Let's go up, up, up, way up, way up and bingo There we have our 121.5 and our 243. MHz We've got some other stuff jumping in here I'm not sure what that's what's going on there, but we can clearly see our two carriers there. There it is 121.5 and 243.

Brilliant. And we should be able to get better performance. Um, on a spectrum analyzer that has gating capability for burst type signals I Don't think this Ryo one has it. or at least I haven't found it yet.

Now if we probe this Uh modulation signal we found before and if we have a look at that, we can see if we trigger off uh, channel two. Here there we go trigger off channel two, we can see our modulation signal. Okay, okay now let's actually have a look at that signal we had coming back before. If you remember from the Uh detection circuitry on the antenna and I'm dual wielding probes here using my third hand and uh, there's the signal.

The yellow one is the return from that detection circuitry there slightly out of phase, but it's the exact signal that's being inputed to the RF circuitry. So if we if we disconnect that and we actually have a look at what's Happening Here the microcontroller is outputting the modulation signal through here through some AC coupling into the oscillator down here which amplitude modulates it and then it goes through the RF transmitter and that's tapping off and that detection circuitry is reading it back and feeding it back in so that microcontroller knows that it's actually transmitting the proper Uh location, transmitting that home in uh, modulation data and presumably um, it'll you know, stop flashing and or do whatever you know, error, error, and uh uh when if it doesn't detect that output signal. So it's when you when it's flashing like that you know that it's it's doing that transmitting and well you know somebody's going to pick it up and they're going to be on their way. So presumably um, if you get nothing on the output means part of the uh RF circuitry is blowing and I presume it will take will turn off the flashing lights.

And of course, there's no need to speculate about that. Let's actually test it. I've removed the AC coupling cap that detects the output signal from the antenna and let's see if this thing actually um continues to flash the LEDs and transmits I Wonder if it does? Let's have a look. oh no.

there you go. No, it's still doing it. Look at that. I have disconnected that AC coupling cap.

Getting desperate now I've removed the AC coupling cap, the 10K resistor and the output transistor so that open collector one. so it's got nothing to uh, switch low WID so that pullup resistor on the circuit there. if we remove that transistor, we remove the AC coupling cap 10K and the transistor. ah, that should.

That should stop that. So let's do it. No, it's not detecting it. There you go.

That's pretty conclusive that it doesn't check that output when it's transmitt. Oh man, it's still modulating. It's it's it. It's still picking it up on the scope.

This is ridiculous. What's going on? Well doll, that's embarrassing I was probing the wrong Trace I got mixed up on which Trace was coming back I was probing that second pin instead of the third one which is the one that comes back. ah don't Anyway, we did prove there that uh, you remov that output um, sampling there that that output to protection circuitry and it's not. It doesn't care about uh sampling that during the Uh transmission period and I've replaced the components and now I am probing the correct pin here.

so that is the return pin instead of what looks like another either another return pin or a transmission pin. and let's take a look at waveform coming back from that detection circuitry and there it is. It is actually demodulating that output signal correctly. So there you have it.

That's the Kti Minisat personal eerb, the old 121.5 MHz analog models and that was just a little bit of playing around with it. There could be some more, uh, interesting stuff going on in the surcy and stuff. So if anyone's got a schematic, uh, we'd love to uh, see it. And if you want to discuss this, jump on over to the Eev blog forum and if you like tear down Tuesday Please give it a big thumbs up.

Catch you next time.